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.
Other research has also supported the idea that aerosols have influenced global temperatures as a whole. Another 2016 paper, also published in Nature Geoscience, suggested that about a third of all the warming that occurred over land areas over the past 50 years was masked—temporarily covered up, in other words—by aerosol pollution.
Collectively, the research indicates that greenhouse gas emissions have had an even greater effect on the climate so far than it appears—it's just that part of it has been obscured by the presence of air pollution. As the air gets cleaner, those masked effects will start to make themselves known.
The new study demonstrates how much extra warming might be expected if all those aerosol emissions suddenly stopped—but it can't predict how long it might take the world to get to that point.
Many nations, including the United States, have made significant strides in cutting down on air pollution—often for health-related reasons—over the last few decades, and other countries are stepping up their efforts now, as well.
Additionally, global efforts to cut down on greenhouse gases are likely to have a spillover effect on aerosols, because air pollution is often a byproduct of the same industrial sources that produce carbon emissions. Reducing one type of emission can help cut down on the other.
So it's likely that aerosols will continue to decline in the atmosphere. At what point they'll disappear entirely—which is where that 0.5 to 1.1 degrees of warming would reach its full potential—is another question.
Some scientists have already pushed for caution when interpreting the new study's results.
"While aerosols are linked to many of the activities related to CO2 emissions (coal burning, deforestation), there is not a one-to-one correspondence," noted climatologist Gavin Schmidt, director of NASA's Goddard Institute for Space Studies, in a Twitter thread commenting on the new research. "You cannot assume that net zero CO2 emissions must also imply zero anthropogenic aerosol emissions."
As a result, he added, it's important to note that anthropogenic aerosols "will not suddenly disappear and make global warming much worse."
Still, other scientists say the new research lends support to the idea that current global goals—particularly the 1.5-degree target—are now almost certain to be overshot, even if the timing is uncertain.
In his own Twitter comment, researcher Glen Peters (also of CICERO, although he is not an author on the new study) suggested that meeting the 1.5-degree target is only possible with the help of geoengineering—using technology to bring global temperature back down below a certain threshold.
In fact, one major proposed form of geoengineering involves using aerosols to cool the climate—although ideally a form that's not hazardous to human health. Other ideas include drawing carbon dioxide back out of the atmosphere.
So far, none of the proposals are anywhere near developed enough to be considered a viable global solution—but some scientists suggest that they deserve more research, and quickly.
For now, Samset said, he hopes the new study can help "inform the next generation of climate scenarios."
The Intergovernmental Panel on Climate Change has established a number of possible future climate trajectories, assuming different levels of climate mitigation efforts and greenhouse gas emissions. It has accounted for future aerosol reductions in all of these scenarios, Samset noted, but with very little variation in the amount of reduction assumed.
Because it's still uncertain exactly how quickly aerosol emissions will decline in the future, some scientists believe a greater variety of possible scenarios should be investigated.
"I think we need to have more variety in those projections, since even matching up the real world today with the emissions projections from 2000 that we used last time shows a significant mismatch," said Schmidt, the NASA scientist (who was not involved with the new research), in an email to E&E News.
What remains clear, though, is that the full extent of human-caused global warming is still revealing itself—and the future may be more severe than the past would seem to suggest.