The scariest thing about climate change? Global cooling.

But fewer 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 air pollution, which has killed tens of millions of people, has also curbed some of the worst effects of global warming.

Tiny particles from burning coal, oil and gas can reflect sunlight and cause clouds to form, 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, as society cleans up the pollution, this cooling effect is diminishing. New regulations have reduced the amount of sulfur aerosols from global shipping traffic across the oceans; China, battling its own air pollution problem, has significantly reduced sulfur pollution over the past decade.

The result is even warmer temperatures – but exactly how much hotter they are is still up for debate. The answer will have lasting impacts on humanity’s ability to achieve its climate goals.

“We’re starting from a very deep area of ​​uncertainty,” said Zeke Hausfather, a climate scientist and head of research for payments company Stripe. “It could be that a full degree of cooling is being masked.”

Most of the cooling from air pollution comes from 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 mirror-like, thereby cooling the Earth.

Coal and oil contain about 1 to 2 percent sulfur – and when humans burn fossil fuels, that sulfur leaks into the atmosphere. It’s deadly: Sulfur dioxide has been linked to respiratory problems and other chronic illnesses, and air pollution contributes to about 1 in 10 deaths worldwide.

In recent 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 reduced its sulfur dioxide emissions by more than 70% since 2005 by installing new technology and scrubbers at fossil fuel power plants. More recently, the International Maritime Organization instituted restrictions in 2020 on the amount of sulfur allowed in transportation fuels. — one of the dirtiest fuels used in transportation. Sulfur dioxide emissions from shipping immediately fell by around 80 percent. Mediterranean countries plan similar maritime transport regulations 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 measures have saved lives – an estimated 200,000 premature babies deaths have already been prevented in China and new shipping regulations could save around 50,000 lives per year. But they also stimulated global temperatures. Scientists estimate that changes in aerosols from the new shipping rule alone could contribute 0.05 to 0.2 degrees Celsius to warming over the coming decades.

Some researchers have suggested that changes to shipping regulations may have contributed significantly to last year’s record heat – and that aerosols may have masked much more heat than previously thought. Satellite images showed that changes in clouds diminished after sulfur emissions decreased.

“Data from NASA satellites show that in regions where this is expected, there is a very large increase in absorbed solar radiation,” said Leon Simons, an independent researcher and member of the Club of Rome from the Netherlands, in designating navigation zones. affected by the new rules. “And also during this period, you see sea surface temperatures increasing in the same region.”

In a new paper, scientists at the University of Maryland say that decreasing aerosols could double the rate of warming in the 2020s, compared to the rate observed since 1980. But other researchers have criticized their results.

Many experts estimate the effect will likely be modest, between 0.05 and 0.1 degrees Celsius. “I don’t think you can get better than a factor of two, in terms of uncertainty,” said Michael Diamond, a professor of meteorology and environmental sciences at Florida State University.

Some scientists see this regulation as an analogy to a method researchers are exploring to stop global warming: deliberately thinning clouds 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 might brighten clouds and reflect sunlight. City officials later halted the project, despite reports showing the experiment was safe.

But the real problem is yet to come. 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 high 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.

If aerosols mask cooling much more than expected, for example, the world could be on the verge of exceeding its climate goals without realizing it.

Nearly 200 countries around the world committed in the Paris agreement to keeping global warming below 2 degrees Celsius (3.6 degrees Fahrenheit), compared to pre-industrial levels. Scientists estimate that many dangerous impacts, from the collapse of coral reefs to the melting of major ice sheets, will occur between 1.5 and 2 degrees Celsius.

“It’s not just about greenhouse gas emissions,” said Robert Wood, a professor of atmospheric sciences at the University of Washington. “Whether you clean up quickly, or just fumble with the same aerosol emissions, could make the difference in whether or not you cross the 2 degree Celsius threshold.”

No scientist is advocating stopping aerosol cleanup efforts: the number of deaths from air pollution is simply too high. “There are very good reasons to want to clean up air pollution,” Diamond said. “The public health benefits are really important. »

But researchers fear that cleaning up air pollution without stopping the use of fossil fuels – such as in China – could lead to even greater and faster warming. “We need to make sure we do this at the same time as removing methane and CO2,” Diamond said. Reducing methane emissions, he noted, could help offset the effects of declining aerosols. Methane has a warming effect, but like aerosols, it does not stay in the atmosphere very long.

Yet many scientific questions remain – and until they are answered, the world will not know exactly how much warming the aerosols will unmask.

Harry Stevens contributed to this report.