We’ve accidentally cooled the planet – and that’s about to stop

It is widely accepted that humans have been warming the planet for more than a century by burning coal, oil and gas. The Earth has already warmed by almost 1.2 degrees Celsius (2.2 degrees Fahrenheit) since pre-industrial times, and the planet is about to race past the hoped-for 1.5 degrees Celsius mark of warming.

But fewer people know that burning fossil fuels not only causes global warming, but also global warming cooling. It is one of the great ironies of climate change that air pollution, which has killed tens of millions of people, has also managed to limit some of the worst effects of a warming planet.

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Small particles from the combustion of coal, oil and gas can reflect sunlight and promote the formation of clouds, shielding the planet from the sun’s rays. Since the 1980s, these particles have offset between 40 and 80 percent of the warming caused by greenhouse gases.

And now that society is cleaning up the pollution, that cooling effect is diminishing. New regulations have reduced the amount of sulfur aerosols in global ocean shipping; China, fighting its own air pollution problem, has dramatically reduced sulfur pollution over the past decade.

The result is even warmer temperatures – but exactly how much warmer is still a matter of debate. The answer will have lasting consequences for 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. “It may be that a full degree of cooling is masked.”

Most of the cooling from air pollution comes via sulfur aerosols in two ways. The particles themselves are reflective, which bounce back the sun’s rays and cast the Earth in shadow. They also make existing clouds brighter and more mirror-like, cooling the Earth.

Coal and oil contain about 1 to 2 percent sulfur — and when people burn fossil fuels, that sulfur is released 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 been working 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 scrubbers at fossil fuel plants. More recently, the International Maritime Organization introduced restrictions on the amount of sulfur allowed in marine fuels in 2020 one of the dirtiest fuels used in transportation. Sulfur dioxide emissions from shipping immediately fell by about 80 percent. Mediterranean countries are planning similar shipping regulations by 2025.

“There has been a pretty steep decline over the last decade,” 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. According to estimates, around 200,000 premature deaths have already been avoided in China, and the new shipping rules could save around 50,000 lives a year. But they have also increased global temperatures. Scientists estimate that the changes in aerosols alone due to the new shipping rule could contribute between 0.05 and 0.2 degrees Celsius of warming in the coming decades.

Some researchers have suggested that changes in maritime 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 after sulfur emissions dropped.

“The data from NASA satellites shows that in regions where this is expected, there is a very strong increase in the absorbed solar radiation,” says Leon Simons, an independent researcher and member of the Club of Rome, pointing to shipping areas affected by the new rules. “And also during this period, you see the temperature of the seawater in the same region rising.”

In a new paper, scientists from the University of Maryland argued that declining aerosols could double the rate of warming in the 2020s, compared with 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 sciences at Florida State University.

Some scientists see shipping regulation as an analogy to a way researchers are exploring to halt global warming: purposefully making clouds brighter using less-polluting methods. In Alameda, California, researchers recently released sea salt aerosols into the atmosphere as a first step to study how the particles can brighten clouds and reflect sunlight. City officials later halted the project, despite reports indicating the experiment was safe.

But the real problem still lies ahead. Currently, the UN Intergovernmental Panel on Climate Change estimates that aerosols mask 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.

For example, if aerosols mask cooling much more than expected, the world could easily exceed climate targets without us realizing it.

Nearly 200 countries around the world 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 consequences, from the collapse of coral reefs to the melting of major ice caps, 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 sciences at the University of Washington. “Whether you clean up quickly or just deal with the same aerosol emissions can make the difference whether you exceed the 2 degrees Celsius threshold or not.”

No scientist is advocating an end 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 very important.”

But researchers worry that cleaning up air pollution without halting fossil fuel use – as in China, for example – could be a recipe for even greater and faster warming. “We need to make sure we do this at the same time as cleaning up methane and cleaning up CO2,” Diamond said. Reducing methane emissions, he noted, could help offset the effects of declining aerosols. Methane has a warming effect, but, like aerosols, does not remain in the atmosphere for long.

Still, many scientific questions remain — and until they’re answered, the world won’t know exactly how many warming falling aerosols will unmask.

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Harry Stevens contributed to this report.

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