New Study Warns of an Imminent Spike of Planetary Warming and Deepens Divides Among Climate Scientists

James Hansen, the scientist who first sounded the climate alarm in Congress, sees a decrease in aerosol pollution driving a surge of warming and criticizes the U.N. climate science panel, drawing a backlash from other researchers.

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Climate scientist and activist James Hansen attends a press conference at the COP 23 United Nations Climate Change Conference on November 6, 2017 in Bonn, Germany. Credit: Sean Gallup/Getty Images
Climate scientist and activist James Hansen attends a press conference at the COP 23 United Nations Climate Change Conference on November 6, 2017 in Bonn, Germany. Credit: Sean Gallup/Getty Images

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During the past year, the needles on the climate dashboard for global ice melt, heatwaves, ocean temperatures, coral die-offs, floods and droughts all tilted far into the red warning zone. In summer and fall, monthly global temperature anomalies spiked beyond most projections, helping to drive those extremes, and they may not level off anytime soon, said James Hansen, lead author of a study published today in the journal Oxford Open Climate Change that projects a big jump in the rate of warming in the next few decades.

But the research was controversial even before it was published, and it may widen the rifts in the climate science community and in the broader public conversation about the severity and imminence of climate impacts, with Hansen criticizing the United Nations Intergovernmental Panel on Climate Change for underestimating future warming, while other researchers, including IPCC authors, lambasted the new study. 

The research suggests that an ongoing reduction of sulfuric air pollution particles called aerosols could send the global average annual temperature soaring beyond the targets of the Paris climate agreement much sooner than expected, which would sharply increase the challenges faced by countries working to limit harmful climate change under international agreements on an already treacherous geopolitical stage.


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Differences about climate science projections is not the main problem, said Jeffery Sachs, director of the Columbia University Earth Institute, who moderated a panel presentation by the authors of the new study.

“We’re in a grim situation,” he said. “And it’s even grimmer that the politicians have failed their responsibility to the world now for quite a long time. We have a massive political failure. Our politicians like wars. They don’t want to save the planet, in the right way.”

Hansen and the international co-authors also reanalyzed paleoclimate records going back several thousand years and found that the planet’s most important ocean heat transport currents could slow or shut down this century because they are more sensitive to increasing freshwater from melting ice than shown by widely used climate models, including those used as the basis for scientific projections by the Intergovernmental Panel on Climate Change, which has also been criticized by other scientists, including some who are authors with the IPCC, for downplaying climate risks. 

The findings suggest that the same widely used models and projections also downplay how fast vast global ice sheets could melt and speed sea level rise to a rate that would be difficult to adapt to, the authors of the new paper said.

Combining the the paleoclimate data with modeling and detailed observations from the last few decades, the team concluded that the world is in for a wild ride of climate impacts, including possible superstorms that could toss house-sized boulders to the top of seaside cliffs, radical changes to global rainfall patterns that would affect agriculture in densely populated regions and possibly several meters of sea level rise by 2100, as compared to the IPCC-projected range of .29 to 1.1 meters.

“Look at what we are seeing the last few months at the current level of warming,” said co-author Leon Simons, a researcher with the Club of Rome, in the Netherlands. “We see the impacts happening now. The forest fires in Canada are a very concrete example, emitting almost 2 billion tons of CO2 and bringing smoke to Europe. That’s just one example. There will be much more of that in the next few years.”

A Surge of Alarming Studies and Public Actions From Researchers

In the last few years, Hansen, Simons and several other research groups have tried to raise awareness about the potential for sudden and unexpected climate shocks in the near future that would affect most people alive right now. For example, studies show the growing risk of multiple simultaneous crop failures in different parts of the world that would seriously threaten global food security.

On Sept. 19, the Stockholm Resilience Center published research showing that six of nine climate-related planetary boundaries have been breached, which “risks the stability of the entire planet,” according to the authors, including Johan Rockström, director of the Potsdam Institute for Climate Impact Research, and Konrad “Koni” Steffen, who researched and warned of the dangers of a Greenland Ice Sheet meltdown until his untimely death in a crevasse there in 2020.

Last week, the Alliance of World Scientists said “moral urgency” compelled them to again warn of a global climate emergency, with the currently projected 3 degrees Celsius of warming by 2100 making global societal collapse a “plausible and dangerously underexplored” possibility, a scenario the researchers wrote about in a December 2022 study

The rise of such warnings coincides with widespread criticism that the IPCC’s scientific process is too slow to help society make decisions to deal with the rapidly changing climate, and that the panel’s key findings are watered down because politically appointed science officials and administrators have the final say over what is included in the panel’s key summary reports that are meant to inform public policy. That vicious cycle of slow and overly restrained science feeds public complacency and justifies government inaction, according to Hansen.

Hansen’s early testimony to the U.S. Congress in 1988 was politically groundbreaking, presenting decision-makers with compelling scientific information that he hoped would prompt action. When, after decades, it had not, he followed up by joining protests against the Keystone Oil Pipeline and getting arrested outside the White House in 2011, and again in 2018 in West Virginia while protesting mountaintop removal for coal mining, all while he was still head of NASA’s Goddard Institute for Space Studies. 

In 2009 he published Storms of My Grandchildren as a climate prophecy about what kind of world is still possible for children born in the 21st century, and is currently finishing a book called Sophie’s Planet, written as a series of letters to his granddaughter with a similar theme—how the choices we make right now will affect future generations.

Hansen retired from NASA and now heads the Climate Science, Awareness and Solutions think tank at Columbia University’s Earth Institute. He remains active in public policy advocacy and has also lent his climate science expertise in several legal cases due to his belief—shared by Abramoff, Kalmus and thousands of other scientists—that knowledge confers a moral imperative to act.

Other researchers have also pressed for more urgency from scientists and scientific organizations recently. Rose Abramoff and Peter Kalmus interrupted a talk at the American Geophysical Union annual conference in Chicago last December, unfurling a banner urging scientists to “GET OUT OF THE LAB AND INTO THE STREETS” and and criticizing the paths outlined by major climate institutions as too slow to avert catastrophe. Abramoff was fired from her job at Oak Ridge National Laboratory for her actions at the conference.

Are Aerosols Driving The Current Surge of Warming?

For several years, Hansen and Simons have proposed that the recent and ongoing surge in a wide range of global climate indicators—not just average global temperatures—may be driven in large part by a sharp reduction in tiny sulfuric particles produced by burning shipping fuels and other fossil fuels, and by other industrial processes. 

Those aerosol particles, spewed into the atmosphere in massive quantities since the start of the industrial revolution, often make clouds brighter and more persistent, so they reflect more of the sun’s incoming heat energy back to space. While carbon dioxide and other greenhouse gases have been warming the planet since at least 1850, other byproducts of burning fossil fuels were cooling the planet at the same time, although the effects of aerosols only last for a fraction of the time that the heat-trapping gases persist in the atmosphere.

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That basic idea is not controversial, but Hansen’s new research about the relative strengths of those competing effects diverges from many other studies by suggesting the cooling effect has been underestimated so that as sulfur aerosols and their effects on clouds are reduced, temperature will increase more than expected. 

How clouds will change in the decades ahead, and their interaction with aerosols, remains the single greatest uncertainty in making accurate projections for future temperature increases, according to most climate scientists. Several key satellite instruments that could have helped answer that question never made it into orbit in the 1980s and 1990s, despite repeated requests, Hansen said.

Gavin Schmidt, who replaced Hansen as head of NASA’s global Institute for Space Studies, said those missions to study aerosol composition and change should have been higher priorities. 

“Jim and others did have to fight hard to get an aerosol polarimeter on the GLORY mission, which would have gone a long way to helping,” Schmidt said. “Unfortunately that did not reach orbit due to a faulty faring on the rocket, which turned out to be due to fraud in the supply chain.”

Schmidt said a new mission called PACE (short for Plankton, Aerosol, Cloud, ocean Ecosystem) could quickly reduce the uncertainties surrounding the effects of aerosols on the climate. That satellite should be in orbit within the coming year. Copernicus, the European Union climate change service, is also launching a new satellite, EarthCARE, also with the goal of measuring the relationship of aerosols, clouds and precipitation to how much of the sun’s radiation reaches Earth to drive global heating.

Absent those data, the new study used a process of elimination to again show reductions in sulfur aerosols were triggering accelerated warming. Comparisons with past climate periods hold some of the clues, showing, for example, that reefs along the Yucatán Peninsula grew upward and shoreward in giant spurts over the course of just a few decades, about 100,000 years ago during the late Eemian geological era. That, Hansen said, is another warning sign that parts of Earth’s climate system, and particularly ice sheets and ice shelves, are more sensitive to warming than we think.

“The IPCC system doesn’t acknowledge the degree to which the aerosol forcing will affect the climate in the next few decades, probably more than anything else,” Simons said. “We hope we’re wrong.” 

Deep Divides Over Climate Communication

The paper drew criticism when it was made public as a draft, shining a light on a long-running rift in the climate science community about how to communicate relative levels of certainty about global warming and its impacts. 

The sharp backlash to the final version published today, as well as Hansen’s direct critique of the IPCC, suggest that the divide is only growing deeper. In a commentary Hansen published describing the new paper, he wrote that reviewers of some of his previous papers have been reticent to endorse full-throated warnings about global warming by choosing, for example, to eliminate words like “dangerous” in a description of the documented climate impacts of 2 degrees Celsius of warming. 

But several well-known climate scientists said Hansen is confusing reticence with insistence on accuracy and robustness.

“His commentary is terrible,” said Piers Forster, director of the Priestley Centre for Climate Futures at the University of Leeds, and lead author of the section of the most recent IPCC report that covered climate feedbacks and climate sensitivity. 

“This is simply a very bad hotchpotch of a paper made out to be a new assessment of climate sensitivity and critique of IPCC,” Forster said. “It is nothing of the sort.” Instead, Forster said the paper is a combination of historical perspectives, geographic modeling and Hansen’s opinions.

The high climate sensitivity estimates cited in Hansen’s paper “all seem quite subjective and not justified by observations, model studies or literature,” he said. “They portray their paper as an alternative but it doesn’t have rigor, systematic analysis or comprehensive literature review. It also does a great disservice to the hundreds of scientists from around the world that give many years of their lives to the IPCC process.”

Kevin Trenberth, a distinguished professor emeritus with the National Center for Atmospheric Research, said he was surprised the new paper was published, noting that the journal is relatively obscure.

“I think it is likely wrong,” Trenberth said. “Hansen has never been involved in the IPCC in any capacity, even as a reviewer. He is not at all collegial and he tends to ignore legitimate criticism.   He moves papers around until they get published. He stuffs papers full of extraneous stuff that would not be publishable by itself.”

Trenberth said the new paper relies on an estimate of paleoclimate sensitivity that was proven wrong by a 2021 study that modeled ancient warm and cold periods, providing a way to accurately assess the climate impact of industrial age greenhouse gas emissions. He said Hansen’s team also seems to have ignored increasing water vapor caused by global warming as a factor in the recent warming surge.

“Water vapor is a major feedback,” he said. “As temperatures go up so does the water-holding capacity of the atmosphere,” which amplifies global warming.

Hansen and Simons are undeterred by the controversy swirling around their research and remain convinced they have a duty to warn and to make policy recommendations, some of which are controversial. Hansen advocates for speeding up decarbonization by instituting a global price on carbon, using more nuclear energy and accelerating the deployment of renewable energy. And he said scientists need to study various forms of geoengineering, including the deliberate dispersal of chemicals in the atmosphere to reflect a small percentage of incoming solar energy, or the brightening of large areas of ocean clouds by spraying salt mists into the sky with a fleet of robot boats.

Those ideas are still highly controversial in the science community, but Hansen said that, because the current generation has painted itself into a tight climate corner, we owe it to future generations to give them as many informed choices as possible, and studying geoengineering is part of that.

“We have to recognize we’re geoengineering the planet right now,” Hansen said. “This is what we’re doing by putting these huge amounts of greenhouse gas into the atmosphere,” which is warming the planet “at a greater rate than has ever occurred in the Earth’s history, as far as we know.” 

That warming has to be slowed, and that will probably require reflecting sunlight because of how hard it is to remove greenhouse gases from the atmosphere once they are there, he added.

Simons said there is a need for specialized science teams to determine if rapid sulfur aerosol reductions are indeed causing the current global temperature spike and how the sudden drop of pollution and the temperature increase interact to affect monsoon rainfall, as well as ocean currents and large-scale atmospheric patterns.“There are no easy choices in this,” Simons recently posted on the social media site X, formerly known as Twitter. “How much warming will the world accept? And how fast can the rate of warming be until we are unable to adapt? And will we have the stability of global governance to face these accumulating challenges?”

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