But fewer people know that burning fossil fuels not only causes global warming, but also causes global cold. It is one of the great ironies of climate change that air pollution, which has killed tens of millions, has also held it back. some of the worst effects of global warming.
Small particles from burning coal, oil and gas can reflect sunlight and promote cloud formation, shading the planet from sunlight. 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 the pollution, that chilling effect is fading. New regulations have reduced the amount of sulfur aerosols from global ocean shipping traffic; China, fighting its own air pollution problem, has dramatically reduced sulfur pollution in the past decade.
The result is even warmer temperatures – but exactly how much warmer is still up for 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 head of research for payments company Stripe. “It could be a whole degree of coolness in disguise.”
Most of the cooling from air pollution comes through sulfur aerosols, in two ways. The particles themselves are reflective, bouncing off sunlight and shading the Earth. They also make existing clouds brighter and more mirror-like, thereby cooling the Earth.
Coal and oil are about 1 to 2 percent sulfur—and when people burn fossil fuels, that sulfur is released into the atmosphere. It’s 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 more than 70 percent since 2005 by installing new technologies and cleaners at fossil fuel plants. Recently, the International Maritime Organization set limits in 2020 on the amount of sulfur allowed in marine transport fuels – one of the dirtiest fuels used in transport. Sulfur dioxide emissions from transportation fell immediately by about 80 percent. Mediterranean countries are planning a similar transport regulation for 2025.
“There’s been a pretty big 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 moves have saved lives – an estimated 200,000 premature Deaths have already been averted in China and new transport regulations could save around 50,000 lives a year. But they have also grown global temperatures. Scientists estimate that changes in aerosols from the new transport rule alone could contribute between 0.05 and 0.2 degrees Celsius of warming over the coming decades.
Some researchers have suggested that changes in shipping regulations may have been a major contributor to last year’s record heat – and that aerosols may have masked much more heat than previously thought. Satellite images have shown that cloud changes decreased as sulfur emissions decreased.
“Data from NASA satellites show that in regions where this should be expected, there is 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. showing transport zones. affected by the new rules. “And also in this period you see sea surface temperatures rising in the same region.”
In a new paper, scientists at the University of Maryland argued that reducing aerosols could double the rate of warming in the 2020s, compared to the rate since 1980. But other researchers have 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 transportation regulation as an analog to a way researchers are exploring to stop global warming: purposefully brightening clouds using less polluting methods. In Alameda, California, researchers recently released sea salt aerosols into the atmosphere as a first step in studying how the particles can light up clouds and reflect sunlight. City officials later halted the project, despite reports that the experiment was safe.
But the real issue is yet to come. Currently, the UN Intergovernmental Panel on Climate Change estimates that aerosols are masking about 0.5 degrees Celsius of global warming. But that value could be as little as 1 degree or as much as 0.2 degrees – and the difference could be the difference between meeting the 2015 targets Paris Agreement or not?
If aerosols have masked much more cooling than expected, for example, the world may be about to exceed its climate targets without realizing it.
Almost 200 of the world’s countries 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 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. “If you clear quickly, or if you just walk along with the same aerosol emissions, it could be the difference whether you pass the 2 degree Celsius threshold or not.”
No scientist is advocating stopping 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 researchers worry that cleaning up air pollution without ending the use of fossil fuels — such as in China — could be a recipe for even greater and faster warming. “We have to make sure we’re doing it at the same time as methane cleanup and CO2 cleanup,” Diamond said. Reducing methane emissions, he noted, could help offset the effects of falling aerosols. Methane has a warming effect, but like aerosols, it does not stay in the atmosphere for long.
However, many scientific questions remain – and until they are answered, the world won’t know exactly how much warming falling aerosols will reveal.
Harry Stevens contributed to this report.