A team of scientists led by former NASA climate researcher James Hansen, who formally raised the alarm about climate change to U.S. government leaders in his 1988 testimony to Congress, is working on a new study that warns of a possible short-term spike of planetary heating 2 degrees Celsius above pre-industrial levels by 2050.
In an irony of climate change, the scientists said the sudden surge of warming—especially since 2010—is driven mainly by steep reduction of climate-cooling sulfate aerosol particles in the past 10 to 20 years, as new regulations limited emissions from the biggest sources, including the burning of coal and heavy ship fuels.
The draft paper has not been peer-reviewed, but Hansen, director of the Climate Science Awareness And Solutions center at Columbia University’s Earth Institute, posted it publicly on May 19 on a scientific discussion website, again drawing public attention to the potential for a shock of short-term warming that could devastate global food production and ecosystems.
Hansen’s previous warning about the potential for short-term heating due to emissions reductions was in 2021, when he said the drop in sulfate aerosol pollution could double the rate of global warming during the next 25 years. In his monthly climate bulletin he explained that sulfate aerosols, cause microscopic water droplets in the atmosphere to multiply, which brightens clouds to reflect heat away from the Earth. The reduced amount of sulfates in the atmosphere allows more heat from the sun to warm ocean and land surfaces.
In the discussion draft of the new paper, the authors predict the rate of warming will double from the observed 0.18 degrees Celsius per decade from 1970 to 2010, to at least 0.27 degrees Celsius per decade since 2010.
“Under the current geopolitical approach to GHG emissions, global warming will likely pierce the 1.5°C ceiling in the 2020s and 2°C before 2050,” the authors wrote. “Impacts on people and nature will accelerate as global warming pumps up hydrologic extremes.” The “enormity of the consequences,” they added, requires trying to reverse global warming and cool the Earth down to the relatively stable range of the past 12,000 years, before carbon dioxide pollution disrupted the climate.
The concentrations of climate-cooling sulfate aerosols have decreased most sharply over oceans in the past 20 years because of pollution-cutting rules imposed on shipping. And the new warning in Hansen’s paper comes at a time when the average ocean surface temperature has soared and stayed well above previous record levels.
That fact is not lost on Leon Simons, a co-author of the draft paper, who recently wrote on Twitter: “North Atlantic is on fire,” and went on to explain the ocean warming with a graph showing how the overheated area overlaps with key shipping lanes where aerosol emissions have declined.
The rapid drop of aerosols is increasing Earth’s energy imbalance so quickly that an acceleration of warming is inevitable, said Simons, a climate researcher and board member of the Club of Rome, a Switzerland-based nonprofit sustainability think tank known for publishing the The Limits to Growth report in 1972, as well as a 50-year followup report last year.
In effect, sulfate aerosol particles shielded the planet’s surface from some of the sun’s heat for decades, and cutting them is removing the shield, leading to a rapid warmup. Other research showing that sulfate aerosols have also masked global precipitation increases driven by greenhouse gases warming the atmosphere.
Like Hansen, Simons said he’s agreed to not directly discuss the paper’s findings. But he said the publicly posted draft “includes observational evidence for the increased rate of warming,” adding that it “seems rather obvious” that the net heating effect of greenhouse gases on the atmosphere “can result in 2 degrees Celsius of warming.
Not everyone agrees. Warming of 2 degrees Celsius (3.6 Fahrenheit) by 2050 is unlikely, said climate scientist Michael Mann, director of the Center for Science, Sustainability & the Media at the University of Pennsylvania.
Mann said he doesn’t think the findings in the draft paper will withstand peer review because the research doesn’t adequately account for the cooling effects of cutting other short-lived climate-warming pollutants, which can offset the heating resulting from the reduction of sulfate aerosols in the atmosphere. Black soot, which absorbs heat from the sun, can warm the atmosphere in the short term, and other extremely potent industrial pollutants emitted in tiny quantities have an outsized climate heating effect.
Calculations of those different effects are included in the most recent global climate assessments by the Intergovernmental Panel on Climate Change that serve as the basis for global climate policy talks, he said.
The IPCC reports show that the competing effects nearly cancel each other out, which would make 2 degrees Celsius warming by 2050 less likely. But those same IPCC projections include big cuts of methane emissions as another key to offsetting the spike in warming from the reduction of atmospheric sulfates. But methane emissions are not declining; they’ve accelerated sharply the past five years, data from the National Oceanic and Atmospheric Administration show.
Claiming that current scientific literature supports the idea that warming can be limited to “well below” 2 degrees Celsius is “egregious,” Hansen said, and shows “uncritical acceptance of models and the assumptions that went into them.”
Publicizing a paper at the pre-review stage is uncommon, and for good reasons, including concerns about unwarranted claims grabbing headlines and public attention, while subsequent corrections or changes often don’t get the same level of attention.
Hansen said the research team won’t answer questions directly about the study until it has been peer reviewed. “If I do an interview before it is accepted (and published), it seems to give the self-appointed ‘experts’ an excuse to blackball our paper,” he said.
But in a May 25 update on his Columbia University website, he responded to some of the initial reactions to the draft study by writing,“There’s no time to get involved in Twitter wars.”
He emphasized that he thinks the Intergovernmental Panel on Climate Change is downplaying some of global warming’s most imminent risks and he elaborated on the “blackball” comment by referring to a peer-reviewed and published 2016 paper that he said the Intergovernmental Panel on Climate Change ignored.
That paper found that the current rate of greenhouse gas emissions would lead crucial climate-regulating Atlantic Ocean currents to shut down, and sea level to rise several meters within 50 to 150 years.
“As yet, little has changed to get us off that path,” he wrote. “You would not know that from the communications of the United Nations COPs (Conferences of the Parties) and their scientific advisory body, the IPCC.” The IPCC’s modeling approach is useful, he said, but he warned that some of its projections seem to assume that “a miracle will occur,” so those models need to be checked against the real world.
“Our research is focused on real world data and comparison with models, with the hope of gaining insights about how the climate system works and where the real world is headed,” he wrote. The “miracle” that limits warming to less than 2 degrees Celsius in the most hopeful IPCC scenarios is based on an “assumption of negative emissions via power plants that burn biofuels, capturing and sequestering the CO2.”
Focusing attention on the paper before it’s reviewed is “mainly to start the scientific discussion and get input from the broader scientific community,” Simons added. “Such a broad paper benefits from this, as the reviewers might be more specialized. With Jim [Hansen], there will of course automatically be media attention, but that’s not the goal. People need to know about the acceleration of warming.”
If the average global temperature warms 2 degrees above pre-industrial times by 2050, it means that temperatures over land will likely increase double that amount, by 4 degrees Celsius, because land surfaces have less heat capacity than the oceans, where some of the heat goes deep down and isn’t immediately expressed as a rise of surface temperature.
This year’s IPCC 6th Assessment Report shows that level of warming rapidly increases the odds of massive, widespread droughts that could wipe out food production in key global crop areas at the same time, as well as severe water shortages and fierce heat waves that would displace millions of people. The combined physical and social impacts would destabilize some regions and possibly stir up conflicts over food and water supplies.
The draft discussion paper posted by Hansen also explores how much warming is locked into the system for thousands of years to come by current atmospheric greenhouse gas concentrations. Those long-term consequences are overlooked in climate policy discussions that don’t look beyond 2100, climate scientists Zeke Hausfather and Andrew Dessler wrote on May 22, in a discussion of the new paper.
“Considering that Jim Hansen’s predictions have often proven correct, it’s important that we pay close attention to what he’s saying,” Hausfather and Dessler wrote.
From 1981 to 2013, Hansen was director of the NASA Goddard Institute for Space Studies in New York City, a part of the Goddard Space Flight Center. He led efforts to methodically analyze temperature data from thousands of global observations, which showed the clearly emerging global warming fingerprint.
Even before retiring from NASA, Hansen started participating in climate demonstrations. He was arrested at an oil pipeline protest in 2011, and in front of the White House in 2013. In 2017 he called for a wave of climate lawsuits against governments and polluting corporations. Hansen’s granddaughter, Sophie Kivlehan, is one of 21 youth plaintiffs who has sued the U.S. government in Juliana v. the United States, alleging the government is infringing on their constitutional rights by not acting to stop global warming.
In the discussion draft of the current study, Hansen’s team suggests that most of the long-term “global warming for today’s atmosphere is still in the pipeline,” adding that current projections for ice sheet melting under today’s atmospheric conditions are “unrealistically lethargic.”
But Mann, also a leading expert in this field, said the draft paper doesn’t account for how much carbon dioxide the oceans will absorb in the decades ahead. Commenting on the same topic last year on Twitter, Mann said that Hansen has “ignored a decade of new science,” and that the incorrect claims about climate sensitivity “won’t survive peer review.” That’s why it’s a bad idea to publicize new research before it’s been submitted for expert verification, he added.
The draft paper’s new warnings are aligned with other recent studies about how the risks of some climate extremes are underestimated. The climate science canon these days often includes phrases like “faster than expected,” “sooner than expected” and “hotter than expected” when it comes to the decline of global ice, increasing temperatures, sea level rise and other impacts.
Even if the dire conclusion of 2 degrees Celsius of warming is affirmed by peer review, it’s not clear if one new research paper would have much impact on global climate policy, said Glen Peters, a senior climate researcher with the Center for International Climate Research (CICERO) in Oslo.
“Policy makers and decision makers do not generally respond to each new paper that is published, no matter how reputable the author,” he said. “They wait for the consensus view from the IPCC published every 5-10 years. Even so, the policy outcome would be much the same. A sharp increase in climate action is needed, beyond the level that politicians already find unpalatable.”
With existing scientific reports already highlighting the grave dangers of continued warming, it’s not likely that a new paper detailing an additional increment of warming would be game-changing, said Dana Fisher, a sociologist and climate activism researcher at the University of Maryland.
“Sadly, I think that no level of knowledge or scary predictions about warming or climate disruption are going to motivate a shift on climate action,” she said. “It will certainly increase the number of people in the streets throwing food and gluing themselves to things, though. And there is the caveat that, as we all experience more frequent and severe climate shocks, more people will mobilize and pressure their governments to act.”
So far, there’s been very little policy at the scale required to deal with the problem, Simons added.
“The increase in disasters will likely have more impact on urgency than almost any paper will,” he said. “ Honestly, I’m both surprised and humbled by how peaceful protesters stay, even when faced with violence themselves.”
And regardless of whether any new paper moves the policy needle, it’s important to clear up the uncertainties around whether there could be a dangerous upward temperature spike in the next two decades, he said.
“Humanity is the first species able to … measure and analyze the incoming and outgoing energy; how the balance is changing and what happens with the accumulating energy,” he said. “Understanding this is a crucial first step in order to act effectively. I believe that with a more thorough understanding of our planet, humanity could become a beneficial force to life on Earth, if we acquire the collective will to do so.”