NASA OCO2 satellite
(Photo credit: NASA)

The last six years have been the six hottest globally ever recorded by humans.

2014 had been the hottest year up to that point, until the record was shattered in 2015, and again in 2016 thanks to a monster El Niño event. El Niños bring hot water up to the ocean surface where it warms the temperature of the surface air that most directly influences and interests humanity. Next came 2017, the second-hottest year recorded by humans, but far and away the hottest that wasn’t influenced by an El Niño event. And then 2018, the fourth-hottest overall, but by far the hottest year on record that was cooled by a La Niña event.

And now 2019, warmed by a moderate El Niño event and as such not as hot as 2016 with its monster El Niño; but it was nevertheless the second-hottest year on record, and quite possibly the second-hottest in the history of human civilization.

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As global warming has continued, so too has the volume of peer-reviewed reports and studies published by climate scientists documenting its accelerating impacts. Here are some of the most influential climate change research papers published in calendar 2019.

United Nations IPCC reports

The U.N. Intergovernmental Panel on Climate Change published two special reports in 2019, one documenting climate change impacts on land and food security, the other on the oceans and ice. Both reports warned that the risks of severe climate change impacts will grow as global temperatures warm beyond the 1.5 and 2 degrees Celsius (2.7 and 3.6 degrees Fahrenheit) targets set in the 2015 Paris climate agreement. Food security will become increasingly threatened, as will important marine species and ecosystems like coral reefs. Melting ice sheets will continue to accelerate sea-level rise, and permafrost, once-but-no-longer-permanently frozen, will release increasing amounts of previously trapped carbon into the atmosphere as it thaws. Those are but a few of the growing climate change threats documented in the IPCC special reports, which were widely characterized as being “bleak.”

A report produced by the United Nations Environment Programme (UNEP) in November 2019 also documented that countries’ planned fossil fuel extraction efforts will far overshoot the Paris climate targets. While global fossil fuel production and associated carbon emissions must peak and begin to fall within a few years to meet those goals, countries instead plan to continue increasing fossil fuel extraction through the year 2040, which would be consistent with a pathway of more than 3 degrees C (5.4 degrees F) hotter than pre-industrial temperatures by the end of the century. This report highlighted an important disconnect between countries’ goals to curb global warming and their plans to continue extracting ever-more fossil fuels.

Scientists are becoming increasingly concerned and outspoken about this disconnect. In November, more than 11,000 scientists signed a letter published in the journal BioScience declaring “clearly and unequivocally that planet Earth is facing a climate emergency.”

According to Altmetric, which tracks which scientific research most captures the public’s interest each year, the BioScience letter was the fourth-most influential scientific paper published in 2019, with coverage in 527 news outlets and 8,290 tweets.

Studies on the climate of the ‘Common Era’

Climate scientists’ best global temperature reconstructions cover the past 2,000 years, a period also known as the “Common Era.” Several studies investigating the details of temperature changes over the Common Era were published in 2019.

In July, the journal Nature published a study that looked for significant natural climate change events during that period, such as the Medieval Warm Period (approximately the years 950 to 1250) and the Little Ice Age (approximately 1300 to 1850). The authors reported that they “find no evidence for preindustrial globally coherent cold and warm epochs … the warmest period of the past two millennia occurred during the twentieth century for more than 98% of the globe. This provides 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.”

According to Altmetric, that Nature report was the 28th-most influential scientific paper of 2019, with coverage in 264 news outlets and 1,930 tweets.

The Past Global Changes consortium of more than 5,000 scientists from some 125 countries published its newest reconstruction of global temperatures over the Common Era in Nature Geoscience in July. That group similarly concluded, “The largest warming trends at timescales of 20 years and longer occur during the second half of the twentieth century, highlighting the unusual character of the warming in recent decades.”

A study published in Quaternary Science Reviews in March 2019 sought to determine whether the arrival of Europeans in the Americas in 1492 and the subsequent large-scale massacres of native populations (an estimated 56 million deaths by 1600, shrinking the indigenous population 90%) had a detectable influence on the global climate. Forests re-grew on land previously altered by humans, which the authors estimated led to “5 ppm CO2 additional uptake into the land surface in the 1500s compared to the 1400s … The Great Dying of the Indigenous Peoples of the Americas resulted in a human-driven global impact on the Earth System in the two centuries prior to the Industrial Revolution.”

According to Altmetric, the Quaternary Science Reviews study was the 46th-most influential scientific paper of 2019, with coverage in 101 news outlets and 4,141 tweets.

Research in 2019 on coral reefs

Coral reefs are among the most diverse ecosystems in the world and a critical source of food and habitat for about 25% of the ocean’s fish. They’re also among the species and ecosystems most vulnerable to the changing climate, and thus are the subject of much scientific research.

Researchers published a study in Nature Climate Change exploring the resilience of Great Barrier Reef corals that had survived an extreme hot year in 2016 only to be hard-hit again by extreme heat the following year. The results offered a rare source of encouragement, finding that corals that survived 2016 without bleaching were also more resistant to bleaching in 2017.

According to Altmetric, that study was the 14th-most influential scientific paper of 2019, with coverage in 47 news outlets and 6,228 tweets celebrating the good news.

The 22nd-most influential paper, with coverage in 98 news outlets with 4,812 tweets, however, painted a bleaker picture for the Great Barrier Reef. Published in Nature in April, authors of the study found that larval recruitment (the settlement of fish and coral larvae necessary for a healthy ecosystem) declined 89% in 2018 after those two extreme hot years. This study received a boost of media attention in November, when in a Nature Communications study, scientists used underwater speakers to replicate the sounds of healthy coral reefs. They found that twice as many fish arrived and stayed as in areas where no sound was played. This finding provides some hope that coral reef ecosystem recovery could be accelerated – if the reefs aren’t continually battered year after year by extreme heat.

The year’s major findings on flora and fauna

Land species also were the subject of several influential papers published in 2019. One published in Science in July estimated the number of trees that could be planted and the amount of carbon they could pull out of the atmosphere. “Ecosystems could support an additional 0.9 billion hectares of continuous forest. This would represent a greater than 25% increase in forested area, including more than 200 gigatonnes of additional carbon at maturity.” Those numbers would represent a removal of about one-third of cumulative human carbon emissions through current times, and 20 years’ worth at the current rate of about 10 billion tons of carbon per year. However, some scientific groups disputed the accuracy of these estimates, which also rely on foresting every available hectare of land. Nevertheless, the study was Altmetric’s 9th-most influential of 2019, with coverage in 330 news outlets and 6,518 tweets.

Two 2019 studies finding alarming rates of species extinctions were also high on Altmetric’s list. One published in Biological Conservation in April found “dramatic rates of decline that may lead to the extinction of 40% of the world’s insect species over the next few decades” as a result of various human factors including climate change, which “is particularly important in tropical regions.” This study was Altmetric’s 13th-most influential of the year, with coverage in 251 news outlets and 4,679 tweets.

In the second study, published in Science in October, researchers surveyed bird species and found “a net loss approaching three billion birds, or 29% of 1970 abundance” as a result of “habitat loss, agricultural intensification, coastal disturbance, and direct anthropogenic mortality, all exacerbated by climate change.” That study was Altmetric’s 34th-most influential paper, with coverage in 259 news outlets and 1,465 tweets.

These studies are consistent with the “bleak” notion that Earth is currently on a path to its sixth mass extinction event. (An upcoming post at this site will look in depth at major 2019 research reports addressing wildlife and climate change.)

The past year’s major findings on melting ice

Several high-profile papers in 2019 also addressed the accelerating melting of ice and its implications. One, published in Science Advances in June, found that since 2001, Himalayan glaciers have been losing ice at a rate twice as fast as they had in the prior 25 years. Those glaciers provide an important source of water for billions of people in China, India, Pakistan, and several other countries in the region. The paper was Altmetric’s 65th-most influential, with coverage in 294 news outlets and 414 tweets.

A January study published in the Proceedings of the National Academy of Sciences found that over the past decade, Antarctica has been losing ice at a rate six times faster than during the 1980s. And another paper published in PNAS in June asking experts about future sea-level rise projections found that because of accelerating ice sheet declines, a rise of more than two meters (about six feet) by the year 2100 remains within the realm of possibility. Those were the 86th- and 70th-most influential papers of 2019, respectively.

Overall, the second-hottest year was, sadly but perhaps unsurprisingly, full of bleak news in climate science research. Climate scientists’ findings and reports increasingly raise the alarm of a “climate emergency” or “climate crisis” – terms that many in the mainstream media began to use regularly in 2019 without feeling the need to qualify or use quotes. Because of climate scientists’ increasingly concerning research findings and language, climate crisis is becoming more widely accepted and used as the norm.

Editor’s note:  This piece was edited 1/10/2020 to state that the BioScience letter was the fourth most widely covered scientific paper published in 2019.

Dana Nuccitelli, research coordinator for the nonprofit Citizens' Climate Lobby, is an environmental scientist, writer, and author of 'Climatology versus Pseudoscience,' published in 2015. He has published...