But fewer people know that burning fossil fuels doesn’t just cause global warming — it also causes global cooling. It is one of the great ironies of climate change that air pollution, which has killed tens of millions, has also curbed some of the worst effects of a warming planet.
Tiny particles from the combustion of coal, oil and gas can reflect sunlight and spur the formation of clouds, shading the planet from the sun’s rays. Since the 1980s, those particles have offset between 40 and 80 percent of the warming caused by greenhouse gases.
And now, as society cleans up pollution, that cooling effect is waning. New regulations have cut the amount of sulfur aerosols from global shipping traffic across the oceans; China, fighting its own air pollution problem, has slashed sulfur pollution dramatically in the last decade.
The result is even warmer temperatures — but exactly how much warmer is still under debate. The answer will have lasting impacts on humanity’s ability to meet its climate goals.
“We’re starting from an area of deep, deep uncertainty,” said Zeke Hausfather, a climate scientist and research lead for the payments company Stripe. “It could be a full degree of cooling being masked.”
Most of the cooling from air pollution comes through sulfur aerosols, in two ways. The particles themselves are reflective, bouncing the sun’s rays away and shading the Earth. They also make existing clouds brighter and more mirror-like, thus cooling the Earth.
Coal and oil are around 1 to 2 percent sulfur — and when humans burn fossil fuels, that sulfur spills into the atmosphere. It is deadly: Sulfur dioxide has been linked to respiratory problems and other chronic diseases, and air pollution contributes to about 1 in 10 deaths worldwide.
Over the past few decades, countries have worked to phase out these pollutants, starting with the United States and the European Union, followed by China and India. China has cut its sulfur dioxide emissions by over 70 percent since 2005 by installing new technologies and scrubbers on fossil fuel plants. More recently, the International Maritime Organization instituted restrictions in 2020 on the amount of sulfur allowed in shipping fuels — one of the dirtiest fuels used in transportation. Shipping emissions of sulfur dioxide immediately dropped by about 80 percent. Mediterranean countries are planning a similar shipping regulation for 2025.
“There has been a pretty steep decline over the last 10 years,” said Duncan Watson-Parris, an assistant professor at the Scripps Institution of Oceanography at the University of California at San Diego.
These moves have saved lives — according to estimates, around 200,000 premature deaths have already been avoided in China, and the new shipping regulations could save around 50,000 lives per year. But they have also boosted global temperatures. Scientists estimate that the changes in aerosols from the new shipping rule alone could contribute between 0.05 and 0.2 degrees Celsius of warming over the next few decades.
Some researchers have suggested that the changes to ocean shipping regulations may have been a big contributor to last year’s record heat — and that aerosols may have been masking much more heat than previously thought. Satellite images have shown that cloud changes declined after sulfur emissions went down.
“The data from NASA satellites shows that in regions where this should be expected, there’s a very strong increase in absorbed solar radiation,” said Leon Simons, an independent researcher and a member of the Club of Rome of the Netherlands, pointing to shipping areas affected by the new rules. “And also in this period you see sea surface temperatures increasing in the same region.”
In one new paper, scientists at the University of Maryland argued that the decrease in aerosols could double the rate of warming in the 2020s, compared to the rate since 1980. But other researchers have critiqued their results.
Many experts believe the effect is likely to be modest — between 0.05 and 0.1 degrees Celsius. “I don’t think it’s possible to get better than a factor of two, in terms of how uncertain we are,” said Michael Diamond, a professor of meteorology and environmental science at Florida State University.
Some scientists see the shipping regulation as an analog to a way that researchers are exploring to halt global warming: purposefully brightening clouds using less polluting methods. In Alameda, Calif., researchers recently released sea salt aerosols into the atmosphere as a first step to study how the particles could brighten clouds and reflect sunlight. City officials later halted the project, despite reports showing that the experiment was safe.
But the real issue is still ahead. Currently, the U.N. Intergovernmental Panel on Climate Change estimates that aerosols are masking about 0.5 degrees Celsius of global warming. But that value could be as high as 1 degree or as low as 0.2 degrees — and the difference could be the difference between meeting the goals of the 2015 Paris agreement or not.
If aerosols have been masking cooling much more than expected, for example, the world could be poised to blow past its climate targets without realizing it.
Almost 200 of the world’s nations pledged in the Paris agreement to keep warming below 2 degrees Celsius (3.6 degrees Fahrenheit), compared to preindustrial levels. Scientists believe that many dangerous impacts, from the collapse of coral reefs to the melting of major ice sheets, will occur somewhere between 1.5 and 2 degrees Celsius.
“It’s not just a story of greenhouse gas emissions,” said Robert Wood, a professor of atmospheric science at the University of Washington. “Whether you clean up rapidly, or whether you just fumble along with the same aerosol emissions, could be the difference of whether you cross the 2-degree Celsius threshold or not.”
No scientists are advocating a halt to aerosol clean up efforts — the death tolls from air pollution are simply too high. “There are really good reasons to want to be cleaning up air pollution,” Diamond said. “The public health benefits are really important.”
But researchers worry that cleaning up air pollution without halting fossil fuel use — as, for example, in China — could be a recipe for even greater and faster warming. “We need to make sure that we’re doing it at the same time as cleaning up methane and cleaning up CO2,” Diamond said. Cutting methane emissions, he noted, could help offset the effects of declining aerosols. Methane has a warming effect, but like aerosols, doesn’t remain in the atmosphere for very long.
Still, a lot of scientific questions remain — and until they are answered, the world won’t know exactly how much warming falling aerosols will unmask.
Harry Stevens contributed to this report.
