It is widely accepted that humans have been warming the planet by burning coal, oil, and natural gas for more than a century. The Earth has warmed almost 1.2 degrees Celsius (2.2 degrees Fahrenheit) since pre-industrial times, and the planet is poised to quickly surpass the hoped-for 1.5 degree Celsius warming limit.
But few people know that burning fossil fuels not only causes global warming, but also causes global warming cooling. It is one of the great ironies of climate change that the air pollution that kills tens of millions of people has also curbed some of the worst effects of a warming planet.
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Tiny particles from the burning of coal, oil and gas can reflect sunlight and encourage clouds to form, 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, as society cleans up pollution, this cooling effect is diminishing. New regulations reduced the amount of sulfur aerosols in global shipping traffic across oceans; China, tackling its own air pollution problem, has significantly reduced sulfur pollution in the last decade.
As a result, temperatures are even higher; but exactly how hot it is is still debated. The answer will have lasting impacts on humanity’s ability to meet its climate goals.
“We’re starting from a place of deep, deep uncertainty,” said Zeke Hausfather, a climate scientist and research leader at payments company Stripe. “Masking can be a total cooling degree.”
Most of the cooling from air pollution is achieved in two ways through sulfur aerosols. The particles themselves are reflective, reflecting the sun’s rays back and shading the Earth. They also cool the Earth by making existing clouds brighter and mirror-like.
Coal and oil contain about 1% to 2% sulfur, and when humans burn fossil fuels, this 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, with the United States and the European Union leading the way, followed by China and India. China has reduced sulfur dioxide emissions by over 70% since 2005 by installing new technologies and scrubbers at fossil fuel plants. More recently, the International Maritime Organization introduced restrictions on the amount of sulfur allowed in shipping fuels in 2020 – It is one of the dirtiest fuels used in transportation. Shipping emissions of sulphur dioxide fell by around 80 percent immediately. Mediterranean countries are planning a similar maritime regulation for 2025.
“It’s been a pretty steep decline in 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 saved lives; estimates have already prevented nearly 200,000 premature deaths in China, and new shipping regulations could save around 50,000 lives a year. But they also increased global temperatures. Scientists estimate that aerosol changes resulting from the new shipping rule could contribute between 0.05 and 0.2 degrees Celsius of warming over the next few decades.
Some researchers have suggested that changes to ocean shipping regulations contributed greatly to last year’s record temperatures, and that aerosols may be masking much more heat than previously thought. Satellite imagery has shown that cloud changes slowed after sulfur emissions dropped.
“Data from NASA satellites show a very strong increase in absorbed solar radiation in regions where this should be expected,” said Leon Simons, an independent researcher and member of the Dutch Club of Rome, pointing to the shipping zones. affected by the new rules. “You also see sea surface temperatures increasing in the same region during this period.”
Scientists at the University of Maryland suggest in a new paper that the reduction in aerosols could double the rate of warming in the 2020s compared to the rate seen since 1980. But other researchers criticized the results.
Many experts believe the impact will likely be modest, between 0.05 and 0.1 degrees Celsius. “Given how uncertain we are, I don’t think it’s possible to get to better than twice as much,” said Michael Diamond, a professor of meteorology and environmental science at Florida State University.
Some scientists liken the shipping regulation to a method researchers are exploring to stop global warming: deliberately brightening clouds using less polluting methods. In Alameda, California, researchers recently released sea salt aerosols into the atmosphere to study how the particles brighten clouds and reflect sunlight. City officials later halted the project despite reports that the experiment was safe.
But the real issue is still before us. Currently, the UN Intergovernmental Panel on Climate Change estimates that aerosols mask about 0.5 degrees Celsius of global warming. But this value could be as high as 1 degree or as low as 0.2 degrees, and the difference could be the difference between whether the 2015 Paris Agreement achieves its goals or not.
For example, if aerosols mask cooling much more than expected, the world may be poised to unknowingly exceed its climate targets.
Almost 200 countries around the world pledged in the Paris agreement to keep warming below 2 degrees Celsius (3.6 degrees Fahrenheit) from pre-industrial levels. Scientists believe that many dangerous effects, from the collapse of coral reefs to the melting of large ice sheets, will occur between 1.5 and 2 degrees Celsius.
“This isn’t just a story of greenhouse gas emissions,” said Robert Wood, a professor of atmospheric science at the University of Washington. “Whether you clean up quickly or clumsily clean up the same aerosol emissions can make a difference in whether you exceed the 2-degree Celsius threshold.”
No scientist is advocating halting aerosol cleanup efforts; Death rates due to air pollution are very high. “There are really good reasons for wanting to clean up air pollution,” Diamond said. “The public health benefits are really significant.”
But researchers worry that cleaning up air pollution without stopping fossil fuel use, as is happening in China, could lead to greater and faster warming. “We need to make sure we do this while removing methane and CO2 at the same time,” Diamond said. Reducing methane emissions could help offset the effects of reduced aerosols, he noted. Methane has a warming effect but does not remain in the atmosphere for very long like aerosols.
Still, many scientific questions remain, and until they are answered, the world won’t know exactly how much warming falling aerosols will cause.
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Harry Stevens contributed to this report.
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