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Open Access

Peer-reviewed

Research Article

Talking about Climate Change and Global Warming

Contributed equally to this work with: Maurice Lineman, Yuno Do

Affiliation College of Natural Sciences, Department of Biological Sciences, Pusan National University, Busan, South Korea

* E-mail: [email protected]

  • Maurice Lineman, 
  • Yuno Do, 
  • Ji Yoon Kim, 
  • Gea-Jae Joo

PLOS

  • Published: September 29, 2015
  • https://doi.org/10.1371/journal.pone.0138996
  • Reader Comments

Fig 1

The increasing prevalence of social networks provides researchers greater opportunities to evaluate and assess changes in public opinion and public sentiment towards issues of social consequence. Using trend and sentiment analysis is one method whereby researchers can identify changes in public perception that can be used to enhance the development of a social consciousness towards a specific public interest. The following study assessed Relative search volume (RSV) patterns for global warming (GW) and Climate change (CC) to determine public knowledge and awareness of these terms. In conjunction with this, the researchers looked at the sentiment connected to these terms in social media networks. It was found that there was a relationship between the awareness of the information and the amount of publicity generated around the terminology. Furthermore, the primary driver for the increase in awareness was an increase in publicity in either a positive or a negative light. Sentiment analysis further confirmed that the primary emotive connections to the words were derived from the original context in which the word was framed. Thus having awareness or knowledge of a topic is strongly related to its public exposure in the media, and the emotional context of this relationship is dependent on the context in which the relationship was originally established. This has value in fields like conservation, law enforcement, or other fields where the practice can and often does have two very strong emotive responses based on the context of the problems being examined.

Citation: Lineman M, Do Y, Kim JY, Joo G-J (2015) Talking about Climate Change and Global Warming. PLoS ONE 10(9): e0138996. https://doi.org/10.1371/journal.pone.0138996

Editor: Hayley J. Fowler, Newcastle University, UNITED KINGDOM

Received: August 18, 2014; Accepted: September 8, 2015; Published: September 29, 2015

Copyright: © 2015 Lineman et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Data Availability: All relevant data are within the paper.

Funding: This study was financially supported by the 2015 Post-Doc Development Program of Pusan National University.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Identifying trends in the population, used to be a long and drawn out process utilizing surveys and polls and then collating the data to determine what is currently most popular with the population [ 1 , 2 ]. This is true for everything that was of merit to the political organizations present, regarding any issue of political or public interest.

Recently, the use of the two terms ‘Climate Change’ and ‘Global Warming’ have become very visible to the public and their understanding of what is happening with respect to the climate [ 3 ]. The public response to all of the news and publicity about climate has been a search for understanding and comprehension, leading to support or disbelief. The two terms while having similarity in meaning are used in slightly different semantic contexts. The press in order to expand their news readership/viewer lists has chosen to use this ambiguity to their favor in providing news to the public [ 4 ]. Within the news releases, the expression ‘due to climate change’ has been used to explain phenomological causality.

These two terms “global warming–(GW)” and “climate change–(CC)” both play a role in how the public at large views the natural world and the changes occurring in it. They are used interactively by the news agencies, without a thought towards their actual meaning [ 3 , 4 ]. Therefore, the public in trying to identify changes in the news and their understanding of those changes looks for the meaning of those terms online. The extent of their knowledge can be examined by assessing the use of the terms in online search queries. Information searches using the internet are increasing, and therefore can indicate public or individual interest.

Internet search queries can be tracked using a variety of analytic engines that are independent of, or embedded into, the respective search engines (google trend, naver analytics) and are used to determine the popularity of a topic in terms of internet searches [ 5 ]. The trend engines will look for selected keywords from searches, keywords chosen for their relevance to the field or the query being performed.

The process of using social media to obtain information on public opinion is a practice that has been utilized with increasing frequency in modern research for subjects ranging from politics [ 6 , 7 ] to linguistics [ 8 – 10 ] complex systems [ 11 , 12 ] to environment [ 13 ]. This variety of research belies the flexibility of the approach, the large availability of data availability for mining in order to formulate a response to public opinion regarding the subject being assessed. In modern society understanding how the public responds regarding complex issues of societal importance [ 12 ].

While the two causally connected terms GW and CC are used interchangeably, they describe entirely different physical phenomena [ 14 ]. These two terms therefore can be used to determine how people understand the parallel concepts, especially if they are used as internet search query terms in trend analysis. However, searching the internet falls into two patterns, searches for work or for personal interest, neither of which can be determined from the trend engines. The By following the searches, it is possible to determine the range of public interest in the two terms, based on the respective volumes of the search queries. Previously in order to mine public opinion on a subject, government agencies had to revert to polling and surveys, which while being effective did not cover a very large component of the population [ 15 – 17 ].

Google trend data is one method of measuring popularity of a subject within the population. Individuals searching for a topic use search keywords to obtain the desired information [ 5 , 18 ]. These keywords are topic sensitive, and therefore indicate the level of knowledge regarding the searched topic. The two primary word phrases here “climate change” and “global warming” are unilateral terms that indicate a level of awareness about the issue which is indicative of the individuals interest in that subject [ 5 , 19 , 20 ]. Google trend data relates how often a term is searched, that is the frequency of a search term can be identified from the results of the Google® trend analysis. While frequency is not a direct measure of popularity, it does indicate if a search term is common or uncommon and the value of that term to the public at large. The relationship between frequency and popularity lies in the volume of searches by a large number of individuals over specific time duration. Therefore, by identifying the number of searches during a specific period, it is possible to come to a proximate understanding of how popular or common a term is for the general population [ 21 ]. However, the use of trend data is more appropriately used to identify awareness of an issue rather than its popularity.

This brings us to sentiment analysis. Part of the connection between the search and the populations’ awareness of an issue can be measured using how they refer to the subject in question. This sentiment, is found in different forms of social media, or social networking sites sites i.e. twitter®, Facebook®, linked in® and personal blogs [ 7 , 22 – 24 ]. Thus, the original information, which was found on the internet, becomes influenced by personal attitudes and opinions [ 25 ]and then redistributed throughout the internet, accessible to anyone who has an internet connection and the desire to search. This behavior affects the information that now provides the opportunity to assess public sentiment regarding the prevailing attitudes regarding environmental issues [ 26 , 27 ]. To assess this we used Google® and Twitter® data to understand public concerns related to climate change and global warming. Google trend was used to trace changes in interest between the two phenomena. Tweets (comments made on Twitter®) were analyzed to identify negative or positive emotional responses.

Comparatively, twitter data is more indicative of how people refer to topics of interest [ 28 – 31 ], in a manner that is very linguistically restricted. As well, twitter is used as a platform for verbal expression of emotional responses. Due to the restrictions on tweet size (each tweet can only be 140 characters in length), it is necessary to be more direct in dealing with topics of interest to the tweeter. Therefore, the tweets are linguistically more emotionally charged and can be used to define a level of emotional response by the tweeter.

The choice of target words for the tweets and for the Google trend searches were the specific topic phrases [ 32 , 33 ]. These were chosen because of the descriptive nature of the phrases. Scientific literature is very specific in its use and therefore has very definitive meanings. The appropriation of these words by the population as a method for describing their response to the variation in the environment provides the basis for the choice as target words for the study. The classification of the words as being positive versus negative lies in the direction provided by Frank Lutz. This politicization of a scientific word as a means of directing public awareness, means the prescription of one phrase (climate change) as being more positive than the other (global warming).

Global warming is defined as the long-term trend of increasing average global temperatures; alternatively, climate change is defined as a change in global or regional climate patterns, in particular a change apparent from the mid to late 20 th century onwards and attributed to the increased levels of atmospheric carbon dioxide arising from the use of fossil fuels. Therefore, the search keywords were chosen based on their scientific value and their public visibility. What is important about the choice of these search terms is that due to their scientific use, they describe a distinctly identifiable state. The more specific these words are, the less risk of the algorithm misinterpreting the keyword and thus having the results misinterpreted [ 34 – 36 ].

The purpose of the following study was to identify trends within search parameters for two specific sets of trend queries. The second purpose of the study was to identify how the public responds emotionally to those same queries. Finally, the purpose of the study was to determine if the two had any connections.

Data Collection

Public awareness of the terms climate change and global warming was identified using Google Trends (google.com/trends) and public databases of Google queries [ 37 ]. To specify the exact searches we used the two terms ‘climate change’ and ‘global warming’ as query phrases. Queries were normalized using relative search volume (RSV) to the period with the highest proportion of searches going to the focal terms (i.e. RSV = 100 is the period with the highest proportion for queries within a category and RSV = 50 when 50% of that is the highest search proportion). Two assumptions were necessary for this study. The first is, of the two terms, climate change and global warming, that which draws more search results is considered more interesting to the general population. The second assumption is that changes in keyword search patterns are indicators of the use of different forms of terminology used by the public. To analyze sentiments related to climate change and global warming, tweets containing acronyms for climate change and global warming were collected from Twitter API for the period from October 12 to December 12, 2013. A total of 21,182 and 26,462 tweets referencing the terms climate change and global warming were collected respectively. When duplicated tweets were identified, they were removed from the analysis. The remaining tweets totaled 8,465 (climate change) and 8,263 (global warming) were compiled for the sentiment analysis.

Data Analysis

In Twitter® comments are emotionally loaded, due to their textually shortened nature. Sentiment analysis, which is in effect opinion mining, is how opinions in texts are assessed, along with how they are expressed in terms of positive, neutral or negative content [ 36 ]. Nasukawa and Yi [ 10 ]state that sentiment analysis identifies statements of sentiment and classifies those statements based on their polarity and strength along with their relationship to the topic.

Sentiment analysis was conducted using Semantria® software ( www.semantria.com ), which is available as an MS Excel spreadsheet application plugin. The plugin is broken into parts of speech (POS), the algorithm within the plugin then identifies sentiment-laden phrases and then scores them from -10 to 10 on a logarithmic scale, and finally the scores for each POS are tabulated to identify the final score for each phrase. The tweets are then via statistical inferences tagged with a numerical value from -2 to 2 and given a polarity, which is classified as positive, neutral or negative [ 36 ]. Semantria®, the program utilized for this study, has been used since 2011 to perform sentiment analyses [ 7 , 22 ].

For the analysis, an identity column was added to the dataset to enable analysis of individual tweets with respect to sentiment. A basic sentiment analysis was conducted on the dataset using the Semantria® plugin. The plugin uses a cloud based corpus of words tagged with sentimental connotations to analyze the dataset. Through statistical inference, each tweet is tagged with a sentiment value from -2 to +2 and a polarity of (i) negative, (ii) neutral, or (iii) positive. Positive nature increases with increasing positive sentiment. The nature of the language POS assignation is dependent upon the algorithmic classification parameters defined by the Semantria® program. Determining polarity for each POS is achieved using the relationship between the words as well as the words themselves. By assigning negative values to specific negative phrases, it limits the use of non-specific negation processes in language; however, the program has been trained to assess non-specific linguistic negations in context.

A tweet term frequency dictionary was computed using the N-gram method from the corpus of climate change and global warming [ 38 ]. We used a combination of unigrams and bigrams, which has been reported to be effective [ 39 ]. Before using the N-gram method, typological symbols were removed using the open source code editor (i.e. Notepad) or Microsoft Words’ “Replace” function.

Differences in RSV’s for the terms global warming and climate change for the investigation period were identified using a paired t-test. Pettitt and Mann-Kendall tests were used to identify changes in distribution, averages and the presence of trends within the weekly RSV’s. The Pettitt and MK tests, which assume a stepwise shift in the mean (a break point) and are sensitive to breaks in the middle of a time series, were applied to test for homogeneity in the data [ 40 ]. Temporal trends within the time series were analyzed with Spearman’s non-parametric correlation analysis. A paired t-test and Spearman’s non-parametric correlation analysis were conducted using SPSS software (version 17.0 SPSS In corp. Chicago IL) and Pettitt and MK tests were conducted using XLSTAT (version 7.0).

To determine the accuracy and reliability of the Sentiment analysis, a Pearson’s chi-square analysis was performed. This test identifies the difference ratio for each emotional response group, and then compares them to determine reliance and probability of interactions between the variables, in this case the terms global warming and climate change.

According to Google trend ( Fig 1 ) from 2004–2014, people searched for the term global warming (n = 8,464; mean ± S.D = 25.33 ± 2.05) more frequently than climate change (n = 8,283; mean ± S.D. = 7.97±0.74). Although the Intergovernmental Panel on Climate Change (IPCC) published its Fourth Assessment Report in 2007 and was awarded the Nobel Prize, interest in the term global warming as used in internet searches has decreased significantly since 2010 (K = 51493, t = 2010-May-23, P<0.001). Further the change in RSV also been indicative of the decreased pattern (Kendall’s tau = -0.336, S = -44563, P<0.001). The use of the term “climate change” has risen marginally since 2006 (K = 38681, t = 2006-Oct-08, P<0.001), as indicated by a slight increase (Kendall’s tau = -0.07, S = 9068, P<0.001). These findings show that the difference in usage of the two terms climate change and global warming has recently been reduced.

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https://doi.org/10.1371/journal.pone.0138996.g001

The sentiment analysis of tweets ( Fig 2 ) shows that people felt more negative about the term global warming (sentiment index = -0.21±0.34) than climate change (-0.068±0.36). Global warming tweets reflecting negative sentiments via descriptions such as, “bad, fail, crazy, afraid and catastrophe,” represented 52.1% of the total number of tweets. As an example, the tweet, “Supposed to snow here in the a.m.! OMG. So sick of already, but Saturday says 57 WTF!” had the lowest score at -1.8. Another observation was that 40.7% of tweets, including “agree, recommend, rescue, hope, and contribute,” were regarded as neutral. While 7.2% of tweets conveyed positive messages such as, “good, accept, interesting, and truth.” One positive global warming tweet, read, “So if we didn’t have global warming, would all this rain be snow!”. The results from the Pearson’s chi-square analysis showed that the relationship between the variables was significant (Pearson’s chi-square –763.98, d.f. = 2, P<0.001). Negative climate change tweets represented 33.1% of the total while neutral tweets totaled 49.8%, while positive climate change tweets totaled 17.1%.

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https://doi.org/10.1371/journal.pone.0138996.g002

Understandably, global warming and climate change are the terms used most frequently to describe each phenomenon, respectively, as revealed by the N-gram analysis ( Table 1 ). When people tweeted about global warming, they repeatedly used associated such as, “ice, snow, Arctic, and sea.” In contrast, tweets referring to climate change commonly used, “report, IPCC, world, science, environment, and scientist.” People seem to think that climate change as a phenomenon is revealed by scientific investigation.

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https://doi.org/10.1371/journal.pone.0138996.t001

Internet searches are one way of understanding the popularity of an idea or meme within the public at large. Within that frame of reference, the public looks at these two terms global warming and climate change and their awareness of the roles of the two phenomena [ 41 ]. From 2004 to 2008, the search volumes for the term global warming far exceeded the term climate change. The range for the term global warming in Relative search volumes (RSV) was more than double that of climate change in this period ( Fig 1 ). From 2008 on the RSV’s began to steadily decrease until in 2014 when the RSV’s for the term global warming were nearly identical to those for the term climate change. From 2008 there was an increase in the RSVs for CC until 2010 at which point the RSVs also began to decline for the term climate change. The decline in the term climate change for the most part paralleled that of the term global warming from 2010 on to the present.

While we are seeing the increases and decreases in RSVs for both the terms global warming and climate change, the most notable changes occur when the gap between the terms was the greatest, from 2008 through to 2010. During this period, there was a very large gap found between the RSVs for the terms global warming and climate change; however, searches for the term climate change was increasing while searches for the tem global warming were decreasing. The counter movement of the RSV’s for the two terms shows that there is a trend happening with respect to term recognition. At this point, there was an increase in the use of the CC term while there was a corresponding decrease in the use of the GW term. The change in the use of the term could have been due to changes in the publicity of the respective terms, since at this point, the CC term was being used more visibly in the media, and therefore the CC term was showing up in headlines and the press, resulting in a larger number of searches for the CC term. Correspondingly, the decrease in the use of the GW term is likely due to the changes in how the term was perceived by the public. The public press determines how a term is used, since they are the body that consistently utilizes a term throughout its visible life. The two terms, regardless of how they differ in meaning, are used with purpose in a scientific context, yet the public at large lacks this definition and therefore has no knowledge of the variations in the terms themselves [ 42 ]. Therefore, when searching for a term, the public may very well, choose the search term that they are more comfortable with, resulting in a search bias, since they do not know the scientific use of the term.

The increase in the use of the CC term, could be a direct result of the release of the fourth assessment report for the IPCC in 2007 [ 43 ]. The publicity related to the release of this document, which was preceded by the release of the Al Gore produced documentary “An Inconvenient Truth”, both of which were followed by the selection by the Nobel committee of Al Gore and the IPCC scientists for the Nobel Prize in 2007 [ 43 ]. These three acts individually may not have created the increased media presence of the CC term; however, at the time the three events pushed the CC term and increased its exposure to the public which further drove the public to push for positive environmental change at the political level [ 44 , 45 ]. This could very well have resulted in the increases in RSV’s for the CC term. This point is more likely to depict accurately the situation, since in 2010 the use of the two terms decline at almost the same rate, with nearly the same patterns.

Thus with respect to trend analysis, what is interesting is that RSVs are paralleling the press for specific environmental events that have predetermined value according to the press. The press in increasing the visibility of the term may drive the increases in the RSV’s for that term. Prior to 2007, the press was using the GW term indiscriminately whenever issues affecting the global climate arose; however, after the movie, the report and then the Nobel prize the terminology used by the press switched and the CC term became the word du jour. This increased the visibility of the word to the public, thereby it may be that increasing public awareness of the word, but not necessarily its import, is the source for the increases in RSV’s between 2008 and 2010.

The decline in the RSV’s then is a product of the lack of publicity about the issue. As the terms become more familiar, there would be less necessity to drive the term publicly into the spotlight; however, occasionally events/situations arise that refocus the issue creating a resurgence in the terms even though they have reached their peak visibility between 2008 and 2010.

Since these terms have such an impact on the daily lives of the public via local regional national and global weather it is understandable that they have an emotional component to them [ 46 ]. Every country has its jokes about the weather, where they come up with cliché’s about the weather (i.e. if you don’t like the weather wait 10minutes) that often show their discord and disjunction with natural climatological patterns [ 47 ]. Furthermore, some sectors of society (farmers) have a direct relationship with the climate and their means of living; bad weather is equal to bad harvests, which means less money. To understand how society represents this love hate relationship with the weather, the twitter analysis was performed. Twitter, a data restricted social network system, has a limited character count to relay information about any topic the sender chooses to relate. These tweets can be used to assess the sentiment of the sender towards a certain topic. As stated previously, the sentiment is defined by the language of the tweet within the twitter system. Sentiment analysis showed that the two terms differed greatly. Based on the predefined algorithm for the sentiment analysis, certain language components carried a positive sentiment, while others carried a negative sentiment. Tweets about GW and CC were subdivided based on their positive, neutral and negative connotations within the tweet network. These emotions regardless of their character still play a role in how humans interacts with surroundings including other humans [ 48 , 49 ] As seen in Fig 2 the different terms had similar distributions, although with different ranges in the values. Global warming showed a much smaller positive tweet value than did climate change. Correspondent to this the respective percentage of positive sentiments for CC was more than double that of GW. Comparatively, the neutral percentiles were more similar for each term with a small difference. However, the negative sentiments for the two terms again showed a greater disparity, with negative statements about GW nearly double those of climate change.

These differences show that there is a perceptive difference in how the public relates to the two terms Global Warming and Climate Change [ 50 , 51 ]. Climate change is shown in a more positive light than global warming simply based on the tweets produced by the public. The difference in how people perceive climate change and global warming is possibly due to the press, personal understanding of the terms, or level of education. While this in itself is indefinable, since by nature tweets are linguistically restrictive, the thing to take from it is that there is a measurable difference in how individuals respond to climatological changes that they are experiencing daily. These changes have a describable effect on how the population is responding to the publicity surrounding the two terms to the point where it can be used to manipulate governmental policy [ 52 ].

Sentiment analysis is a tool that can be used to determine how the population feels about a topic; however, the nature of the algorithm makes it hard to effectively determine how this is being assessed. For the current study, the sentiment analysis showed that there was a greater negative association with the term global warming than with the term climate change. This difference, which while being an expression of individual like or dislike at the time the tweet was created, denotes that the two terms were either not understood in their true form, or that individuals may have a greater familiarity with one term over the other, which may be due to a longer exposure to the term (GW) or the negative press associated with the term (GW).

Conclusions

Trend analysis identified that the public is aware of the terminology used to describe climatological variation. The terminology showed changes in use over time with global warming starting as the more well-known term, and then its use decreased over time. At the same time, the more definitive term climate change had less exposure early on; however, with the increase of press exposure, the public became increasingly aware of the term and its more accurate definition. This increase appeared to be correspondent with the increasing publicity around three very powerful press exposure events (a documentary, a scientific report release and a Nobel Prize). The more the term was used the more people came to use it, this included searches on the internet.

Comparatively sentiment analysis showed that the two terms had differential expressions in the population. With climate change being seen in a more positive frame than global warming. The use of sentiment analysis as a tool to evaluate how the population is responding to a feature is an important tool. However, it is a tool that measures, it does not define.

Social network systems and internet searches are effective tools in identifying changes in both public awareness and public perception of an issue. However, in and of itself, these are bell ringers they can be used to determine the importance of an issue, but not the rationale behind the why it is important. This is an important fact to remember when using analytical tools that evaluate social network systems and their use by the public.

Acknowledgments

This study was financially supported by the 2015 Post-Doc. Development Program of Pusan National University

Author Contributions

Conceived and designed the experiments: YD GJJ. Performed the experiments: ML YD. Analyzed the data: ML YD. Contributed reagents/materials/analysis tools: JK YD. Wrote the paper: ML YD GJJ.

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global warming paper research

Roz Pidcock

Which of the many thousands of papers on climate change published each year in scientific journals are the most successful? Which ones have done the most to advance scientists’ understanding, alter the course of climate change research, or inspire future generations?

On Wednesday, Carbon Brief will reveal the results of our analysis into which scientific papers on the topic of climate change are the most “cited”. That means, how many times other scientists have mentioned them in their own published research. It’s a pretty good measure of how much impact a paper has had in the science world.

But there are other ways to measure influence. Before we reveal the figures on the most-cited research, Carbon Brief has asked climate experts what they think are the most influential papers.

We asked all the coordinating lead authors, lead authors and review editors on the last Intergovernmental Panel on Climate Change (IPCC) report to nominate three papers from any time in history. This is the exact question we posed:

What do you consider to be the three most influential papers in the field of climate change?

As you might expect from a broad mix of physical scientists, economists, social scientists and policy experts, the nominations spanned a range of topics and historical periods, capturing some of the great climate pioneers and the very latest climate economics research.

Here’s a link to our summary of who said what . But one paper clearly takes the top spot.

Winner: Manabe & Wetherald ( 1967 )

With eight nominations, a seminal paper by Syukuro Manabe and Richard. T. Wetherald published in the Journal of the Atmospheric Sciences in 1967 tops the Carbon Brief poll as the IPCC scientists’ top choice for the most influential climate change paper of all time.

Entitled, “Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity”, the work was the first to represent the fundamental elements of the Earth’s climate in a computer model, and to explore what doubling carbon dioxide (CO2) would do to global temperature.

Manabe & Wetherald (1967), Journal of the Atmospheric Sciences

Manabe & Wetherald (1967), Journal of the Atmospheric Sciences

The Manabe & Wetherald paper is considered by many as a pioneering effort in the field of climate modelling, one that effectively opened the door to projecting future climate change. And the value of climate sensitivity is something climate scientists are still grappling with today .

Prof Piers Forster , a physical climate scientist at Leeds University and lead author of the chapter on clouds and aerosols in working group one of the last IPCC report, tells Carbon Brief:

This was really the first physically sound climate model allowing accurate predictions of climate change.

The paper’s findings have stood the test of time amazingly well, Forster says.

Its results are still valid today. Often when I’ve think I’ve done a new bit of work, I found that it had already been included in this paper.

Prof Steve Sherwood , expert in atmospheric climate dynamics at the University of New South Wales and another lead author on the clouds and aerosols chapter, says it’s a tough choice, but Manabe & Wetherald (1967) gets his vote, too. Sherwood tells Carbon Brief:

[The paper was] the first proper computation of global warming and stratospheric cooling from enhanced greenhouse gas concentrations, including atmospheric emission and water-vapour feedback.

Prof Danny Harvey , professor of climate modelling at the University of Toronto and lead author on the buildings chapter in the IPCC’s working group three report on mitigation, emphasises the Manabe & Wetherald paper’s impact on future generations of scientists. He says:

[The paper was] the first to assess the magnitude of the water vapour feedback, and was frequently cited for a good 20 years after it was published.

Tomorrow, Carbon Brief will be publishing an interview with Syukuro Manabe, alongside a special summary by Prof John Mitchell , the Met Office Hadley Centre’s chief scientist from 2002 to 2008 and director of climate science from 2008 to 2010, on why the paper still holds such significance today.

Joint second: Keeling, C.D et al. ( 1976 )

Jumping forward a decade, a classic paper by Charles Keeling and colleagues in 1976 came in joint second place in the Carbon Brief survey.

Published in the journal Tellus under the title, “Atmospheric carbon dioxide variations at Mauna Loa observatory,” the paper documented for the first time the stark rise of carbon dioxide in the atmosphere at the Mauna Loa observatory in Hawaii.

A photocopy of Keeling et al., (1976) Source: University of California, Santa Cruz

A photocopy of Keeling et al., (1976) Source: University of California, Santa Cruz

Dr Jorge Carrasco , Antarctic climate change researcher at the University of Magallanes  in Chile and lead author on the cryosphere chapter in the last IPCC report, tells Carbon Brief why the research underpinning the “Keeling Curve’ was so important.

This paper revealed for the first time the observing increased of the atmospheric CO2 as the result of the combustion of carbon, petroleum and natural gas.

Prof David Stern , energy and environmental economist at the Australian National University and lead author on the Drivers, Trends and Mitigation chapter of the IPCC’s working group three report, also chooses the 1976 Keeling paper, though he notes:

This is a really tough question as there are so many dimensions to the climate problem – natural science, social science, policy etc.

With the Mauna Loa measurements continuing today , the so-called “Keeling curve” is the longest continuous record of carbon dioxide concentration in the world. Its historical significance and striking simplicity has made it one of the most iconic visualisations of climate change.

Source: US National Oceanic and Atmospheric Administration (NOAA)

Source: US National Oceanic and Atmospheric Administration (NOAA)

Also in joint second place: Held, I.M. & Soden, B.J. ( 2006 )

Fast forwarding a few decades, in joint second place comes a paper by Isaac Held and Brian Soden published in the journal Science in 2006.

The paper, “Robust Responses of the Hydrological Cycle to Global Warming”, identified how rainfall from one place to another would be affected by climate change. Prof Sherwood, who nominated this paper as well as the winning one from Manabe and Wetherald, tells Carbon Brief why it represented an important step forward. He says:

[This paper] advanced what is known as the “wet-get-wetter, dry-get-drier” paradigm for precipitation in global warming. This mantra has been widely misunderstood and misapplied, but was the first and perhaps still the only systematic conclusion about regional precipitation and global warming based on robust physical understanding of the atmosphere.

Extract from Held & Soden (2006), Journal of Climate

Held & Soden (2006), Journal of Climate

Honourable mentions

Rather than choosing a single paper, quite a few academics in our survey nominated one or more of the Working Group contributions to the last IPCC report. A couple even suggested the Fifth Assessment Report in its entirety, running to several thousands of pages. The original IPCC report , published in 1990, also got mentioned.

It was clear from the results that scientists tended to pick papers related to their own field. For example, Prof Ottmar Edenhofer , chief economist at the Potsdam Institute for Climate Impact Research and co-chair of the IPCC’s Working Group Three report on mitigation, selected four papers from the last 20 years on the economics of climate change costs versus risks, recent emissions trends, the technological feasibility of strong emissions reductions and the nature of international climate cooperation.

Taking a historical perspective, a few more of the early pioneers of climate science featured in our results, too. For example, Svante Arrhenius’ famous 1896 paper  on the Greenhouse Effect, entitled “On the influence of carbonic acid in the air upon the temperature of the ground”, received a couple of votes.

Prof Jonathan Wiener , environmental policy expert at Duke University in the US and lead author on the International Cooperation chapter in the IPCC’s working group three report, explains why this paper should be remembered as one of the most influential in climate policy. He says:

[This is the] classic paper showing that rising greenhouse gas concentrations lead to increasing global average surface temperature.

Svante Arrhenius (1896), Philosophical Magazine

Svante Arrhenius (1896), Philosophical Magazine

A few decades later, a paper by Guy Callendar in 1938  linked the increase in carbon dioxide concentration over the previous 50 years to rising temperatures. Entitled, “The artificial production of carbon dioxide and its influence on temperature,” the paper marked an important step forward in climate change research, says Andrew Solow , director of the Woods Hole Marine Policy centre and lead author on the detection and attribution of climate impacts chapter in the IPCC’s working group two report. He says:

There is earlier work on the greenhouse effect, but not (to my knowledge) on the connection between increasing levels of CO2 and temperature.

Though it may feature in the climate change literature hall of fame, this paper raises a question about how to define a paper’s influence, says Forster. Rather than being celebrated among his contemporaries, Callendar’s work achieved recognition a long time after it was published. Forster says:

I would loved to have chosen Callendar (1938) as the first attribution paper that changed the world. Unfortunately, the 1938 effort of Callendar was only really recognised afterwards as being a founding publication of the field … The same comment applies to earlier Arrhenius and Tyndall efforts. They were only influential in hindsight.

Guy Callendar and his 1938 paper in Quarterly Journal of the Royal Meteorological Society

Guy Callendar and his 1938 paper in Quarterly Journal of the Royal Meteorological Society

Other honourable mentions in the Carbon Brief survey of most influential climate papers go to Norman Phillips, whose 1956 paper described the first general circulation model, William Nordhaus’s 1991 paper on the economics of the greenhouse effect, and a paper by Camile Parmesan and Gary Yohe in 2003 , considered by many to provide the first formal attribution of climate change impacts on animal and plant species.

Finally, James Hansen’s 2012 paper , “Public perception of climate change and the new climate dice”, was important in highlighting the real-world impacts of climate change, says Prof Andy Challinor , expert in climate change impacts at the University of Leeds and lead author on the food security chapter in the working group two report. He says:

[It] helped with demonstrating the strong links between extreme events this century and climate change. Result: more clarity and less hedging.

Marc Levi , a political scientist at Columbia University and lead author on the IPCC’s human security chapter, makes a wider point, telling Carbon Brief:

The importance is in showing that climate change is observable in the present.

Indeed, attribution of extreme weather continues to be at the forefront of climate science, pushing scientists’ understanding of the climate system and modern technology to their limits.

Look out for more on the latest in attribution research as Carbon Brief reports on the Our Common Futures Under Climate Change conference taking place in Paris this week.

Pinning down which climate science papers most changed the world is difficult, and we suspect climate scientists could argue about this all day. But while the question elicits a range of very personal preferences, stories and characters, one paper has clearly stood the test of time and emerged as the popular choice among today’s climate experts – Manabe and Wetherald, 1967.

Main image: Satellite image of Hurricane Katrina.

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Global Warming 101

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What is global warming?

What causes global warming, how is global warming linked to extreme weather, what are the other effects of global warming, where does the united states stand in terms of global-warming contributors, is the united states doing anything to prevent global warming, is global warming too big a problem for me to help tackle.

A: Since the Industrial Revolution, the global annual temperature has increased in total by a little more than 1 degree Celsius, or about 2 degrees Fahrenheit. Between 1880—the year that accurate recordkeeping began—and 1980, it rose on average by 0.07 degrees Celsius (0.13 degrees Fahrenheit) every 10 years. Since 1981, however, the rate of increase has more than doubled: For the last 40 years, we’ve seen the global annual temperature rise by 0.18 degrees Celsius, or 0.32 degrees Fahrenheit, per decade.

The result? A planet that has never been hotter . Nine of the 10 warmest years since 1880 have occurred since 2005—and the 5 warmest years on record have all occurred since 2015. Climate change deniers have argued that there has been a “pause” or a “slowdown” in rising global temperatures, but numerous studies, including a 2018 paper published in the journal Environmental Research Letters , have disproved this claim. The impacts of global warming are already harming people around the world.

Now climate scientists have concluded that we must limit global warming to 1.5 degrees Celsius by 2040 if we are to avoid a future in which everyday life around the world is marked by its worst, most devastating effects: the extreme droughts, wildfires, floods, tropical storms, and other disasters that we refer to collectively as climate change . These effects are felt by all people in one way or another but are experienced most acutely by the underprivileged, the economically marginalized, and people of color, for whom climate change is often a key driver of poverty, displacement, hunger, and social unrest.

A: Global warming occurs when carbon dioxide (CO 2 ) and other air pollutants collect in the atmosphere and absorb sunlight and solar radiation that have bounced off the earth’s surface. Normally this radiation would escape into space, but these pollutants, which can last for years to centuries in the atmosphere, trap the heat and cause the planet to get hotter. These heat-trapping pollutants—specifically carbon dioxide, methane, nitrous oxide, water vapor, and synthetic fluorinated gases—are known as greenhouse gases, and their impact is called the greenhouse effect .

Though natural cycles and fluctuations have caused the earth’s climate to change several times over the last 800,000 years, our current era of global warming is directly attributable to human activity—specifically to our burning of fossil fuels such as coal, oil, gasoline, and natural gas, which results in the greenhouse effect. In the United States, the largest source of greenhouse gases is transportation (29 percent), followed closely by electricity production (28 percent) and industrial activity (22 percent). Learn about the natural and human causes of climate change .

Curbing dangerous climate change requires very deep cuts in emissions, as well as the use of alternatives to fossil fuels worldwide. The good news is that countries around the globe have formally committed—as part of the 2015 Paris Climate Agreement —to lower their emissions by setting new standards and crafting new policies to meet or even exceed those standards. The not-so-good news is that we’re not working fast enough. To avoid the worst impacts of climate change, scientists tell us that we need to reduce global carbon emissions by as much as 40 percent by 2030. For that to happen, the global community must take immediate, concrete steps: to decarbonize electricity generation by equitably transitioning from fossil fuel–based production to renewable energy sources like wind and solar; to electrify our cars and trucks; and to maximize energy efficiency in our buildings, appliances, and industries.

A: Scientists agree that the earth’s rising temperatures are fueling longer and hotter heat waves, more frequent droughts, heavier rainfall, and more powerful hurricanes .

In 2015, for example, scientists concluded that a lengthy drought in California—the state’s worst water shortage in 1,200 years —had been intensified by 15 to 20 percent by global warming. They also said the odds of similar droughts happening in the future had roughly doubled over the past century. And in 2016, the National Academies of Science, Engineering, and Medicine announced that we can now confidently attribute some extreme weather events, like heat waves, droughts, and heavy precipitation, directly to climate change.

The earth’s ocean temperatures are getting warmer, too—which means that tropical storms can pick up more energy. In other words, global warming has the ability to turn a category 3 storm into a more dangerous category 4 storm. In fact, scientists have found that the frequency of North Atlantic hurricanes has increased since the early 1980s, as has the number of storms that reach categories 4 and 5. The 2020 Atlantic hurricane season included a record-breaking 30 tropical storms, 6 major hurricanes, and 13 hurricanes altogether. With increased intensity come increased damage and death. The United States saw an unprecedented 22 weather and climate disasters that caused at least a billion dollars’ worth of damage in 2020, but 2017 was the costliest on record and among the deadliest as well: Taken together, that year's tropical storms (including Hurricanes Harvey, Irma, and Maria) caused nearly $300 billion in damage and led to more than 3,300 fatalities.

The impacts of global warming are being felt everywhere. Extreme heat waves have caused tens of thousands of deaths around the world in recent years. And in an alarming sign of events to come, Antarctica has lost nearly four trillion metric tons of ice since the 1990s. The rate of loss could speed up if we keep burning fossil fuels at our current pace, some experts say, causing sea levels to rise several meters in the next 50 to 150 years and wreaking havoc on coastal communities worldwide.

A: Each year scientists learn more about the consequences of global warming , and each year we also gain new evidence of its devastating impact on people and the planet. As the heat waves, droughts, and floods associated with climate change become more frequent and more intense, communities suffer and death tolls rise. If we’re unable to reduce our emissions, scientists believe that climate change could lead to the deaths of more than 250,000 people around the globe every year and force 100 million people into poverty by 2030.

Global warming is already taking a toll on the United States. And if we aren’t able to get a handle on our emissions, here’s just a smattering of what we can look forward to:

  • Disappearing glaciers , early snowmelt, and severe droughts will cause more dramatic water shortages and continue to increase the risk of wildfires in the American West.
  • Rising sea levels will lead to even more coastal flooding on the Eastern Seaboard, especially in Florida, and in other areas such as the Gulf of Mexico.
  • Forests, farms, and cities will face troublesome new pests , heat waves, heavy downpours, and increased flooding . All of these can damage or destroy agriculture and fisheries.
  • Disruption of habitats such as coral reefs and alpine meadows could drive many plant and animal species to extinction.
  • Allergies, asthma, and infectious disease outbreaks will become more common due to increased growth of pollen-producing ragweed , higher levels of air pollution , and the spread of conditions favorable to pathogens and mosquitoes.

Though everyone is affected by climate change, not everyone is affected equally. Indigenous people, people of color, and the economically marginalized are typically hit the hardest . Inequities built into our housing , health care , and labor systems make these communities more vulnerable to the worst impacts of climate change—even though these same communities have done the least to contribute to it.

A: In recent years, China has taken the lead in global-warming pollution , producing about 26 percent of all CO2 emissions. The United States comes in second. Despite making up just 4 percent of the world’s population, our nation produces a sobering 13 percent of all global CO2 emissions—nearly as much as the European Union and India (third and fourth place) combined. And America is still number one, by far, in cumulative emissions over the past 150 years. As a top contributor to global warming, the United States has an obligation to help propel the world to a cleaner, safer, and more equitable future. Our responsibility matters to other countries, and it should matter to us, too.

A: We’ve started. But in order to avoid the worsening effects of climate change, we need to do a lot more—together with other countries—to reduce our dependence on fossil fuels and transition to clean energy sources.

Under the administration of President Donald Trump (a man who falsely referred to global warming as a “hoax”), the United States withdrew from the Paris Climate Agreement, rolled back or eliminated dozens of clean-air protections, and opened up federally managed lands, including culturally sacred national monuments , to fossil fuel development. Although President Biden has pledged to get the country back on track, years of inaction during and before the Trump administration—and our increased understanding of global warming’s serious impacts—mean we must accelerate our efforts to reduce greenhouse gas emissions.

Despite the lack of cooperation from the Trump administration, local and state governments made great strides during this period through efforts like the American Cities Climate Challenge and ongoing collaborations like the Regional Greenhouse Gas Initiative . Meanwhile, industry and business leaders have been working with the public sector, creating and adopting new clean-energy technologies and increasing energy efficiency in buildings, appliances, and industrial processes. Today the American automotive industry is finding new ways to produce cars and trucks that are more fuel efficient and is committing itself to putting more and more zero-emission electric vehicles on the road. Developers, cities, and community advocates are coming together to make sure that new affordable housing is built with efficiency in mind , reducing energy consumption and lowering electric and heating bills for residents. And renewable energy continues to surge as the costs associated with its production and distribution keep falling. In 2020 renewable energy sources such as wind and solar provided more electricity than coal for the very first time in U.S. history.

President Biden has made action on global warming a high priority. On his first day in office, he recommitted the United States to the Paris Climate Agreement, sending the world community a strong signal that we were determined to join other nations in cutting our carbon pollution to support the shared goal of preventing the average global temperature from rising more than 1.5 degrees Celsius above preindustrial levels. (Scientists say we must stay below a 2-degree increase to avoid catastrophic climate impacts.) And significantly, the president has assembled a climate team of experts and advocates who have been tasked with pursuing action both abroad and at home while furthering the cause of environmental justice and investing in nature-based solutions.

A: No! While we can’t win the fight without large-scale government action at the national level , we also can’t do it without the help of individuals who are willing to use their voices, hold government and industry leaders to account, and make changes in their daily habits.

Wondering how you can be a part of the fight against global warming? Reduce your own carbon footprint by taking a few easy steps: Make conserving energy a part of your daily routine and your decisions as a consumer. When you shop for new appliances like refrigerators, washers, and dryers, look for products with the government’s ENERGY STAR ® label; they meet a higher standard for energy efficiency than the minimum federal requirements. When you buy a car, look for one with the highest gas mileage and lowest emissions. You can also reduce your emissions by taking public transportation or carpooling when possible.

And while new federal and state standards are a step in the right direction, much more needs to be done. Voice your support of climate-friendly and climate change preparedness policies, and tell your representatives that equitably transitioning from dirty fossil fuels to clean power should be a top priority—because it’s vital to building healthy, more secure communities.

You don’t have to go it alone, either. Movements across the country are showing how climate action can build community , be led by those on the front lines of its impacts, and create a future that’s equitable and just for all .

This story was originally published on March 11, 2016 and has been updated with new information and links.

This NRDC.org story is available for online republication by news media outlets or nonprofits under these conditions: The writer(s) must be credited with a byline; you must note prominently that the story was originally published by NRDC.org and link to the original; the story cannot be edited (beyond simple things such as grammar); you can’t resell the story in any form or grant republishing rights to other outlets; you can’t republish our material wholesale or automatically—you need to select stories individually; you can’t republish the photos or graphics on our site without specific permission; you should drop us a note to let us know when you’ve used one of our stories.

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Exxon disputed climate findings for years. its scientists knew better..

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Alice McCarthy

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Research shows that company modeled and predicted global warming with 'shocking skill and accuracy' starting in the 1970s

Projections created internally by ExxonMobil starting in the late 1970s on the impact of fossil fuels on climate change were very accurate, even surpassing those of some academic and governmental scientists, according to an analysis published Thursday in Science by a team of Harvard-led researchers. Despite those forecasts, team leaders say, the multinational energy giant continued to sow doubt about the gathering crisis.

In “Assessing ExxonMobil’s Global Warming Projections,” researchers from Harvard and the Potsdam Institute for Climate Impact Research show for the first time the accuracy of previously unreported forecasts created by company scientists from 1977 through 2003. The Harvard team discovered that Exxon researchers created a series of remarkably reliable models and analyses projecting global warming from carbon dioxide emissions over the coming decades. Specifically, Exxon projected that fossil fuel emissions would lead to 0.20 degrees Celsius of global warming per decade, with a margin of error of 0.04 degrees — a trend that has been proven largely accurate.

“This paper is the first ever systematic assessment of a fossil fuel company’s climate projections, the first time we’ve been able to put a number on what they knew,” said Geoffrey Supran, lead author and former research fellow in the History of Science at Harvard. “What we found is that between 1977 and 2003, excellent scientists within Exxon modeled and predicted global warming with, frankly, shocking skill and accuracy only for the company to then spend the next couple of decades denying that very climate science.”

“This paper is the first ever systematic assessment of a fossil fuel company’s climate projections, the first time we’ve been able to put a number on what they knew,” said Geoffrey Supran, lead author.

File photo by Stephanie Mitchell/Harvard Staff Photographer

“We thought this was a unique opportunity to understand what Exxon knew about this issue and what level of scientific understanding they had at the time,” added co-author Naomi Oreskes , Henry Charles Lea Professor of the History of Science whose work looks at the causes and effects of climate change denial. “We found that not only were their forecasts extremely skillful, but they were also often more skillful than forecasts made by independent academic and government scientists at the exact same time.”

Allegations that oil company executives sought to mislead the public about the industry’s role in climate change have drawn increasing scrutiny in recent years, including lawsuits by several states and cities and a recent high profile U.S. House committee investigation.

Harvard’s scientists used established Intergovernmental Panel on Climate Change (IPCC) statistical techniques to test the performance of Exxon’s models. They found that, depending on the metric used, 63-83 percent of the global warming projections reported by Exxon scientists were consistent with actual temperatures over time. Moreover, the corporation’s own projections had an average “skill score” of 72 percent, plus or minus 6 percent, with the highest scoring 99 percent. A skill score relates to how well a forecast compares to what happens in real life. For comparison, NASA scientist James Hansen’s global warming predictions presented to the U.S. Congress in 1988 had scores from 38 to 66 percent.

The researchers report that Exxon scientists correctly dismissed the possibility of a coming ice age, accurately predicted that human-caused global warming would first be detectable in the year 2000, plus or minus five years, and reasonably estimated how much CO 2 would lead to dangerous warming.

The current debate about when Exxon knew about the impact on climate change carbon emissions began in 2015 following news reports of internal company documents describing the multinational’s early knowledge of climate science.  Exxon disagreed with the reports, even providing a link to internal studies and memos from their own scientists and suggesting that interested parties should read them and make up their own minds.

“That’s exactly what we did,” said Supran, who is now at the University of Miami. Together, he and Oreskes spent a year researching those documents and in 2017 published a series of three papers analyzing Exxon’s 40-year history of climate communications . They were able to show there was a systematic discrepancy between what Exxon was saying internally and in academic circles versus what they were telling the public. “That led us to conclude that they had quantifiably misled the public, by essentially contributing quietly to climate science and yet loudly promoting doubt about that science,” said Supran.

“I think this new study is the smoking gun, the proof, because it shows the degree of understanding … this really deep, really sophisticated, really skillful understanding that was obscured by what came next,” said Harvard Professor Naomi Oreskes.

Harvard file photo

In 2021, the team published a new study in One Earth using algorithmic techniques to identify ways in which ExxonMobil used increasingly subtle but systematic language to shape the way the public talks and thinks about climate change — often in misleading ways.

These findings were hardly a surprise to Oreskes, given her long history of studying climate communications from fossil fuel companies, work that drew national attention with her 2010 bestseller, “Merchants of Doubt.” In it she and co-author, Caltech researcher Erik Conway, argued that Exxon was aware of the threat of carbon emissions on climate change yet waged a disinformation campaign about the problem.  Despite the book’s popularity and the peer-reviewed papers with Supran, however, some continued to wonder whether she could prove the effect these campaigns had, if they indeed made a difference.

“I think this new study is the smoking gun, the proof, because it shows the degree of understanding … this really deep, really sophisticated, really skillful understanding that was obscured by what came next,” Oreskes said. “It proves a point I’ve argued for years that ExxonMobil scientists knew about this problem to a shockingly fine degree as far back as the 1980s, but company spokesmen denied, challenged, and obscured this science, starting in the late 1980s/early 1990s.”

Added Supran: “Our analysis here I think seals the deal on that matter. We now have totally unimpeachable evidence that Exxon accurately predicted global warming years before it turned around and publicly attacked climate science and scientists.”

The authors of this research were supported by a Rockefeller Family Fund grant and Harvard University Faculty Development funds.

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The Economic Geography of Global Warming

Global warming is a worldwide and protracted phenomenon with heterogeneous local economic effects. In order to evaluate the aggregate and local economic consequences of higher temperatures, we propose a dynamic economic assessment model of the world economy with high spatial resolution. Our model features a number of mechanisms through which individuals can adapt to global warming, including costly trade and migration, and local technological innovations and natality rates. We quantify the model at a 1° × 1° resolution and estimate damage functions that determine the impact of temperature changes on a region’s fundamental productivity and amenities depending on local temperatures. Our baseline results show welfare losses as large as 15% in parts of Africa and Latin America but also high heterogeneity across locations, with northern regions in Siberia, Canada, and Alaska experiencing gains. Our results indicate large uncertainty about average welfare effects and point to migration and, to a lesser extent, innovation as important adaptation mechanisms. We use the model to assess the impact of carbon taxes, abatement technologies, and clean energy subsidies. Carbon taxes delay consumption of fossil fuels and help flatten the temperature curve but are much more effective when an abatement technology is forthcoming.

We thank Klaus Desmet, Per Krusell and participants at numerous seminars and conferences for their feedback. We also thank the International Economics Section at Princeton University for financial support. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.

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Climate Change, Health and Existential Risks to Civilization: A Comprehensive Review (1989–2013)

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Background: Anthropogenic global warming, interacting with social and other environmental determinants, constitutes a profound health risk. This paper reports a comprehensive literature review for 1989–2013 (inclusive), the first 25 years in which this topic appeared in scientific journals. It explores the extent to which articles have identified potentially catastrophic, civilization-endangering health risks associated with climate change. Methods: PubMed and Google Scholar were primarily used to identify articles which were then ranked on a three-point scale. Each score reflected the extent to which papers discussed global systemic risk. Citations were also analyzed. Results : Of 2143 analyzed papers 1546 (72%) were scored as one. Their citations (165,133) were 82% of the total. The proportion of annual papers scored as three was initially high, as were their citations but declined to almost zero by 1996, before rising slightly from 2006. Conclusions : The enormous expansion of the literature appropriately reflects increased understanding of the importance of climate change to global health. However, recognition of the most severe, existential, health risks from climate change was generally low. Most papers instead focused on infectious diseases, direct heat effects and other disciplinary-bounded phenomena and consequences, even though scientific advances have long called for more inter-disciplinary collaboration.

1. Introduction

In 1988 the leading climate scientist James Hansen, of the National Aeronautics and Space Administration, with three other senior researchers, testified to a U.S. Congressional committee that it was 99 percent certain that the warming trend in Earth’s temperature that was then observed was not natural variation but was caused by the accumulation of carbon dioxide and other “greenhouse” gases. This testimony was reported prominently in the New York Times [ 1 , 2 ]. Hansen was criticized then, and many times since, for his “adventurous” interpretation of climate data, however the publicity which followed his testimony, itself reflecting a decade of growing agitation about the geo-political impacts of climate change [ 2 ] may have influenced health workers to think more deeply about the issues. In any case, within a year, a Lancet editorial discussed health and the “greenhouse effect” [ 3 ], possibly the first such publication in a health journal, eight years after a chapter concerning climate change and parasitic disease appeared [ 4 ]. At least six other chapters on this topic were published in the 1980s, as well as at least two reports. For details, see [ 5 ]. Two other journal articles concerning climate change and health were also published in 1989 [ 6 , 7 ].

The 1989 editorial stated “global warming, increased ultraviolet flux, and higher levels of tropospheric ozone will reduce crop production, with potentially devastating effects on world food supplies. Malnutrition (sic) might then become commonplace, even among developed nations, and armed conflicts would be more likely as countries compete for a dwindling supply of natural resources” [ 3 ]. In the New England Journal of Medicine, Leaf warned, also in 1989, of sea level rise, especially in the south-eastern U.S. state of Florida, higher precipitation, millions of environmental refugees, an increased risk of drought and the possibility that warming at higher latitudes would not fully compensate any climate change related loss of agricultural productivity towards the equator [ 6 ]. The third paper published that year [ 7 ] was even more direct, warning of “catastrophic” consequences to human health and well-being.

In the early 1990s, warnings of potentially catastrophic consequences of climate change continued to dominate. Yet, by the turn of the millennium, the author had formed the impression that the scientific publishing milieu was becoming less receptive to the message that climate change and other forms of “planetary overload” [ 8 ] pose existential, civilization-wide risks. This was disturbing, as my own confirmation bias seemed to support the case that the evidence of existential risk was continuing to rise [ 9 , 10 ].

That the health risks from climate change are indeed extraordinarily high was stressed in the 2009 publication of the lengthy (41 page) article by the Lancet and University College London Institute for Global Health Commission, which described climate change as the “biggest global health threat of the 21st century” [ 11 ]. Yet, although this paper attracted considerable attention at the time, the long-term outlook for climate change and health has since continued to deteriorate.

By existential, I mean related to the word “existence”. But it is not the continued existence of Earth that is in doubt, but instead the existence of a high level of function of civilization, one in which prospects of “health for many” (though no longer “health for all”) are realistic and even improving [ 12 ]. Existential risk does not necessarily mean that global civilization will collapse. Nor does it exclude pockets of order and even prosperity enduring for generations, from which global or quasi-global civilization may one day emerge, provided worst case scenarios are avoided, such as runaway climate change and nuclear war leading to nuclear winter [ 13 ]. Compared to today, such prospects should be recognized as catastrophic. Unchecked climate change could generate similar, or bleaker, global futures. Seeking to minimize such possibilities should be seen as a major responsibility for all workers concerned with sustaining and improving global public health.

There is reticence [ 14 , 15 ], shared by many authors, reviewers, journals, funders and media outlets to discuss the possibility of such existential risks. Nonetheless, the consequences for health are so vast that discussion is warranted. This paper seeks to do that, in the process conducting the largest review on the topic of climate change and health yet to be published.

1.1. Climate Change Science, Risk and the 2015 Paris Agreement

The scientific knowledge that gases, accumulating mainly from the burning of fossil fuels and the clearing of forests, add to the natural “greenhouse effect” has been known since the 19th century [ 16 ]. In 1957 scientists observed “human beings are now carrying out a largescale geophysical experiment of a kind which could not have happened in the past nor be reproduced in the future. Within a few hundred years we are returning to the air and oceans the concentrated organic carbon stored over hundreds of millions of years” [ 17 ].

In 2015 the Paris climate change agreement, negotiated by representatives of 196 parties (195 nations and the European Union) committed countries (thus, effectively, civilization), upon ratification, to actions that would seek to restrict average global warming to “well below” 2 °C above “pre-industrial” levels and to “pursue efforts” to limit the rise to 1.5 °C. The text of the Paris Agreement defines neither the pre-industrial temperature nor the time for this baseline, but most experts agree that it means the temperature in the late 18th or 19th century, soon after the start of the industrial revolution, when coal burning increased. This time is after the end of the Little Ice Age, which itself was accompanied by a rebound in average temperatures, independent of the slow rise in greenhouse gases (chiefly methane and nitrous oxide as well as carbon dioxide) that occurred throughout the 19th century. Estimates of global warming for the period 1861–1880 until 2015 range from 0.93 °C [ 18 ] to 1.12 °C [ 19 ].

Although the goal of 1.5 °C is widely known, there is less understanding that meeting this challenge would not guarantee safety from a climate change perspective [ 20 ]. Indeed, if it were to be more widely accepted that climate change has already contributed to the Syrian war [ 21 , 22 ], to the rise in global food prices which accompanied the 2010 drought and heatwave in Russia [ 23 , 24 ], and the 2018 wildfire season in the Northern Hemisphere, then the threshold of danger might already be widely seen as having long been exceeded.

In recent years the science concerning the physical impacts of climate has continued to expand and to disturb. Average global temperatures continue to rise [ 25 ], apparently in a process more “stepped” than as a trend [ 26 ] with record average global heat in both El Niño and La Niña years. Loss of ice from both Antarctica and Greenland is increasing and the rate of sea level rise is consequently accelerating [ 27 ]. Property values in parts of the U.S. East Coast may soon fall, due to sea level rise [ 28 ]. There is growing concern about more intense rainfall [ 29 , 30 ], fires worsened by heat and drought [ 31 ], a weakening Gulf Stream [ 32 ] and increased sinuosity of the jet stream, which can cause unusual cold at lower latitudes, even if the average global temperature is rising [ 33 , 34 ]. The projected trend toward a weaker and poleward-shifted jet stream is also consistent with projections of a significantly increased risk of worsening extreme heat and dryness in the Northern Hemisphere [ 35 ].

There is also growing evidence of greenhouse effect-intensifying feedbacks in the Earth system [ 36 ] that might release enormous quantities of carbon dioxide and methane, independent of fossil fuel combustion, agriculture or deforestation, from sources including warming tundra and increased fires, both of peat and forests [ 37 , 38 ]. Such releases could dwarf the climate saving made possible by the putative implementation of the Paris climate agreement. The strength of the oceanic carbon sink is also weakening [ 39 ]. If this intensifies it is likely to accelerate warming of the atmosphere, ocean and land.

1.2. Interaction, Attribution, and Causation

All, or virtually all, environmental health effects interact with social and technological factors as well as other “purely” environmental determinants. For example, the effects of heat upon individual health are influenced by temperature, humidity, exercise, hydration, age, pre-existing health status, and also by occupation, clothing, behavior, autonomy, vulnerability, and sense of obligation. Does the person affected by heat, perhaps a brick maker in India, have the capacity to regulate her heat exposure; or might they be an elite athlete or emergency worker voluntarily pushing their limits? Other factors influencing the heath impact of heat include housing quality, the presence of absence of affordable air conditioning and energy subsidies, if any. In turn, these factors are influenced by governance and socio-economic status. Thus, the health-harming effects of heat can be seen to have many contributing causes, of which climate change is only one. As McMichael (and before him David Hume, among others) pointed out, causal attribution is to an extent philosophical; it is influenced by the “focal depth” of the examiner’s “causal lens” [ 40 ]. Consider a mass shooting in a school: Some will see underlying social and legal factors as contributing; others may see only the shooter. Yet, a major role and goal of public health is to seek to identify and reduce “deep” or “underlying” causes [ 41 ]. A world in which only the most “proximal” causes are identified will not function well.

Attributing the fraction of human-caused (anthropogenic) climate change to physical events such as storms, floods and heatwaves is similarly contested and assumption-dependent. The contribution of climate change to more indirect, strongly socially mediated effects such as migration, famine or conflict is even more difficult and contentious [ 22 , 42 , 43 ]. Perhaps in part because of these causal complications, issues such as famine, genocide, large-scale population dislocation and conflict have, with rare exceptions [ 44 ], been peripheral to public health. This is despite the obvious large-scale adverse health effects of these phenomena.

Rigorous methods have been developed to detect and attribute the health effects of phenomena that are more directly or obviously related to climate change, such as heat and infectious diseases [ 45 ]. However, excessive caution risks a type II error, the overlooking of genuine effects [ 46 , 47 ]. To reduce this risk, the authors of a recent study on attribution acknowledged the role for “well-informed judgments, based on understanding of underlying processes and matching of patterns of health, climate, and other determinants of human well-being” [ 45 ]. This paper makes many such judgments.

1.3. Integrative Risk and the Sustainability of Civilization

Publications in health journals about nuclear war and health date at least to 1962 [ 48 ]. In 1992 the Union of Concerned Scientists coordinated the “World Scientist’s warning to humanity”, signed by over 1700 leading scientists (but no public health workers) [ 49 ]. This warning was repeated in 2017, with far more signatories (including many health workers) [ 50 ].

Many authors outside health have warned of the fragility of modern civilization [ 51 , 52 ]. However, comparatively few writers with a health background have contributed [ 9 , 10 , 53 , 54 ]. Tony McMichael, who led the first Intergovernmental Panel on Climate Change chapter on health [ 55 ] frequently wrote and spoke of eroding “life support mechanisms” [ 56 , 57 ], a term probably introduced into the health literature in 1972 by Sargent [ 58 ]. Certainly, McMichael wanted to convey, when using this term, a profound risk to human well-being and health.

If civilization is to collapse then effects such as conflict, population displacement and famine are likely to be involved. A heatwave, on its own, is unlikely to cause the collapse of civilization, nor even ruin an economy for a decade. It needs social co-factors to do this. For example, a series of heatwaves damaging crop yields and contributing to internal migration has been postulated as contributing to the Syrian civil war that started in 2011 [ 21 , 22 , 59 , 60 , 61 , 62 ]. Prolonged heat, especially if in a humid setting, could cause some regions to be completely abandoned [ 63 , 64 , 65 ].

A severely damaged health system, allied with worsening undernutrition and poverty, could provide a milieu for a devastating epidemic, including a resurgence of HIV/AIDS [ 66 ]. An increase in infectious diseases, if of sufficient scale, could contribute to integrative cascades of failure generating regional or even global civilization collapse. Infectious diseases, as well as unfavorable eco-climatic change, contributed to the collapse of the Roman Empire [ 67 ].

While such consequences may seem far-fetched to some, the prospect of sea level rise of one meter or more by 2100 (perhaps sooner), proliferating nuclear weapons, millions of refugees, xenophobia and tribalism which limits integration, and growing cases of state failure is disquieting. Few, if any, formal scenarios, as exercises by senior scientists, are as bleak, but funding and other pressures constrain the realism of such exercises [ 15 ]. Already, the number of forcibly displaced people exceeds 68 million [ 68 ], a rise that has been linked with tightening limits to growth, including climate change [ 69 ].

It is stressed, again, that the idea that any single climate related event, such as heat, drought, sea level rise, conflict or migration will cause the collapse of civilization is simplistic. It is far more plausible to conceive that collapse (or quasi-collapse) could arise via a “milieu” of multi-factorial risk, enhancing, inflaming and interacting with climate change and other factors [ 43 , 70 ].

1.4. Hypothesis

This article seeks to test the hypothesis that the early literature relevant to climate change and health was more willing to describe catastrophic, potentially civilization disrupting health effects including famine, mass migration and conflict than it was to become, at least until 2014.

To explore this hypothesis, a database of articles relevant to climate change and health was assembled, relying mainly on PubMed and Google Scholar. This had six steps (see Appendix for details). Due to limited resources, the main search was restricted to the period 1980–2013, and the terms “climate change” and health or “global warming” and health. After eliminating duplicates, remaining papers were checked to see if they met eligibility criteria (see Box 1 ).

inclusion and exclusion criteria.

Included: Articles, editorials, commentaries, journalistic pieces with bylines.

Excluded: Reports, books, book sections including e-chapters, letters, factsheets, monographs, un-credited journalistic entries, non-English publications, papers concerning stratospheric ozone depletion, podcast transcripts, journalistic pieces that could not easily be recovered.

The search was not restricted to health or to multidisciplinary journals. However, papers outside health journals had to meet more exacting requirements to be included. They had to include health (or a synonym such as nutrition) in their title, abstract, keywords or text, even if they focused on an effect with health implications, such as population displacement, conflict or food insecurity.

The title of each identified paper was read, followed by the abstract of each paper, assessed as possibly eligible. If a score was still unclear, the full text was obtained and searched for words and phrases that suggested a broader interpretation of the indirect effects of climate change, such as “population displacement”, “migration”, “conflict”, “war”, “famine”, and “food insecurity”.

Eligible papers were scored as one if they exclusively concerned an effect other than conflict, migration, population displacement or large-scale undernutrition or famine. They also needed to exclude statements (even if introductory) such as “climate change has been recognized as the greatest risk to health in the 21st century”.

Papers were scored as two if they either mentioned such an effect and/or contained statements recognizing the potentially enormous scale of the health impacts from climate change. A synonym for this understanding was the phrase eroding “life support mechanisms”.

Papers were scored as three if they included a more detailed explanation or assertion of the future (or current) existence and importance of conflict, migration or famine, perhaps suggesting an interaction among them. A score of three was more likely if they also warned of the general severity of climate change. The score was also influenced by the tone of the language, and the space devoted to these issues (see Appendix for further details).

In addition, PubMed was searched for papers published from 2014–2017 matching the criteria “climate change” and “health”. A sample of 156 of these articles was randomly selected, approximately 5% in each year, after the elimination of a proportion of ineligible articles. Each was then scored, using the method described above for papers published from 1989 to 2013 (inclusive). Bootstrapping was then used to estimate the average score and 95% confidence interval of these articles, by taking ten thousand resamples, each of 156 papers, with replacement from this set (so that in each iteration some papers will appear more than once, while others will not appear at all).

A total of 2143 unique articles and journalistic essays satisfied the inclusion criteria, for the period 1989–2013 inclusive. The full database is available in the supplementary material . This shows the year, lead author (at least), journal, title and primary search method. It also lists the number of Google Scholar citations and the date these were identified. Table A1 ( Appendix ) tabulates the primary search method of papers, by each year.

No paper published before 1989 was eligible for retention in the final database. One potential publication [ 71 ] was cited by Kalkstein and Smoyer [ 5 ] as published in 1988, but it could not be located. About half the total papers (1142 or 53%) were published since 2009 (see Figure 1 ). Most papers (1546 papers, 72%) were scored as one, while only 189 (3.3%) were scored as three. The difference in these scores is statistically significant ( p < 0.01 ANOVA). The average score of these 2143 papers was 1.37 (see Table A2 in Appendix ).

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Number of papers in each category. Since 1989 the number of papers concerning climate change and health has expanded considerably, particularly since 2008. As this article did not review the entire literature, the actual number of papers published, even in English, is more than shown. The average score of these papers declined from 1.9 in the first quintile to 1.34 in the final five years.

The increase in the size of literature reflects growing awareness of the risks to health from climate change. Over 50% of the papers published in the first quintile (1989–1993) were scored as two or three, although the total number in that time (27) was small (see Figure 1 ). Since 1993 the majority of papers have focused on effects such as heat, infectious diseases, allergies or asthma. The number of papers scored as two or three increased slightly after its trough (23%) in the third quintile (1999–2004) but was only 26% for 2009–2013 inclusive.

Papers scored as three were particularly uncommon in the third quintile (1999–2003), representing only 2.6% of the total published papers in that period. Even in the first quintile (1989–1993) most citations were for papers scored as one (see Figure 2 ).

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Number of citations per annum for each score of paper. Most citations were for papers scored as one. Note that in 2005–2007 three extensively cited papers were scored as two (these are discussed in the Appendix A ).

3.1. Citations

Citation data were available for 2105 papers (98%). Over 201,000 citations were identified by Google Scholar (see Table A3 in Appendix ). Thirty two percent of these citations were for papers published since 2009 (see Figure 2 ). Of these citations, the great majority (82%) were for papers scored as one, each of which was cited an average of 107 times. Papers scored 2 were cited an average of 73 times, representing 15% of the total. Papers scored as three were cited 35 times each on average and accounted for 3% of the total. The difference in these citation scores is also statistically significant ( p < 0.01 ANOVA). Citations for papers scored as three from 1995 to 2008 inclusive were even lower, accounting for less than 1% of the total citations in each year of this period (see Figure 3 ). The fraction of the literature discussing existential risk remained lower in the last 5 years of this database than in the first five years (see Figure 1 ). The shift in the ratio of annual citations from the early period to the more recent years is evident in Figure 3 . Until 1991, the majority of citations were for papers scored as three. From 1994 the fraction of citations for papers scored as three was almost zero (3% or less) in every year until 2009. In 2013 it again fell to 3%.

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The proportion of citations each year for papers scored as one and three. Since 1991 most citations have been for papers scored as 1. The Lancet UCL paper published in 2009 [ 11 ] led to a resurgence of citations for papers scored as 3, but this effect declined. Three individual papers, each scored as two (published in 2005, 2006 and 2007), were disproportionately cited. In each year at least some papers scored two or three, but their proportion of citations fell steeply after the first quintile. In 2003 no paper was scored as three, and for almost a decade (1997–2005 inclusive) virtually no papers scored as three were cited.

3.2. Coverage of Topics

All papers published in 1989 discussed multiple potential health effects of climate change. However, from 1990, journal articles focusing exclusively on infectious diseases and climate change appeared [ 72 , 73 , 74 ]. Early papers also focused on heat [ 75 ] and allergies [ 76 ]. From 2000, the foci of concerns expanded greatly. Additional topics included reduced micronutrient concentrations in food [ 77 ], asthma [ 78 ], thunderstorm asthma [ 79 ], chronic diseases and obesity [ 80 ], toxin exposure (such as from increased concentrations in Arctic mammals [ 81 ] and increased algal blooms [ 82 ]), forest fires [ 83 ], mental health [ 84 ] and respiratory [ 85 ], cardio-vascular [ 86 ], renal [ 87 ], fetal [ 88 ], genito-urinal [ 89 ] and skin conditions [ 90 ]. By 2000, papers were also appearing arguing that the impact of climate change for malaria was overstated [ 91 , 92 ].

Articles also appeared on the impact of climate change on groups such as indigenous people [ 93 ], children [ 94 ], the elderly [ 95 ] and regions and locations, including cities [ 96 ], the Arctic [ 97 ] and small island states [ 98 ] as well as many individual nations. Other themes appeared, including on how the health sector might reduce its carbon footprint [ 99 ], on “co-benefits” [ 100 ], on climate change as a great opportunity to improve public health [ 101 ], on medical education [ 102 ], pharmaceuticals [ 103 ] and on the health risks of adaptation and geoengineering, including of carbon capture and storage [ 104 ].

3.3. The Leadership Role of Some Journals

Many journals played prominent, even campaigning roles, especially the Lancet, BMJ and Environmental Health Perspectives. Several journals had special issues, including Global Health Action, the American Journal of Preventive Medicine, the Asia Pacific Journal of Public Health and Health Promotion International. Seven journals published at least 28 articles each, including editorials and news items (see Table A4 in Appendix ). At least 34 journals published editorials, which, with an average score of 2.2, were more likely to be scored as two or three than journal articles (average score 1.3). News items and other journalistic pieces had an average score of 1.6. At least 21 articles were published in nursing journals, with an average score of 1.67.

3.4. Papers for the Period 2014–2017

A total of 3377 papers were identified by PubMed as published from 2014–2017. Of these, 346 were found to be ineligible, although the true number would be higher, if all candidates were examined. Of the potentially eligible remainder, 113 papers were published in 2018, but recorded by PubMed as e-published in 2017. Slightly over five percent of the articles for each year was randomly selected, resulting in 156 articles (see Table A5 in Appendix ). Their average score and 95% confidence interval, estimated by bootstrapping, was 1.29 (95% CI 1.21–1.39) (see Figure A2 in Appendix ). Details of these 156 papers are in the supplementary material . Note that their citations were not checked.

4. Discussion

This paper describes the first published analysis of the extent to which the literature on climate change and health has described or in other ways engaged with “existential” risk. By including 2000 articles, 60 editorials and 83 news items (2143 “papers” in total) on climate change and health, it is by far the largest review of the climate change and health literature to have so far been published. Lack of resources currently prevents an extension of the fuller analysis to more recent years. However, a randomly selected sample of 156 articles for papers identified by PubMed as published in the period 2014–2017 found that these papers had an average score lower than the average score for any quintile from 1989–2013, other than for 1999–2003 (see Table A2 and Table A5 in Appendix ).

Several systematic and other reviews of topics related to climate change and health have been published, but on a much smaller scale, and with different research questions. Ford and Pearce systematically reviewed 420 papers, published between 1990 and 2009, exploring the topic of climate change vulnerability in the Canadian western Arctic [ 105 ]. Two systematic reviews concerned heat. Huang et al. [ 106 ] searched for papers published between 1980 and July 2010, projecting the heat related mortality under climate change scenarios. Only 14 papers were included in their final analysis. Xu et al. [ 107 ] explored the relationship between heat waves and children’s health, but selected twelve, an even small number. A systematic review into dengue fever and climate change (for the period 1991–2012) included 16 studies [ 108 ].

Nichols et al. (2009) [ 109 ] undertook a systematic review on health, climate change and energy vulnerability, searching for papers published in English between 1998 and 2008. They retrieved 114 papers but included only 36 in their final analysis. Bouzid et al. (2013) undertook a “systematic review of systematic reviews” to explore the effectiveness of public health interventions to reduce the health impact of climate change [ 110 ]. This identified over 3100 unique records, but of these, only 85 full papers were assessed, with 33 included in the final review.

This may also be the first review paper concerning climate change and health to use a citation analysis [ 111 ] as an indicator of influence. Citations in Google Scholar were used for convenience and cost. Although such citations are prone to error, and include essays in the gray literature, they still reflect influence. Some reports in the gray literature may be more widely read and more influential than more scholarly work.

4.1. Selection and Other Forms of Bias

A systematic review was not undertaken. However, all papers identified by searching using PubMed and at least 100 papers for each year identified by Google Scholar were considered for inclusion. The search term relevant to health was restricted to a single word, rather than synonyms such as “disease”, “morbidity”, “illness”, or “mortality”. Undoubtedly, a search using additional terms will identify more papers, as would a systematic review.

To examine the possibility that a more extensive search strategy would alter the conclusions, PubMed was also searched for the terms “climate change” and “morbidity” for papers published in 2013. This strategy identified 261 papers, compared to 496 when searching for “climate change” and “health”. Of these 261 papers, 30 had not previously been identified by the other search methods used, and met the other inclusion criteria. However, all of these additional papers were scored as one. Their inclusion in the final analysis was considered likely to bias the paper away from the null hypothesis, by accentuating the fraction of papers not scored as two or three. This bias towards papers scored one (i.e., identified by searching for “morbidity”) seems plausible because the term morbidity may be more likely to be associated with specific diseases than the term “health”. These papers therefore were not added to the analysis.

The search was supplemented by the addition of 17 papers first identified from the author’s own database, but not later found by the search strategy using Google Scholar or PubMed (steps 2–3) as described in Figure A1 . Eight of these 17 papers, five of which the author wrote or co-wrote, were scored as three. Their average score was 2.17, far higher than for the balance (1.23). This group also includes two editorials, one published in the Lancet, one in the BMJ. The inclusion of one of these editorials (scored as three, published in 1989) has biased the findings in favor of the hypothesis that highly scored papers were more common in the early period of this literature. Note, however, that no citations were recorded for this editorial.

The inclusion of these higher scoring papers later in the period of analysis has biased the result to the null, that is, away from the hypothesis that fewer such papers were published from about 2000. The most influential of these 17 papers, judged by Google Scholar citations, was cited 272 times. It was the first to report that rising levels of carbon dioxide depress micronutrient concentrations in food [ 77 ]. The other 16 papers were cited 405 times between them, an average of 25, which is low compared to the average citation number (94). Twenty eight other papers were included, mostly identified from special issues. Their average score was 1.9. One paper was identified post-review, by chance. It was scored as two (perhaps generously) and was included because it was judged that to exclude it would bias the result away from the null hypothesis.

Bias is also likely to have been introduced in the scoring process, but not to the extent that it could challenge the main conclusions. The rigor of this paper would be improved if the scores could be checked by a third party, blind to the first score. Unfortunately, no resources were available for this purpose. Some classification errors are likely, especially for papers for which the author had no previous familiarity, and if published after 2009, when, due to time pressure, many papers were scored rapidly. On the other hand, in the process of ranking over 2000 papers the author became skilled at making rapid decisions, especially for most papers scored as one. The difference between papers scored one and two was generally more apparent than for papers scored between two and three. In cases of doubt a higher score was always selected.

The likelihood of bias and error is unlikely to explain the difference in the character of the papers in the early period and those which later dominated. Although the widely cited paper by Costello et al. [ 11 ] (1583 citations as of June 2018) may have refreshed appreciation of the potentially catastrophic nature of climate change, the majority of papers and their citations published between 2010 and 2013 continued to focus on specific issues. This trend appears to have persisted in the years since, judged by the analysis of a randomly selected sample, identified by PubMed as published between 2014 and 2018.

4.2. Reasons for the Apparent Conservatism of the Literature

There are several plausible, overlapping and interacting explanations for the decline in the proportion of papers scored as two or three (and for their comparatively fewer citations) following 1996, and also in the failure for papers published since 2009 to fully amplify the most severe warnings. One likely contributing explanation is self-censorship. The topic of climate change and health is unfamiliar territory for many health editors and writers. Climate change has become politicized in many English-speaking countries, especially in the U.S. and Australia. Although comparatively few health workers have expertise concerning climate change and health, the readership of some health journals seems judged, by their editor, to be skeptical of, or even to reject climate science. For example, one editor, defending the decision to publish a paper (scored, possibly generously, as two) [ 112 ] seemed almost apologetic, writing “On its face, the paper by Hess and colleagues is largely a political commentary and a departure from the types of articles found in Academic Emergency Medicine” [ 113 ].

Thus, for some health workers and editors, even broaching the topic of climate change and health may be a courageous act. The publication of papers in health journals that describe potential pathways that could threaten civilization would appear even bolder. It is unsurprising that such papers are still fairly uncommon, at least until 2014, and particularly in journals which do not yet have a long tradition of publishing papers or editorials on this topic.

In the early period of the climate and health literature (1989–1993) some of the most outspoken articles were editorials. Perhaps at that time, there was a certain sense of shock concerning climate change, which has since waned. It was also a time when concerns about overpopulation were slightly less taboo [ 114 , 115 , 116 ]. However, editorials in more years also tend to have a higher index of concern than other articles.

Another likely contributor to the comparative degree of restraint is the view, backed by some research, that an excess of fear is counter-productive [ 117 ]. However, the smell of smoke in a theater requires the sounding of a vigorous alarm. Compounding the difficulty of communicating the risk over climate change is the lag between the whiff of smoke and the onset of visible fire. Hansen warned of great danger over thirty years ago, and he, with others, have issued many warnings since [ 118 ]. Sceptics are still waiting to see the metaphorical “flames” of climate change, even disputing the link between literal flames (fires) and climate change.

On the other hand, science, though not infallible, has delivered countless miracles such as antisepsis, anesthesia, penicillin and the jet engine. It has long warned of the physical changes of climate change. We who work in health should not be amazed if the predictions of climate and Earth scientists prove broadly accurate. Social science is less precise than climatology [ 43 ], however the links between food insecurity, drought, sea level rise, migration and, in some places, conflict are, also, surely not far-fetched. Papers that fail to express appreciation of the extraordinary risks we face as civilization may be judged by people of the future as having failed in their duty of care to protect health.

Another likely reason for the general restraint in the literature is the fragmentation of science and limited funding for multidisciplinary work. Comparatively few authors, other than if collaborating in large, multidisciplinary teams (rare for most authors primarily concerned with health), are rewarded or funded for thinking systemically. This problem is possibly worsening. Related to this, many recent papers are by sub-disciplines of health that have not previously published on the topic of climate change. Such papers are probably less likely to discuss existential risk.

As the effects of climate change have become increasingly clear the need for adaptation has become overwhelming. A stress on adaptation does not necessarily reflect any underestimation of the eventual severity of climate change. However, a stress on adaptation at the expense of mitigation may do so. In many countries, political leadership favors adaptation.

5. Conclusions

In 1989, thirty two years after the International Geophysical Year, the first papers on global warming and health appeared in the world’s leading medical journals [ 3 , 6 , 7 ]. All three of these early papers warned of severe, even existential risk and were each scored as three.

In 1990 McCally and Cassel warned that “progression of these environmental changes could lead to unprecedented human suffering” [ 119 ]. Also, in 1990, Fiona Godlee, then deputy editor of the BMJ, wrote “Countries in the developing world would suffer both the direct effects of drought and flood and the knock-on effect of agricultural and economic decline in the West. The already present problems of feeding the world’s growing population would be compounded by the increasing numbers of displaced people unable to grow their own food” [ 120 ]. In 1992 Powles observed “It is possible that adverse lagged effects of current industrial (and military) activities will disrupt the habitat of future generations of our species through processes such as stratospheric ozone depletion, global warming and others as yet unpredicted” [ 121 ]. However, in the following years, this sense of urgency largely dissipated, until the long paper by Costello et al. in 2009 [ 11 ].

Conditioned by growing up during the Cold War, the author has long been apprehensive about civilization’s survival. However, my timeline for global health disaster has always been multi-decadal. Civilizational collapse, if it is to occur, will not necessarily be in my own lifetime [ 54 ]. My concerns are not based solely on climate change. Climate change, by itself, is most unlikely to cripple civilization. A well-functioning global society, motivated to do so, could easily eliminate hunger and poverty, not only today, but under all but worst-case climate change. Refugees from inundated islands, war-torn Syria or the drought-stricken Chad basin [ 122 ] could easily be accommodated in more fertile and more elevated parts of the world. Unfortunately, humans currently do not co-operate on such a scale, and this behavior may, in part, be driven by inborn, “hard-wired”, evolutionary-shaped traits [ 123 ]. If civilization is to endure we may need to collectively overcome our seemingly deep wiring for tribalism and separation.

Acknowledgments

My thanks to John Potter for his help with locating obscure references, and to Andy Morse and Kristie Ebi for their very helpful comments, and Joseph Guillaume for his statistical advice. I especially thank Ivan Hanigan for the bootstrap analysis. I also thank three anonymous reviewers.

Supplementary Materials

The following are available online at http://www.mdpi.com/1660-4601/15/10/2266/s1 .

Appendix A.1. Detailed Methods and Results

The search method had six steps (see Figure A1 ). Initial exploration used the author’s Endnote database, of over 35,000 references, to find relevant articles. The second step was to search, using Google Scholar, for up to the first 100 results for each year in the search period (1980–2013), using the terms “climate change” and health or “global warming” and “health”. For the first decade in which relevant articles were found (1989–1998) both pairs of terms were used, but from 1999 to 2013 inclusive, only the former terms were used (“climate change” and “health”). In the third step, the search was expanded by seeking the same terms, using PubMed, for the same period; 1980–2013 (inclusive). After eliminating duplicates, all remaining papers were checked to ensure that they met the eligibility criteria listed in Box 1 . In stage 4, several papers were included if they appeared in special issues of journals, together with articles identified by PubMed, or suggested by colleagues. In stage 5, the BMJ database for news items about climate change and health was searched explored, because although PubMed found a few the proportion it identified was low. Finally, in stage 6, several other papers were found by chance, such as in reviews, in the references of cited papers, or by searching for other papers.

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Outline of the six stage search strategy for papers published from 1989–2013.

Appendix A.2. Further Scoring Details

The following details are provided in order to provide additional information about the scoring process. It discusses the scoring process for three highly cited papers (from 2005–2007), each of which was scored as two. The first (cited 2059 times) had no mention of population displacement or conflict, but included the sentence “Projections of the effect of climate change on food crop yield production globally appear to be broadly neutral, but climate change will probably exacerbate regional food supply inequalities” [ 124 ]. This statement was assessed as accepting the possibility of a degree of food scarcity judged to be more severe than that described by many papers (particularly concerning the Arctic) which discuss a likely impairment in regional nutrition, but do not forecast insufficient calories or nutrients, let alone famine. Although the conclusion regarding overall global food security in this paper was reassuring, there are already four acknowledged famines in African nations and one in Yemen [ 125 ]. Any exacerbation of regional food supply inequalities is therefore likely to result in aggravated famines, unless future famines are eliminated; an unlikely prospect. Because this paper was cited so frequently a lower score would impact the overall result. If there is a bias from scoring this paper as two it is towards the null hypothesis.

In 2006 a widely cited paper [ 126 ] stated “Other important climatic risks to health, from changes in regional food yields, disruption of fisheries, loss of livelihoods, and population displacement (because of sea-level rise, water shortages, etc.) are less easy to study than these factors and their causal processes and effects are less easily quantified”. This is a more comprehensive list of civilization-endangering effects than the paper discussed above, but the language is restrained and brief. It was scored as a two.

In 2007 another widely cited paper included the sentences “Climate change will, itself, affect food yields around the world unevenly. Although some regions, mostly at mid-to-high latitude, could experience gains, many (e.g., in sub-Saharan Africa) are likely to be adversely affected, with impairment of both nutrition and incomes. Population displacement and conflict are also likely, because of various factors including food insecurity, desertification, sea-level rise, and increased extreme weather events” [ 127 ]. Of the three papers discussed here this provided the most comprehensive list of such effects and also explores their interaction. However, it did not speculate about civilization collapse, nor describe climate change as the biggest threat to global public health.

A gradient exists between papers scored two or three, rather than a clear threshold. Papers were not scored as three simply by including a more detailed explanation or assertion of the existence and importance of conflict, migration or famine, even if an interaction among them was suggested. They needed something extra. For example, one paper [ 128 ] stated (referring to Costello et al. [ 11 ]) “a watershed paper … suggests that climate change represents the biggest potential threat to human health in the twenty-first century … a recent report … also estimates that four billion people are vulnerable and 500 million people are at extreme risk”. This paper was scored as three even though the paper focused on medical education. Although the phrase “the biggest potential threat to human health in the twenty-first century” can, with repetition, lose its capacity to shock, its meaning, if taken literally, is surely sufficiently dire to be scored as three.

Another paper (scored as three) stated “global health, population growth, economic development, environmental degradation, and climate change are the main challenges we face in the 21st century” [ 129 ]. It also stated that “significant mass migration is likely to occur in response to climate change”.

The interpretation of papers was not excessively generous. For example, a paper that noted: “Changes in the frequency and intensity of extreme weather and climate events have had profound effects on both human society and the natural environment” was scored as one because there was no discussion of this aspect in the abstract or further in the text. It was also considered that the words “have had profound” was insufficiently clear. Nor did the paper discuss conflict, migration or famine.

In contrast, two papers about climate change and health in Nepal were scored as two, as they included the statements “Climate change is becoming huge threat to health especially for those from developing countries” (sic) [ 130 ] and “Climate change is a global issue in this century which has challenged the survival of living creatures affecting the life supporting systems of the earth: atmosphere, hydrosphere and lithosphere” [ 131 ].

Appendix A.3. Sources (Detailed)

Seventeen articles were identified from the author’s database, but not found via PubMed or Google Scholar. Other sources are shown in Table A4 .

This shows the primary source of the 2146 included articles. 18 articles were from special issues, 5 were found accidentally, 1 was from a review and 1 was from a colleague. Many articles were found using multiple methods. The papers listed here in the GS column were not found by PM but may also have been identified by CB. Abbreviations: PM = PubMed, GS = Google Scholar, CB = Colin Butler.

Appendix A.4. Score, Citation and Journal Details

This shows the number of articles and their average score for each quintile from 1989–1993.

This shows the number of papers and citations in each category divided into five quintiles for the 25 years of analysis. Note that in the third quintile (1999–2003) only 5 articles were ranked as three. Ironically, the paper scored as three in 2002 was a news item which quoted Andrew Sims, policy director of the New Economics Foundation as lamenting “Health is not even being talked about here [Delhi], although the potential health impact is a devastating one, almost unimaginable” [ 132 ].

Ten journals published at least 22 articles on climate change and health in the period 1989–2013.

Appendix A.5. Additional Papers 2014–2018

PubMed was searched for the terms “climate change” and “health” for the period 2014–2017 inclusive. This found 3377 papers, which were grouped by year of publication and listed alphabetically, by surname of the first author. Every 20th paper (in each year) was then examined. If a paper was found to be ineligible, successive consecutive (alphabetical) candidates were examined until at least 5% of the total maximum number for each year had been found eligible and analyzed. In total, 156 papers were scored. This sample represented 5.1% of the 3036 papers which remained after 341 of the original pool had been eliminated. More would be excluded, given a more thorough inspection. The average score of these 156 articles and their 95% confidence interval, determined by bootstrapping, was 1.29 (1.21–1.39). The average score of these papers is lower than for the papers published from 2009–2013 (1.37). Although the 95% confidence interval for the period 2014–2018 overlaps with this, there is no evidence to suggest that the more recent literature better recognizes existential risk. See Table A5 and Figure A2 .

This shows the number, number analyzed and scores for the 156 papers that were analyzed for the period 2014–2018, tabulated by year. Note that some of the candidate papers would be culled after further examination.

An external file that holds a picture, illustration, etc.
Object name is ijerph-15-02266-g0A2.jpg

This shows the density of means and distributions for each year (2014–2017), based on 10,000 bootstrapped resamples (with replacement from the set for each year) and also for papers from 2013–2018 inclusive.

This research received no external funding.

Conflicts of Interest

The author declares no conflict of interest.

ENCYCLOPEDIC ENTRY

Global warming.

The causes, effects, and complexities of global warming are important to understand so that we can fight for the health of our planet.

Earth Science, Climatology

Tennessee Power Plant

Ash spews from a coal-fueled power plant in New Johnsonville, Tennessee, United States.

Photograph by Emory Kristof/ National Geographic

Ash spews from a coal-fueled power plant in New Johnsonville, Tennessee, United States.

Global warming is the long-term warming of the planet’s overall temperature. Though this warming trend has been going on for a long time, its pace has significantly increased in the last hundred years due to the burning of fossil fuels . As the human population has increased, so has the volume of fossil fuels burned. Fossil fuels include coal, oil, and natural gas, and burning them causes what is known as the “greenhouse effect” in Earth’s atmosphere.

The greenhouse effect is when the sun’s rays penetrate the atmosphere, but when that heat is reflected off the surface cannot escape back into space. Gases produced by the burning of fossil fuels prevent the heat from leaving the atmosphere. These greenhouse gasses are carbon dioxide , chlorofluorocarbons, water vapor , methane , and nitrous oxide . The excess heat in the atmosphere has caused the average global temperature to rise overtime, otherwise known as global warming.

Global warming has presented another issue called climate change. Sometimes these phrases are used interchangeably, however, they are different. Climate change refers to changes in weather patterns and growing seasons around the world. It also refers to sea level rise caused by the expansion of warmer seas and melting ice sheets and glaciers . Global warming causes climate change, which poses a serious threat to life on Earth in the forms of widespread flooding and extreme weather. Scientists continue to study global warming and its impact on Earth.

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Global warming research paper.

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Global Warming and Politics 

    The existing information that we have on global warming and climate change offers little guidance on the amount of damage the globe is suffering. The increase of internal global temperature has been over thousands of years, however, it has been increasing lately. Since the Industrial revolution around the 1760s, the amount of carbon dioxide and other kinds of gasses damaged the ozone cap and increase the greenhouse effect making the world hotter. And increasing climate change with it. The elected head of our country was not a perfect example a couple of years back o on how to treat this cause, but it has gotten better over the few months. There are many ways that we can stop this effect on the world before it gets to 1.5 degrees over the global’s normal temperature.

The meteor that will extinguish the human race is one that we have predicted from years ago. We know how to stop it, but we do nothing about it. It’s called Global Warming. 

In a NASA article called “The Causes of Climate” change that was published when NASA started studying climate change around 1980. It argues the way global warming started, causes, and effects between other topics. The purpose of this article is to inform regular people about the causes and effects of global warming, what we have done to stop it and what we can still do to reduce this temperature. This paper adopts a formal and assertive tone to make it strong and understandable for most of the population to understand and interact with. “Scientists attribute the global warming trend observed since the mid-20 th century to the human expansion of the “greenhouse effect” 1 — warming that results when the atmosphere traps heat radiating from Earth toward space.” (Page #1 Sentences 1-3). This explains why the temperature is increasing, that is the greenhouse effect. Gasses get caught in the ozone layer and make some light get into the globe and some do not let the light leave. 

The authors Yangyang Xu, Veerabhadran Ramanathan, and David G. Victor. In their article called “Global Warming Will Happen Faster Than We Think” which was published in December 2018 addresses the topic of the increase of global temperature at an amazing and worrying speed and argues future predictions on how much time will it take to kill the human race outrunning our adaptation abilities. The purpose of these incredible authors is to make reasoning into the people and get into their feelings. They predicted the amount of temperature that will be around the world in 10 years, 20 years which is incredibly fast. Their children will suffer the consequences of our actions and their children’s children. They adopt a more aggressive tone to mess with people’s heads and turn them against this very dangerous topic. 

https://www.nature.com/articles/d41586-018-07586-5

Here is an image showing the amount of temperature that the globe will be above its normal level in the next couple of years. Even with some variation, there is a critical and deadly increase expected that will kill every live spice in the world, slowly. 

The paper “Climate policy: Ditch the 2 °C warming goal” published in 2014 by two professors of the University of California demonstrates how global warming affects climate change. The number of greenhouse effects also has a big impact on the difference in climate change. “A single index of climate-change risk would be wonderful. Such a thing, however, cannot exist. Instead, a set of indicators is needed to gauge the varied stresses that humans are placing on the climate system and their possible impacts. Doctors call their basket of health indices vital signs. The same approach is needed for the climate”(page 2, sentences 67-71). This explains what humans are doing to increase the randomness and dangerousness of climate change. It also comments how the sea reflects a lot on global warming. “The oceans are taking up 93% of the extra energy being added to the climate system, which is stoking sea-level rise and other climate impacts” 

https://www.nature.com/articles/514030a

    “The Real Clime Debate” This Paper has a very straightforward and informative tone, by responding to questions that the general public has and can easily understand. It was published in October 2017 by Peter Agre, Mario Molina , and Steven Chu, persons with different occupations but with the same goal. What does politics have to do with climate change and global warming? They think that scientists should make a big part of politics. Even though some scientists have been in the government, there should be a little more and with bigger influence within the public, they think that with this, the public will have a bigger impact and take this situation more seriously. 

Talking about politics, we have “President Trump announced on Thursday that the United States would withdraw from the Paris climate accord, weakening efforts to combat global warming and embracing isolationist voices in his White House who argued that the agreement was a pernicious threat to the economy and American sovereignty” (Publish by the New York Times newspaper in June 2017). This paragraph illustrates that President Trump did not care about global warming and climate change. He tried to get the United States out of the Paris climate accord. “At what point does America get demeaned? At what point do they start laughing at us as a country?” Mr. Trump said. “We don’t want other leaders and other countries laughing at us anymore. And they won’t be.” These are words that The president said. He thought that the other countries were laughing at him and his country for trying to stop global warming. His mind was just on his country and even though it sounds selfish, he was just taking care of the job he was assigned to. This is a good example of a lack of information about this topic and that scientists need to do a better job at informing the general public. 

In conclusion, global warming is a major challenge for our current global society. There is no doubt that global warming will change our lives in the next couple of decades. The question is if we are going to be able to decrease its speed and be able to adapt to these new changes or if it will outspeed us and kill the whole human race. Every single person in this world should be making this stop, either by doing something to the environment like taking a bike instead of your car or voting each time so you know at least you are voting for a person that has knowledge about this and has the initiative to stop it. 

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David Wallace-Wells

Just how many people will die from climate change.

global warming paper research

By David Wallace-Wells

Opinion Writer

How deadly could climate change be? Last fall, in an idiosyncratic corner of the internet where I happen to spend a lot of time, an argument broke out about how to quantify and characterize the mortality impact of global warming. An activist named Roger Hallam — a founder of Extinction Rebellion who now helps lead the harder-line group Just Stop Oil — had told the BBC that, if global temperatures reach two degrees Celsius above the preindustrial average, “mainly richer humans will be responsible for killing roughly one billion mainly poorer humans.”

Hallam was quoting from a somewhat obscure paper, published by an engineer and a musicologist and focused less on climate impacts than on climate justice. The claim was quickly picked apart by experts: “An oft-quoted adage within the climate-modeler community is that garbage in equals garbage out,” the climate advocate Mark Lynas wrote. “Getting the science right will strengthen rather than weaken the case for climate activism, both in the public mind and in court.”

These are inarguable principles, and I don’t think it’s right to suggest that reaching two degrees of warming (which now looks very likely) will mean a billion people dead. Certainly that isn’t scientific consensus. But it did make me wonder: How big would the number have to be to strike you as really big? And how small to seem acceptable?

I ask because many more rigorous estimates, while lower, are still quite shocking. Some calculations run easily into the tens of millions. If you include premature deaths from the air pollution produced by the burning of fossil fuels, you may well get estimates stretching into the hundreds of millions. These are all speculations, of course. Estimating climate mortality involves a huge range of calculations and projections, all of which are shrouded by large clouds of uncertainty — it’s literally a climate-scale puzzle, with billions of human variables and many more political and environmental ones, and settling on a number also requires separating the additional impact of warming from the ongoing mortality produced by social and environmental systems running continuously in the background today.

In a recent commentary for Nature Medicine, the Georgetown University biologist Colin Carlson used a decades-old formula to calculate that warming had already killed four million people globally since 2000 just from malnutrition, floods, diarrhea, malaria and cardiovascular disease. As Carlson notes, this means that, since the turn of the millennium, deaths from climate change have already exceeded those from all World Health Organization global-health emergencies other than Covid-19 combined. “Vanishingly few of these deaths will have been recognized by the victims’ families, or acknowledged by national governments, as the consequence of climate change,” he says.

Going forward, most estimates suggest the impact should grow along with global temperature. According to one 2014 projection by the W.H.O., climate change is most likely to cause 250,000 deaths annually from 2030 to 2050. According to research by the Climate Impact Lab, a moderate emissions trajectory, most likely leading to about two degrees of warming by the end of the century, would produce by that time about 40 million additional deaths.

Other work is even more striking. In a recent paper published in The Proceedings of the National Academy of Sciences, a team led by Drew Shindell of Duke University calculated that heat exposure alone is already killing more than 100,000 Indians and about 150,000 Chinese each year. Not all of these deaths are attributable to warming — people died from heat exposure in the preindustrial past, of course — but the trends for all the examined countries were clear and concerning. By the end of the century, the team calculated, even in a low-emissions, low-warming scenario, annual mortality from heat exposure could reach 500,000 in India and 400,000 in China. This is just from heat, remember, and as Shindell points out, there are plenty of known climate impacts that are so hard to model that they are often simply not modeled. “There’s all kinds of stuff missing, and we still get big numbers,” Shindell says. “That should actually be scary.”

One thing that is almost always left out is air pollution. This is the research area for which Shindell is best known, and his most notorious finding on the subject is that simply burning the additional fossil fuel necessary to bring the planet from 1.5 degrees of warming up to two degrees would produce air pollution that would prematurely kill an estimated 153 million people.

If that number shocks you, consider that, according to the new paper, the present-day figures are more than two and a half million Chinese deaths each year, more than two million in India and about 200,000 annually in Pakistan, Bangladesh and the United States each. Even given rapid decarbonization, Shindell and his co-authors find that, by the end of the century, particulate pollution might be responsible for the annual premature deaths of four million Indians, two million Chinese, 800,000 Pakistanis, 500,000 Bangladeshis and 100,000 Americans.

Not all of the particulate pollution is a result of the burning of fossil fuels. (And even fossil-fuel pollution isn’t, technically, a climate impact, though it is produced by the same activities that produce the lion’s share of warming.) But over the course of the century, even in a low-emissions scenario, the total mortality impact of air pollution in just those five countries could reach half a billion.

Now, air pollution is probably not what you have in mind when you picture significant climate change; probably diarrhea and malnutrition aren’t either, or the elevated risk of stroke or respiratory disease that comes, empirically, with higher temperatures. Instead, you’re likely to imagine mass heat death or a world-historical storm. But that is a major lesson of the research on mortality and warming: that our climate fantasies can lead us astray, pulling us toward apocalyptic visions of environmental disaster rather than the simple but tragic accumulation of what today look like ordinary, if unfortunate, events — heat waves like those we’ve already lived through, infectious-disease outbreaks like those we’ve already read about, air-pollution problems like those we’ve mostly left behind in places like the United States. Climate scientists worry a lot about what they often call “discontinuities” or “nonlinearities.” But the world is a very large place, and you don’t need a major phase-shift in our experience of climate to produce a harrowing death toll. You just need things that kill people now to be made worse by warming.

Perversely, it’s also the case that some of the increased death toll can be seen as a sign of more general social progress. Everyone dies of something, mortality researchers like to point out, and you get to die of environmental causes only if you don’t die earlier from something else: childbirth, say, or measles, or smoking. Over the course of the century, Shindell says, he expects the share of overall death attributable to environmental factors to grow — not just because those conditions will worsen but also because other measures of human health and well-being will improve globally. There may well be catastrophic surprises in store, as well — extreme disasters, underestimated impacts and rapidly passed tipping points. But the science of climate mortality today suggests a different experience, of even large-scale climate mortality softening into a grim sort of background noise, never quite deafening, no matter how loud it gets.

When, a few years ago, in the midst of a period of intense climate alarm, a few more hardheaded climate minds invoked instead the analogy of planetary “diabetes,” they got a whiplash of criticism from activists in response. But while we can’t really see the deep future with much clarity, disease may prove a more precise analogy than apocalypse. This is not to say that the size of the impact will be small. It’s to say that imagining a climate future dominated by sudden ruptures and overwhelming catastrophes is perhaps to risk preparing for the wrong future — and remaining oblivious, in the meantime, to the death and suffering of the present.

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  • Published: 27 October 2020

Global warming due to loss of large ice masses and Arctic summer sea ice

  • Nico Wunderling   ORCID: orcid.org/0000-0002-3566-323X 1 , 2 , 3 ,
  • Matteo Willeit 1 ,
  • Jonathan F. Donges   ORCID: orcid.org/0000-0001-5233-7703 1 , 4 &
  • Ricarda Winkelmann   ORCID: orcid.org/0000-0003-1248-3217 1 , 2  

Nature Communications volume  11 , Article number:  5177 ( 2020 ) Cite this article

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  • Climate and Earth system modelling
  • Cryospheric science
  • Projection and prediction

Several large-scale cryosphere elements such as the Arctic summer sea ice, the mountain glaciers, the Greenland and West Antarctic Ice Sheet have changed substantially during the last century due to anthropogenic global warming. However, the impacts of their possible future disintegration on global mean temperature (GMT) and climate feedbacks have not yet been comprehensively evaluated. Here, we quantify this response using an Earth system model of intermediate complexity. Overall, we find a median additional global warming of 0.43 °C (interquartile range: 0.39−0.46 °C) at a CO 2 concentration of 400 ppm. Most of this response (55%) is caused by albedo changes, but lapse rate together with water vapour (30%) and cloud feedbacks (15%) also contribute significantly. While a decay of the ice sheets would occur on centennial to millennial time scales, the Arctic might become ice-free during summer within the 21st century. Our findings imply an additional increase of the GMT on intermediate to long time scales.

Introduction

Extensive changes have been observed in large-scale cryosphere elements during the last decades such as the Arctic summer sea ice, mountain glaciers, the Greenland and West Antarctic Ice Sheet 1 , 2 , 3 , 4 , 5 .

From the late 1970s to the mid-2000s, the Arctic summer sea ice area has declined by more than 10% per decade, as satellite measurements reveal 1 . If this trend continues, the Arctic could become ice-free in summer for the first time within the 21st century. Projections with CMIP-5 6 (Coupled Model Intercomparison Project Phase 5) models show that this could be the case as early as 2030 to 2050 for higher emission scenarios such as RCP8.5 (Representative Concentration Pathway) 7 . Some GCMs (global circulation models) show an ice-free Arctic for the first time within this century also for the moderate emission scenarios at a warming of 1.7 °C above pre-industrial 8 , 9 . Furthermore, observations reveal that the Arctic summer sea ice declines faster than expected in experiments from GCMs 1 .

At the same time, mountain-glaciers world-wide have retreated, with an average weight equivalent ice loss of approximately 250 ± 30 Gt per year between 1901 and 2009 2 , 10 . This translates, in the same time span, into a loss of 21% of the glaciated volume of mountain glaciers worldwide, excluding (Sub-)Antarctic peripheral glaciers, as found in model simulations 11 . During this time, it is estimated that approximately 600 glaciers have disappeared and many more are likely to follow in the future (IPCC-AR5, Chapter 4 6 ). 36  ±  8% of today’s glacier mass is already committed to be lost in response to past greenhouse gas emissions 12 and it has been found that many mountain glaciers are currently in disequilibrium and will be subject to further ice loss 13 .

Moreover, both the West Antarctic and the Greenland Ice Sheet have lost mass at an accelerating pace in the past decades 3 , 4 , 5 . With progressing global warming, ice loss from the polar ice sheets and subsequent sea-level rise is expected to further increase 14 , 15 . Beyond a critical temperature threshold, large parts of the Greenland Ice Sheet might melt, accelerated by positive feedbacks such as the ice-albedo and melt-elevation feedbacks 16 , 17 . From model simulations, this threshold temperature is suggested to range between 0.8 and 3.2 °C above pre-industrial levels 18 .

Parts of the West Antarctic Ice Sheet might already have crossed a point of instability: the grounding lines of several glaciers in the Amundsen basin are rapidly retreating and have likely become unstable, causing sustained ice discharge from the entire basin which could lead to more than 1 m of global sea-level rise 19 . Similar dynamics might be induced in other parts of the Antarctic Ice Sheet and could eventually lead to its complete disintegration under unmitigated climate change 20 .

Anthropogenic climate change has already caused a rise in global mean temperature (GMT) by 0.9 °C comparing 1850–1900 to 2006–2015 21 , with observable impacts on the cryosphere elements mentioned above 6 . It has also been suggested that these regions are likely to change dramatically with ongoing climate warming and some of these changes are suspected to possess some degree of irreversibility 22 , 23 .

Following these recent developments of the cryosphere components, it seems possible that they might be lost at lower temperatures than commonly thought, potentially as low as 1.5 °C above pre-industrial levels 23 . The disintegration of these elements is associated with feedbacks that impact back on GMT, for instance via a change in albedo, clouds or lapse rate, among others, which has not been quantified comprehensively so far. Therefore, we assess the additional global warming caused by disintegration of the Greenland Ice Sheet, the West Antarctic Ice Sheet, the mountain glaciers and the Arctic summer sea ice. Although the Arctic summer sea ice is implemented in more complex Earth system models and its loss part of their simulation results (e.g. in CMIP-5), it is one of the fastest changing cryosphere elements whose additional contribution to global warming is important to be considered. Therefore, we compute and separate its contribution to GMT increase. On the other side, the temperature feedbacks of ice sheets like Greenland, West Antarctica and mountain glaciers are not yet fully integrated in assessments such as CMIP-5.

We base our simulations on the Earth system model of intermediate complexity, CLIMBER-2 24 , 25 because it is computationally efficient and allows a systematic analysis of the decay of the cryosphere components. CLIMBER-2 includes atmosphere, ocean, sea ice, vegetation and land-ice model components and has been applied extensively to understand past and future climate changes 26 , 27 .

In large ensembles of equilibrium model simulations, constrained by fast climate feedbacks strength from global circulation models 28 (see “Methods”), we compare the long-term GMT change in idealised scenarios, where the cryosphere elements are removed, to scenarios where they remain intact. The uncertainty in the additional warming in our simulations is constrained by the uncertainty of the feedback strength in the GCM simulations which we used to mimic the more complex behaviour of GCMs 28 (Supplementary Fig.  1 ). To change the feedback strengths, we alter CLIMBER-2 model parameters that act on the strength of the feedbacks themselves, particularly in the structure of the troposphere and the clouds (atmospheric changes) as well as in the snow albedo (see Supplementary Table  1 ). With reasonably altered parameters in CLIMBER-2, we arrive at an equilibrium climate sensitivity of 2.0–3.75 °C for our ensemble leading to smaller temperature responses than the full range from CMIP-5 (2.0–4.7 °C) or CMIP-6 (1.8–5.6 °C) would 29 . Details on the calibration process are given in the methods section: uncertainty estimates.

In our experiments the state of the Greenland Ice Sheet, the West Antarctic Ice Sheet and mountain glaciers is simply prescribed in the model and affects both, ice cover and topography. In our simulations for the Arctic summer sea ice, the albedo during the summer months (June, July, August) is lowered to average values for open ocean waters instantaneously similar to Blackport and Kushner 30 , while keeping the computation of ice-covered areas dynamic, such that the experiment does not violate energy and water conservation.

In this study, we find that global warming is amplified by the decay of the Earth’s cryosphere as expected from theory and quantify the contribution of each of the four cryosphere components. We further separate the GMT response into contributions from albedo, lapse rate, water vapour and clouds in terms of perturbation of the net radiation at the top of the atmosphere 31 . Here, we focus on the purely radiative effects and neglect freshwater contributions to feedbacks and warming. Thus, our estimates are long-term equilibrium responses when the large ice masses are disintegrated. However, transient warming responses would be reduced due to freshwater input from the West Antarctic and Greenland Ice Sheet on centennial time-scales 32 , 33 , 34 , 35 .

Additional global and regional warming

We consider several different climate scenarios, with atmospheric CO 2 concentrations ranging from the pre-industrial 280 ppm up to 700 ppm and run the model forward until it reaches equilibrium. If not stated otherwise, our findings are shown for a reference simulation at a fixed CO 2 concentration of 400 ppm in equilibrium after 10,000 years. 400 ppm corresponds to an equilibrium GMT increase of 1.5 °C above pre-industrial in CLIMBER-2 simulations. Upon this, we evaluate the additional regional and global warming caused by the large-scale loss of the Arctic sea ice during summer, mountain glaciers, and the polar ice sheets. While this ad-hoc loss of the ice masses poses a hypothetical scenario, it allows us to separate the additional warming through the ice-climate feedbacks from other effects. In our experiments, we report the median value of the ensemble and the brackets represent the interquartile range unless stated otherwise.

In our simulations, we find that global warming is increased by the decay of the Earth’s cryosphere. The disintegration of the Arctic summer sea ice and the retreat of mountain glaciers, the Greenland and the West Antarctic Ice Sheets together cause an additional GMT increase of 0.43 °C (0.39–0.46 °C) for a baseline-scenario of 1.5 °C warming above pre-industrial levels, which translates into an additional warming of 29% (26–31%).

Locally, the loss of each element induces a very strong warming signal, which is consistent with previous studies on polar and Arctic amplification 36 , 37 . Local warming around the cryosphere components is up to 5 °C stronger, particularly around Greenland and West Antarctica (Fig.  1 a). However, the ice loss causes significant warming also in lower latitudes, with values of 0.2 °C around the equator.

The warming results from our simulations are consistent in magnitude and polar amplification with past warm periods, particularly the Mid-Pliocene Warm Period, during which the large ice sheets were at least partially disintegrated 38 , 39 . Still, the distribution among the feedback processes in these paleoclimate states remains uncertain.

Under ongoing global warming, further ice loss is to be expected for all of the four cryosphere components considered here; however, the corresponding time scales differ by several orders of magnitude. While substantial ice loss from Greenland or Antarctica might be triggered by anthropogenic climate change within the current century, these changes would manifest over several centuries to millennia 15 . Ice-free Arctic summers on the other side might already occur in the next decades 1 , 7 , 9 . Therefore, we also consider the regional warming caused solely by the loss of the Arctic summer sea ice (Fig.  1 b). The additional warming in the Arctic region on a yearly average accounts for more than 1.5 °C regionally and for 0.19 °C globally. The meltdown of the Arctic sea ice and its regional warming effect is also simulated by CMIP-5 runs dependent on the future anthropogenic CO 2 forcing scenarios, the RCP scenarios 6 , 9 .

figure 1

a Regional warming for the whole Earth if Arctic summer sea ice (ASSI) in June, July and August, mountain glaciers (MG), Greenland Ice Sheet (GIS) and West Antarctic Ice Sheet (WAIS) vanish at a global mean temperature of 1.5 °C above pre-industrial. b Same as in ( a ) with an additional zoom-in of the Arctic region if only the Arctic summer sea ice vanishes, which might happen until the end of the century. The light blue line indicates the region of removed Arctic summer sea ice extent, where its concentration in CLIMBER-2 is 15% or higher. In all panels, the average additional warming on top of 1.5 °C is shown in absolute degree.

With CLIMBER-2, we are able to distinguish between the respective cryosphere elements and can compute the additional warming resulting from each of these (Fig.  2 ). The additional warmings are 0.19 °C (0.16–0.21 °C) for the Arctic summer sea ice, 0.13 °C (0.12–0.14 °C) for GIS, 0.08 °C (0.07–0.09 °C) for mountain glaciers and 0.05 °C (0.04–0.06 °C) for WAIS, where the values in brackets indicate the interquartile range and the main value represents the median. If all four elements would disintegrate, the additional warming is the sum of all four individual warmings resulting in 0.43 °C (0.39–0.46 °C) (thick dark red line in the Fig.  2 ). Our results regarding the amount of warming are of comparable magnitude to previous efforts computed for late Pliocene realisations (PRISM) of the ice sheets 40 , 41 . Both studies show a pronounced warming in the proximity of the locations where ice is removed, which is in good agreement with our results (see Fig.  1 and Supplementary Fig.  2 ). The disintegration of all elements at the same time can very closely be approximated by the sum of single elements disintegrated indicating that their effects on GMT add up linearly. This can be found in Fig.  3 , where we also show the warming for CO 2 concentrations from 280 to 700 ppm. Fig.  2 highlights the additional warming of 1.5 °C above pre-industrial.

figure 2

The additional warming for the cryosphere components is shown for a scenario consistent with global warming levels of 1.5 °C. Radially outward, the temperature anomaly is displayed which arises from the disappearance of the cryosphere elements. The thick dark red line indicates the maximum effect of additional warming in case all cryosphere elements lose stability. All values are the medians of the ensemble.

figure 3

Additional warming plotted against CO 2 concentration. Disintegration of of cryosphere components separately for ( a ) the Arctic summer sea ice, ( b ) the mountain glaciers, ( c ) the Greenland Ice Sheet, ( d ) the West Antarctic Ice Sheet, ( e ) the sum of all additional warmings from the separately disintegrated cryosphere elements and ( f ) the disintegration of all four elements at the same time. The grey bars match the red bars within their errors which means, according to CLIMBER-2, that the warming effect of singular disintegrated cryosphere elements can linearly be added up to the effect of all four elements disintegrated at the same time. Here we show median, interquartile range and full ensemble spread for each CO 2 concentration. The upper horizontal axis shows the temperature increase above pre-industrial, where a least-square fit converting CO 2 concentration to temperature with python’s function scipy.optimize.curve_fit was used. The respective fitted temperatures arise from full ensemble simulations at prescribed CO 2 concentrations, but without removed cryosphere elements.

Warming from the Arctic summer sea ice

We obtain that the warming results are independent from the CO 2 concentration forcing between 280 and 700 ppm apart from the Arctic summer sea ice (see Fig.  3 a), which shows a decreasing additional warming for higher CO 2 concentrations (Fig  4 ). This can, in turn, be explained: In CLIMBER-2 simulations we find, with increasing prescribed CO 2 concentrations corresponding to increasing GMT, that the Arctic summer sea ice area declines in a linear way, which was also found in observational records 42 and in GCM simulations 9 . For a CO 2 concentration of 400 ppm corresponding to 1.5 °C in CLIMBER-2 above pre-industrial GMT levels, the additional warming is 0.19 °C (0.16–0.21 °C). The actual minimal sea ice cover observed by NERSC (Nansen Environmental & Remote Sensing Center) as an average area from 1979 to 2006 is on the order of 5.5–6.5 × 10 6  km 2 which would correspond to a warming of approximately 0.15 °C in our simulations (see Fig.  4 ). In Supplementary Fig.  3 , we show the sea ice area over the course of 1 year for the control and the perturbed run.

figure 4

Box whiskers plot of global mean temperature (ΔGMT) versus Arctic summer sea ice area with error boxes (error bars) representing the interquartile range (full spread) of the ensemble at the according GMT over the CLIMBER-2 ensemble runs. The additional warming when the Arctic summer sea ice disappears is represented by a second y-axis computed via a least-square fit from the corresponding summer sea ice area. The relationship between summer sea ice area and additional warming is slightly nonlinear. This means that a doubling of the ice area does not quite translate into a doubling of the additional warming. The x-axis shows ΔGMT above pre-industrial computed via a GMT-CO 2 concentration least-square fit. The shaded area shows the mean Arctic sea ice area as observed by NERSC (Nansen Environmental & Remote Sensing Center) from 1979 to 2006, where the uncertainty indicates one standard deviation: 6.0 ± 0.5 × 10 6  km 2 .

Radiative perturbations at the top of the atmosphere

For each cryosphere element, we are able to deconvolve the net change of radiative perturbations at the top of the atmosphere into several components that affect the radiative balance of the Earth: water vapour, clouds, lapse rate and albedo. These factors can be quantified in CLIMBER-2 (Table  1 ).

The values for water vapour, lapse rate and clouds in Table  1 can to a very good approximation directly be interpreted as feedback factors once they are divided by the respective warming, e.g., by 0.43 °C in case all investigated cryosphere elements are removed. However, it is important to note that the perturbation arising from albedo changes is both, a forcing and a feedback. The forcing component originates from the prescribed removal of the cryosphere elements. On the other side, the feedback component derives from responses of the surface albedo to the additional warming as for instance through changes in the extent of snow covered area or changes in vegetation cover. Thus both, the feedback and the forcing contribute to the measured radiative perturbation quantified in Table  1 .

Change in surface albedo is the dominant additional radiative perturbation for each considered cryosphere element. It is mainly caused by the albedo change of large ice-covered areas from ice to other non ice-covered surface types, but also by other land cover changes. In total around 55% of the radiative perturbations can be attributed to the change of the albedo.

Two more additional radiative perturbations which are evaluated together as they are anti-correlated are the lapse rate and the water vapour fast climate feedback 28 , 31 . The lapse rate change arises from non-uniform temperature changes in the vertical atmospheric column and subsequent changes in outgoing longwave radiation. The water vapour change describes the capacity of the air to sustain water vapour in the air. The capacity to sustain water vapour is increased by 7% per degree of warming as can be computed using the Clausius–Clapeyron equation. Since the GMT is increasing through the removal of the cryosphere elements, the air can sustain more water vapour which then in turn leads to an additional warming. Together, the additional radiative perturbation of water vapour and lapse rate combine for approximately 30% of the complete radiative perturbation.

For the cloud feedbacks, the IPCC AR5 and newer studies hypothesised that the feedback from clouds is likely positive 6 , 43 as we also find here. It is responsible for 15% of the total radiative perturbation.

Within our experimental setting, it can be expected that the radiative perturbation from albedo changes is very high due to the prescribed removal of the respective cryosphere element. However, the radiative perturbation related to different fast climate feedbacks such as water vapour, lapse rate and clouds also play an important role as drivers of additional warming. Together they account for more than 40% of the total radiative perturbation on average.

Similar investigations on the additional radiative perturbation from albedo changes have been performed for the removal of Arctic sea ice. For a removal of one month during summer an additional radiative perturbation of 0.3 W/m 2 is reported 44 which is in good agreement with Flanner et al. (2011) 45 . We find a slightly higher value of 0.49 W/m 2 for albedo plus clouds value when the Arctic summer sea ice is removed (Table  1 ). This value probably is higher since we have low sea ice for approximately five months (Supplementary Fig.  3 ) in our perturbed experiments instead of one as in Hudson 44 , but parts of the deviation might also be due to the slightly different experimental setup.

In Supplementary Fig.  4 a, we show the latitudinal distribution of the additional radiative perturbation at the top of the atmosphere. The contribution from albedo as well as from lapse rate and water vapour are higher in polar regions and thus contribute to polar amplification which is also apparent in the corresponding zonal mean surface warming (see Supplementary Fig.  4 b). On the other  hand, the additional cloud feedback does not strongly contribute to polar amplification in our simulations. These trends for clouds and albedo have also been found by other studies 36 , 46 . Further studies mention that the lapse rate feedback plays a major role in polar amplification 47 . This seems to be the case here as well (see Supplementary Fig. 4a), but we can only make this statement for the combined feedbacks of lapse rate and water vapour since we do not separate them in our analysis.

Our results concern short and long term effects on GMT due to the disintegration of cryosphere elements which experienced significant changes within the last decades and are likely to also change strongly in the future due to global warming.

On shorter time scales, the decay of the Arctic summer sea ice would exert an additional warming of 0.19 °C (0.16–0.21 °C) at a uniform background warming of 1.5 °C (=400 ppm) above pre-industrial. On longer time scales, which can typically not be considered in CMIP projections, the loss of Greenland and West Antarctica, mountain glaciers and the Arctic summer sea ice together can cause additional GMT warming of 0.43 °C (0.39–0.46 °C). This effect is robust for a whole range of CO 2 emission scenarios up to 700 pm and corresponds to 29% extra warming relative to a 1.5 °C scenario.

In fact, some feedbacks will also be at play before the complete disintegration of the large ice sheets, for instance due to increased ice-drainage from the Amundsen region in West Antarctica 19 , 48 , 49 . Furthermore, it has been shown for WAIS and GIS that transgressing their critical thresholds is likely not reversible due to hysteresis effects 18 , 50 , 51 .

The additional commitment to global warming that we study here represents a long-term, mean-field effect which is separated from possible direct interactions between the elements such as the freshwater input into the thermohaline circulation from the large ice sheets. In other words, the disintegration of the ice sheets has a direct increasing temperature impact on the GMT via the feedbacks quantified here.

Earth system model

For our analysis, we use the Earth system model of intermediate complexity (EMIC) CLIMBER-2 24 , 25 on a coarse spatial resolution of 10 × 52° (lat × lon) resolution. CLIMBER-2 includes a 2.5-D dynamical-statistical atmosphere and a multi-basin, zonally averaged ocean model including sea ice as well as a dynamic model of the terrestrial biosphere. CLIMBER-2 also includes a model for ice sheets, a global carbon cycle model and an atmosphere surface interaction coupler, which are not used in this study since ice sheets and atmospheric CO 2 are prescribed in our experiments. In CLIMBER-2, changes in the cloud fraction are possible. Apart from that, cloud top height can change following changes in the height of the tropopause. The cloud optical thickness parameterisation includes a dependence on the cumulus cloud fraction in addition to a prescribed increase of optical thickness with latitude. With this representation of clouds, CLIMBER-2 is able to reproduce the planetary albedo as observed from CERES (see Supplementary Fig.  5 ) 52 . We benefit from the use of an EMIC as it is highly computationally efficient and allows for a systematic analysis of the impact of disintegration of the cryosphere elements on GMT. With CLIMBER-2 we are able to distinguish different feedbacks and are able to run a robustness analysis using systematic parameter studies. CLIMBER-2 is a good representative of other EMICs 53 .

Model initialisation

In preparation of the model runs, we set up the ice sheets inbuilt in CLIMBER-2. For distinguishing the West and East Antarctic Ice Sheet, we created a mask based on the Antarctic drainage basins 54 . We also included a mountain glacier mask with data from the Randolph glacier inventory 55 . Since we are interested in the climatological behaviour of the disintegration of one or more of the cryosphere elements, we artificially change the setup of CLIMBER-2 depending on which element we remove: In case of WAIS and GIS, the topography of the ice sheet itself is removed together with the ice sheet as the height of the ice sheet is several thousand metres thick and thus might play an important role on the feedbacks. The albedo is replaced by the albedo of bare land or ocean (where appropriate) at first, but can then change freely into any kind of vegetation or snow cover during the simulation run. For our simulations, isostatic rebound is neglected.

For the Arctic summer sea ice and the mountain glaciers, the topography is not taken into account as either the height of the ice or the spacial extent of high thickness regions is very low. To remove the Arctic summer sea ice during the summer months (June, July and August: JJA), the surface covered by sea ice is darkened and the albedo in this region is replaced by the ocean albedo. With this procedure the energy conservation law is not violated since the ice is not just removed and still retains its function as boundary layer between ocean and atmosphere. Thus we are able to compute the effect of summer sea ice in an energetically self-consistent manner. Note that CLIMBER-2 is mass conserving. Our procedure is similar to the experimental setup of Blackport and Kushner 30 , who also reduce albedo values of the sea ice instantaneously. They do this for the whole year and all sea ice compared to our setup, where the albedo is changed only in the northern hemisphere in the summer months.

Model calibration

To emulate the behaviour of more complex general circulation models (GCMs) we created a model ensemble by perturbing several parameters with the target to cover the range of strength of the fast climate feedbacks found by Soden and Held 28 using an ensemble of GCMs. Equally, this could have been done with the feedbacks stated in the IPCC assessment report 5 (AR5), but changes in the reported feedback strengths are small except for the cloud feedback which is less well constrained in AR5 (see IPCC on page 819 for a direct comparison between AR5 values and the values given in Soden and Held 28 ). Thus, our ensemble and our results can be expected to stay the same. The fast climate feedbacks include the water vapour, the lapse rate, the cloud and the albedo feedback. Each of our 39 ensemble members, that we end up with, is constructed from a pair of simulations: one control run at 280 ppm and one perturbed run at a CO 2 doubling of 560 ppm. We then compute the magnitude of the fast climate feedbacks between these pairs of runs (see Supplementary Fig.  1 a). Here, we evaluate the feedbacks using the partial radiation perturbation method 31 , 56 . In this method partial derivatives of model top of the atmosphere radiation with respect to changes in model parameters (such as water vapour, lapse rate and clouds) are determined by diagnostically rerunning of the model radiation code.

The water vapour feedback added to the lapse rate feedback is supposed to lie in the range of 0.8–1.2 W/m 2 /K. These two feedbacks are evaluated together as they are correlated negatively 28 , 57 . The cloud feedback is supposed to range between 0.3 and 1.1 W/m 2 /K and the albedo feedback between 0.2 and 0.45 W/m 2 /K. Furthermore, we put a constraint on the minimal summer sea ice cover in the northern hemisphere to 1.5–6.5 km 2 (see Supplementary Fig.  1 d). In Soden and Held 28 , the albedo value is constraint to values between 0.2 and 0.4 W/m 2 /K, but in our calibration run, it is necessary to increase the upper limit to 0.45 W/m 2 /K since vegetation shifts are considered and otherwise the ensemble gets distorted to small summer sea ice values in the control run.

On top of the fast climate feedbacks, we require each ensemble member (each pair of runs) to possess an equilibrium climate sensitivity above 1.5 and below 4.5 °C, where the equilibrium climate sensitivity is the global warming per doubling of atmospheric CO 2 concentration (see Supplementary Fig.  1 b). It is important to note that our ensemble members span the range from 2.0 to 3.75 °C. This leads to smaller temperature response ranges than the full range from 1.5 to 4.5 °C would. Furthermore, a last constraint is applied at a CO 2 concentration of 280 ppm. The temperature difference between the runs with perturbed parameters and the reference run with unperturbed parameters (brackets in Supplementary Table  1 ) should be less or equal than  ±1.0 °C (see Supplementary. Fig.  1 c). After the application of all these constraints, we find 39 pairs of runs that match our restrictions.

For covering the uncertainty ranges of the feedbacks we perturb parameters (within their experimental uncertainty range) influencing lapse rate together with the water vapour, cloud and albedo feedbacks similarly to Deimling et al. 57 (Supplementary Table  1 ). With this procedure, we are able to reconstruct the uncertainty ranges of the four fast climate feedbacks stated in Soden and Held 28 fairly well.

Uncertainty estimates

We used these 39 calibrated runs, which also represent the uncertainty of our results, as initialisation for our large-scale ensemble simulations. For each of the cryosphere elements, i.e., WAIS, GIS, Arctic summer sea ice and mountain glaciers, as well as all together, we performed the following experiments: (i) Control runs: the respective cryosphere element(s) is/are kept and (ii) experiment runs: removed cryosphere element(s).

We performed the experiments in (i) and (ii) for different atmospheric CO 2 concentrations as external forcing. We chose the CO 2 concentration parameter since it is the one which is most probably increasing in future climate change scenarios. Each of the experiments is performed as a long term equilibrium run for 10,000 simulation years with today’s boundary conditions, i.e., astrophysical parameters like eccentricity and obliquity, and fixed CO 2 concentration. The results are taken as the mean over the last 4000 simulated years since this cancels out minor fluctuations in the equilibrium state. In the end we subtract the experimental run (ii) from the control run (i) to retrieve the temperature difference. Since we are reporting these differences between perturbed (experimental) and control run throughout the main manuscript, the uncertainties given as interquartile ranges are small, also compared to the calibration (see Supplementary Fig.  1 ). This means that our CLIMBER-2 ensemble is robust against the same perturbations in the cryosphere components. We constructed our ensemble aiming at covering a range of sensitivities and different strengths of the feedbacks by the variation of the parameters in Supplementary Table  1 .

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Code availability

There is no comprehensively documented code for the Earth system model CLIMBER-2 available owing to a lack of comprehensive technical description, but the code is available upon request from M.W.

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Acknowledgements

This work has been carried out within the framework of the IRTG 1740/TRP 2015/50122-0 funded by DFG and FAPESP. N.W. and R.W. acknowledge their support. M.W. acknowledges support from the BMBF through the project PalMod. J.F.D. is grateful for financial support by the Stordalen Foundation via the Planetary Boundary Research Network (PB.net), the Earth League’s EarthDoc programme, and the European Research Council Advanced Grant project ERA (Earth Resilience in the Anthropocene; grant ERC-2016-ADG-743080). We are thankful for support by the Leibniz Association (project DominoES). The authors gratefully acknowledge the European Regional Development Fund (ERDF), the German Federal Ministry of Education and Research and the Land Brandenburg for supporting this project by providing resources on the high performance computer system at the Potsdam Institute for Climate Impact Research.

Open Access funding enabled and organized by Projekt DEAL.

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Earth System Analysis, Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, Potsdam, D-14473, Germany

Nico Wunderling, Matteo Willeit, Jonathan F. Donges & Ricarda Winkelmann

Institute of Physics and Astronomy, University of Potsdam, Potsdam, D-14476, Germany

Nico Wunderling & Ricarda Winkelmann

Department of Physics, Humboldt University of Berlin, Berlin, D-12489, Germany

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N.W., M.W., J.F.D. and R.W. designed the study and wrote the text including revisions. N.W. conducted the model simulation runs and prepared the figures. M.W. designed the model calibration and prepared CLIMBER-2 for simulations.

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Correspondence to Nico Wunderling or Ricarda Winkelmann .

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Wunderling, N., Willeit, M., Donges, J.F. et al. Global warming due to loss of large ice masses and Arctic summer sea ice. Nat Commun 11 , 5177 (2020). https://doi.org/10.1038/s41467-020-18934-3

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Published : 27 October 2020

DOI : https://doi.org/10.1038/s41467-020-18934-3

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