Pollution in the atmosphere is having an unexpected consequence, scientists say—it’s helping to cool the climate, masking some of the global warming that’s occurred so far.
That means efforts worldwide to clean up the air may cause an increase in warming, as well as other climate effects, as this pollution disappears.
New research is helping to quantify just how big that effect might be. A study published this month in the journal Geophysical Research Letters suggests that eliminating the human emission of aerosols—tiny, air-polluting particles often released by industrial activities—could result in additional global warming of anywhere from half a degree to 1 degree Celsius.
This would virtually ensure that the planet will warm beyond the most stringent climate targets outlined in the Paris climate agreement. World leaders have set an ambitious goal of keeping global temperatures within 1.5 to 2 degrees Celsius of their preindustrial levels. But research suggests the world has already warmed by about 1 degree—meaning even another half a degree of warming could push the planet into dangerous territory.
“Since we’re trying to keep to a 1.5- or 2-degree target, then this is something we still need to keep in mind,” said Bjørn Samset, a climate scientist at Norway’s CICERO Center for International Climate Research and the study’s lead author.
The research also suggests that removing aerosols could have striking regional consequences by causing major changes in precipitation and other weather patterns in certain parts of the world. Aerosols don’t linger in the atmosphere for very long, meaning they don’t have time to spread around the world the way carbon dioxide and some other greenhouse gases do. Their effects tend to be strongest in the regions where they were emitted in the first place.
This means the places where air pollution is most severe are likely to experience some of the greatest effects if that pollution were to disappear. East Asia, where aerosol emissions are some of the highest in the world, would be likely to experience a strong increase in precipitation and extreme weather events. To a certain extent, these effects might carry over to other parts of the Northern Hemisphere, which are connected to Asia via major atmospheric currents.
“We also see that the impact that these aerosols have on temperature in Asia really transports northwards to the Arctic region, northern Europe, Norway, the northern U.S.,” Samset noted. “That part of the world is also quite sensitive to the changes in aerosols in Asia.”
Scientists have long known that some types of pollution can actually help cool the climate. Certain aerosols—sulfate, for instance—can reflect sunlight away from the Earth or enhance sun-reflecting cloud cover. As nations around the world have begun to crack down on air pollution, scientists have grown interested in figuring out how much extra warming might be expected as they disappear. This is critical information for strategizing ways to meet global climate goals, like the 2-degree target.
The new study relied on four global climate models, which the researchers used to simulate the effects of removing all human-caused emissions of the major aerosols, including sulfate and carbon-based particles like soot. The resulting global warming, they concluded, would be anywhere from 0.5 to 1.1 degrees Celsius.
These results are in line with other studies that have investigated the cooling “mask” of aerosols. A 2016 paper published in Nature Geoscience found that up to a half-degree Celsius of the warming that has been observed in the Arctic—the most rapidly warming region on the planet—since 1980 was caused by pollution reductions in Europe. Like the new study, those findings speak to both the considerable cooling effect aerosols have had on the climate and to the atmospheric linkages between different regions of the Northern Hemisphere.