Last year saw the rise of climate change protests, with Greta Thunberg, school strikes and Extinction Rebellion generating global news coverage.
Amidst an extraordinary year for media coverage of climate change, scientists and researchers were busily publishing thousands of peer-reviewed journal papers on their latest findings.
These were reported around the world in news articles and blogs and shared on social media platforms, such as Twitter, Facebook, LinkedIn and Reddit. Tracking all these “mentions” was Altmetric, an organisation that scores and ranks papers according to the attention they receive. (Full details of how the Altmetric scoring system works can be found in an earlier article.)
Using Altmetric data for 2019, Carbon Brief has compiled its annual list of the 25 most talked-about climate change-related papers that were published the previous year. The infographic above shows which ones made it into the Top 10.
According to Altmetric, the two highest-scoring climate papers in 2019 are commentaries. These are “World scientists’ warning of a climate emergency” in the journal BioScience, with a score of 10,950, and “Climate tipping points – too risky to bet against” in Nature, which scored 8,552.
The two papers were the third and fifth highest-scoring, respectively, of any “research outputs” published in 2019.
The BioScience “viewpoint” piece declares “clearly and unequivocally that planet Earth is facing a climate emergency”. The paper has five authors, but it was the 11,258 scientist signatories from 153 countries that particularly attracted media attention.
The Nature “comment” paper has a similarly stark message, warning that “evidence from tipping points alone suggests that we are in a state of planetary emergency: both the risk and urgency of the situation are acute”.
Despite their high scores, as they are commentaries rather than research papers, they are not included in Carbon Brief’s leaderboard. (Commentaries are typically commissioned by journal editors, rather than being part of an open submission process. They are also not routinely peer-reviewed. Carbon Brief does include review and perspective articles in the leaderboard, however, as these tend to follow a more traditional editorial process, though this varies by journal. For more on the different types of journal articles, see the guidelines given by Nature and Science as examples.)
Landing the coveted number one spot in 2019 is “New elevation data triple estimates of global vulnerability to sea level rise and coastal flooding”, published in Nature Communications in October, with an Altmetric score of 7,136. This paper was placed seventh in Altmetric’s own Top 100 research papers of 2019.
The study, which was led by Dr Scott Kulp and Dr Benjamin Strauss of the research and reporting website Climate Central, estimates the global population at risk from sea level rise using a new digital elevation model (DEM). The authors argue that global DEMs based on satellite data can overestimate the elevation of coastal areas because the satellites are seeing the tops of buildings and vegetation, rather than the true ground level.
Using the new model, the study revises upwards previous estimates of global vulnerability to rising seas – tripling estimates made using Nasa’s dataset. Under a low emissions scenario, the study suggests that 190 million people “currently occupy global land below projected high tide lines for 2100”. While for a very high emissions scenario, this rises to almost half a billion.
Such large numbers generated a huge amount of coverage. The paper was mentioned in 318 online news articles, on 30 blogs and in more than 21,000 tweets. The outlets the covered the study include BBC News, Reuters, Guardian, iNews, New Scientist, Daily Mirror, Independent, Daily Telegraph, CNN, an interactive feature in the New York Times, the MailOnline and even LADbible.Collection of some of the news headlines for the paper “New elevation data triple estimates of global vulnerability to sea level rise and coastal flooding”. Credit: Tom Prater for Carbon Brief
The Top 5
Just missing out on top spot, in second place is “The global tree restoration potential” in the journal Science, with an Altmetrics score of 6,354.
This study, led by Dr Jean-Francois Bastin – a postdoctoral researcher at the Crowther Lab at ETH Zurich – mapped “the global potential tree coverage to show that 4.4bn hectares of canopy cover could exist under the current climate”. The study estimates that there is “room for an extra 0.9bn hectares” of trees, which has the potential to store 205bn tonnes of carbon.
The paper’s original abstract stated that global tree restoration is “our most effective climate change solution to date” and this eye-catching conclusion drew a lot of attention. The study featured in 582 news articles – more than any other in Carbon Brief’s Top 25, including the sea level rise paper in top spot.
Headlines declared that “Tree planting ‘has mind-blowing potential’ to tackle climate crisis” (Guardian) and “Planting a forest the size of the US could stop climate change” (Sky News). Elsewhere, the study also featured in articles by the Associated Press, National Geographic, Scientific American, CNN, Vox and Quartz, as well as on 68 blogs and in more than 7,000 tweets.
However, the study has also attracted criticism from other scientists. Science has published five “technical comments” responding to the paper, which suggest the original study overestimated the mitigation potential of mass tree-planting and question whether it is indeed the “most effective” method of cutting CO2 when compared to, for example, “energy efficiency and deployment of non-fossil energy sources”. The authors responded to these comments in Science, and also reworded their original abstract to say that global tree restoration is “one of”, rather than “the”, most effective carbon drawdown solutions.
In third place, with a score of 5,429, is a paper in Biological Conservation entitled “Worldwide decline of the entomofauna: A review of its drivers“. The study, by Dr Francisco Sánchez-Bayo of the University of Sydney and Dr Kris Wyckhuys of the University of Queensland, “reveals dramatic rates of decline that may lead to the extinction of 40% of the world’s insect species over the next few decades”.
The systematic review says “habitat loss by conversion to intensive agriculture” as the main driver of the declines, with climate change, pollution and invasive species identified as “additional causes”.
Landing in fourth place is “Food in the Anthropocene: the EAT-Lancet Commission on healthy diets from sustainable food systems“, which was published – unsurprisingly – in the Lancet.
The paper, which has an Altmetric score of 4,560, argues that a “Great Food Transformation is both necessary and achievable” in order to “feed a global population of nearly 10 billion people a healthy diet within food production boundaries by 2050”. It also notes:
“Furthermore, reaching the Paris Agreement of limiting global warming to well below 2C, aiming for 1.5C, is not possible by only decarbonising the global energy system. Transitioning to food systems that can provide negative emissions (ie, function as a major carbon sink instead of a major carbon source) and protecting carbon sinks in natural ecosystems are both required to reach this goal.”
Completing the Top 5 is the Nature paper “Committed emissions from existing energy infrastructure jeopardise 1.5C climate target“, with a score of 4,434.
The study, led by Dr Dan Tong, a postdoctoral scholar at the University of California, Irvine, finds that the committed CO2 emissions from existing and proposed energy infrastructure around the world would be enough to blow the carbon budget for meeting the 1.5C global warming limit.
The Top 10
Just outside of the Top 5, in sixth place, is the Nature paper “Global warming impairs stock-recruitment dynamics of corals“, by lead author Prof Terry Hughes, director of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University.
The study details how the recruitment of new coral larvae in the Great Barrier Reef fell by 89% in 2018 “as a consequence of mass mortality of adult brood stock in 2016 and 2017 owing to heat stress”. The findings suggest the “extent to which the Great Barrier Reef will be able to recover from the collapse in stock-recruitment relationships remains uncertain”, the paper says, “given the projected increased frequency of extreme climate events over the next two decades”.
In seventh place is another Nature paper, this one entitled “No evidence for globally coherent warm and cold periods over the preindustrial Common Era”. Using global palaeoclimate reconstructions, the study finds “strong evidence that anthropogenic global warming is not only unparalleled in terms of absolute temperatures, but also unprecedented in spatial consistency within the context of the past 2,000 years”.
Landing between seventh and eighth is the Science paper “Decline of the North American avifauna”. The study documents “a net loss approaching 3bn birds” across North America, a reduction of 29% since 1970. While the paper did accrue an Altmetrics score of 3,331, climate change gets only a brief mention. Therefore, it earns an honourable mention rather than a place on the leaderboard.
In eighth place, therefore, is “Earth system impacts of the European arrival and Great Dying in the Americas after 1492” in Quaternary Science Reviews. The paper finds that a drop in atmospheric CO2 concentrations in the 16th century was in part due to carbon uptake from abandoned farmland, which was itself left following widespread deaths of Native Americans caused by diseases introduced by European settlers.
Between eighth and ninth is a Nature commentary entitled “Permafrost collapse is accelerating carbon release”. Its authors warn that “the sudden collapse of thawing soils in the Arctic might double the warming from greenhouse gases released from tundra”. Despite its Altmetric score of 3,016, it does not make it into the final list because it is a comment paper.
In ninth is the second Lancet paper in the Top 10: “The global syndemic of obesity, undernutrition, and climate change: The Lancet Commission report”. The paper discusses the three “pandemics” of obesity, undernutrition, and climate change”, noting that global warming can be considered a pandemic “because of its sweeping effects on the health of humans and the natural systems we depend on”.
And rounding off the Top 10 is a Science perspective article “How fast are the oceans warming?” – which features Carbon Brief’s climate science contributor Zeke Hausfather as one of its authors. This paper discusses how observed data show that the world’s oceans are heating up, noting that “multiple lines of evidence from four independent groups thus now suggest a stronger observed OHC [ocean heat content] warming”.
Elsewhere in the Top 25
Just outside the Top 10, in 11th place with a score of 2,767, is the Science Advances paper “Acceleration of ice loss across the Himalayas over the past 40 years”. Using satellite imagery, the study finds a “doubling of the average loss rate” of Himalayan ice during 2000-16 compared to 1975-2000. The study featured in 349 news stories – more than any other in the list outside the Top 4.
In 12th is another Lancet paper, with the longest title of any in the Top 25: “The 2019 report of The Lancet Countdown on health and climate change: ensuring that the health of a child born today is not defined by a changing climate”.
The Lancet Countdown is an “international, multidisciplinary collaboration, dedicated to monitoring the evolving health profile of climate change, and providing an independent assessment of the delivery of commitments made by governments worldwide under the Paris Agreement”. Carbon Brief reported on the 2018 edition of the report, which warned that extreme heat threatens “systemic failure” of the world’s hospitals.
There are also several papers in the Top 25 on the state of the world’s ice sheets – all of which are published in the Proceedings of the National Academy of Sciences (PNAS). In 13th and 14th, respectively, are “Ice sheet contributions to future sea-level rise from structured expert judgment” and “Four decades of Antarctic Ice Sheet mass balance from 1979-2017”. And taking the 25th and final place is “Forty-six years of Greenland Ice Sheet mass balance from 1972 to 2018”.
In 15th is an eye-catching Nature Climate Change paper entitled: “The impact of high ambient temperatures on delivery timing and gestational lengths”. The study of 20 years of delivery data from the US finds that more babies were born around periods of extreme heat, reducing the length of pregnancies by as much as two weeks. It also estimates that, without adaptation, “climate projections suggest additional losses of 250,000 days of gestation per year by the end of the century”.
And in 23rd is another paper by Carbon Brief’s Zeke Hausfather: “Evaluating the performance of past climate model projections” in Geophysical Research Letters. The study concludes that “climate models published over the past five decades were generally quite accurate in predicting global warming in the years after publication, particularly when accounting for differences between modelled and actual changes in atmospheric CO2 and other climate drivers”. The research was inspired by an analysis piece that Hausfather wrote for Carbon Brief in 2017.
All the final scores for the Top 25 climate papers of 2019 can be found in this spreadsheet.
And, finally, looking at which journals feature most frequently in the Top 25, in joint-first place is Nature and PNAS, each represented by four papers. Nature also took top spot in 2018, 2017 and 2015.
Next is Science and the Lancet – each with three – and then Nature Climate Change and Nature Communications with two each. There are seven journals with one paper in the Top 25.Chart shows the frequency that each journal appears in the Top 25 climate papers for 2019. Chart created by Tom Prater using Highcharts
Infographic by Tom Prater for Carbon Brief.
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