However, fewer people know that the burning of fossil fuels not only causes global warming, but also global warming cooling. It is one of the great ironies of climate change that the air pollution that has killed tens of millions of people has also reduced some of the worst effects of global warming.
Tiny particles from burning coal, oil and gas can reflect sunlight and encourage cloud formation, shading the planet from the sun’s rays. Since the 1980s, these particles have offset 40 to 80 percent of the warming caused by greenhouse gases.
And now that society is cleaning up the pollution, that cooling effect is weakening. New regulations have reduced sulfur aerosols from global ocean shipping; Battling its own air pollution problem, China has dramatically reduced sulfur pollution over the past decade.
The result is even hotter temperatures – but exactly how much hotter is still up for debate. The answer will have lasting effects on humanity’s ability to meet its climate goals.
“We’re coming out of an area of ​​deep, deep uncertainty,” said Zeke Hausfather, a climate scientist and head of research for payments company Stripe. “It could be masking the full degree of cooling.
Most of the cooling from air pollution comes via sulfur aerosols, in two ways. The particles themselves are reflective, reflecting the sun’s rays and shading the Earth. They also make existing clouds brighter and more specular, cooling the Earth.
Coal and oil contain about 1 to 2 percent sulfur—and when people burn fossil fuels, that sulfur escapes into the atmosphere. It’s deadly: Sulfur dioxide is 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 more than 70 percent since 2005 by installing new technologies and gas scrubbers at fossil fuel power plants. Recently, the International Maritime Organization introduced a limit on the amount of sulfur allowed in marine fuels in 2020 — one of the dirtiest fuels used in transport. Ship emissions of sulfur dioxide immediately fell by about 80 percent. Mediterranean countries are planning similar regulation of shipping for 2025.
“There’s 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, San Diego.
These steps have saved lives – an estimated 200,000 premature China has already avoided deaths and the new transport regulations could save around 50,000 lives a year. But they also strengthened global temperatures. Scientists estimate that changes in aerosols from the new transport rule alone could contribute 0.05 to 0.2 degrees Celsius of warming over the next few decades.
Some researchers suggested that changes to 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 showed that changes in cloud cover decreased after sulfur emissions fell.
“Data from NASA satellites shows that there is a very strong increase in absorbed solar radiation in areas where this would be expected,” said Leon Simons, an independent researcher and member of the Dutch Club of Rome, pointing to the transport areas. affected by the new rules. “And also during this period, you see sea surface temperatures rising in the same area.”
In one new paper, researchers at the University of Maryland argued that a decline in aerosols could double the rate of warming in 2020 compared to the rate since 1980. But other researchers criticized 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 analogy for a way researchers are exploring to stop global warming: targeted clearing of clouds using less polluting methods. In Alameda, California, researchers recently released aerosols of sea salt into the atmosphere as a first step in studying how the particles could lighten clouds and reflect sunlight. City officials later halted the project, despite reports that the experiment was safe.
But the real problem is still ahead. Currently, the United Nations Intergovernmental Panel on Climate Change estimates that aerosols mask about 0.5 degrees Celsius of global warming. But that value could be as low as 1 degree or as low as 0.2 degrees – and the difference could be the difference between meeting the 2015 targets Paris Agreement or not.
For example, if aerosols are masking cooling much more than expected, the world could be poised to exceed its climate goals without realizing it.
Nearly 200 countries around the world have pledged in the Paris Agreement to keep warming below 2 degrees Celsius (3.6 degrees Fahrenheit) compared to pre-industrial levels. Scientists believe that many dangerous impacts, from the collapse of coral reefs to the melting of large ice sheets, will occur somewhere between 1.5 and 2 degrees Celsius.
“It’s not just a story about greenhouse gas emissions,” said Robert Wood, a professor of atmospheric science at the University of Washington. “Whether you clean up quickly or just fumble with the same aerosol emissions can make the difference between crossing the 2 degree Celsius threshold or not.”
No scientists are advocating a halt to aerosol cleanup efforts—the death toll from air pollution is simply too high. “There are really good reasons to want to clean up air pollution,” Diamond said. “The public health benefits are really important.”
But scientists fear that eliminating air pollution without ending the use of fossil fuels — as in China, for example — could be a recipe for even greater and faster warming. “We have to make sure we’re doing this at the same time as methane cleanup and CO2 cleanup,” Diamond said. Reducing methane emissions, he noted, could help offset the effects of declining aerosols. Methane has a warming effect, but like aerosols, it doesn’t stay in the atmosphere very long.
But many scientific questions still remain—and until they’re answered, the world won’t know exactly how much warming falling aerosols will reveal.
Harry Stevens contributed to this report.