In a paradox, cleaner air is now adding to global warming | Science

It’s one of the paradoxes of global warming. Burning coal or gasoline releases the greenhouse gases that drive climate change. But it also lofts pollution particles that reflect sunlight and cool the planet, offsetting a fraction of the warming. Now, however, as pollution-control technologies spread, both the noxious clouds and their silver lining are starting to dissipate.

Using an array of satellite observations, researchers have found that the climatic influence of global air pollution has dropped by up to 30% from 2000 levels. Although this is welcome news for public health—airborne fine particles, or aerosols, are believed to kill several million people per year—it is bad news for global warming. The cleaner air has effectively boosted the total warming from carbon dioxide emitted over the same time by anywhere from 15% to 50%, estimates Johannes Quaas, a climate scientist at Leipzig University and lead author of the study. And as air pollution continues to be curbed, he says, “There is a lot more of this to come.”

“I believe their conclusions are correct,” says James Hansen, a retired NASA climate scientist who first called attention to the “Faustian bargain” of fossil fuel pollution in 1990. He says it’s impressive scientific detective work because no satellite could directly measure global aerosols over this whole period. “It’s like deducing the properties of unobserved dark matter by looking at its gravitational effects.” Hansen expects a flurry of follow-up work, as researchers seek to quantify the boost to warming.

Some aerosols, such as black carbon, or soot, absorb heat. But reflective sulfate and nitrate particles have a cooling effect. For many years, they formed from polluting gases escaping from car tailpipes, ship flues, and power plant smokestacks. Technologies to scrub or eliminate this pollution have spread slowly from North America and Europe to the developing world. Only in 2010 did air pollution in China begin to decline, for example, and international restrictions on sulfur-heavy ship fuel have come just in the past few years.

The new study, submitted as a preprint to Atmospheric Chemistry and Physics in April and expected for publication in the next few months, grew directly out of last year’s U.N. climate assessment. It included studies showing aerosol declines in North America and Europe but no clear global trends. Quaas and his co-authors thought two NASA satellites, Terra and Aqua, operating since 1999 and 2002, might be able to help.

The satellites tally Earth’s incoming and outgoing radiation, which has enabled several research groups, including Quaas and his colleagues, to track the increase in infrared heat trapped by greenhouse gases. But one instrument on Aqua and Terra has also shown a decline in reflected light. Models suggested a decrease in aerosols is partly responsible, says Venkatachalam Ramaswamy, director of the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory. “It’s very hard to find alternate reasons for this,” he says.

Quaas and his co-authors have now taken things a step further with two instruments on Terra and Aqua that record the haziness of the sky—and therefore its aerosol load. From 2000 to 2019, haze over North America, Europe, and East Asia clearly declined, although it continued to thicken over coal-dependent India.

Aerosols don’t just reflect light on their own; they can also alter clouds. By serving as nuclei on which water vapor condenses, pollution particles reduce cloud droplet size and increase their number, making clouds more reflective. Reducing pollution should undo the effect—and using the same instruments, Quaas and his team found a clear decrease in cloud droplet concentrations in the same regions where aerosols declined.

The evidence in the paper is clear, says Joyce Penner, an atmospheric scientist at the University of Michigan, Ann Arbor. “It’s remarkable that we’re seeing this already,” she says. “This is contributing a lot to the climate changes we’re seeing in the current era.”

Just how much this declining reflectivity has boosted recent warming is hard to quantify, says Stuart Jenkins, a doctoral student at the University of Oxford who is also studying the aerosol decline. In forthcoming work, Jenkins will show there’s just too much natural variability in the past 20 years to pick out the effect of clearer skies.

Whatever the exact contribution, it is sure to grow as air quality continues to improve around the world. The answer isn’t to keep polluting, says Jan Cermak, a remote-sensing scientist at the Karlsruhe Institute of Technology. “Air pollution kills people. We need clean air. There is no question about that.” Instead, efforts to reduce greenhouse gases need to be redoubled, he says.

But with Earth having warmed by some 1.2°C since preindustrial times, Hansen thinks there’s little hope of cutting emissions fast enough to meet the 1.5°C target he and other scientists have called for. And so the solution, he says, could come back to aerosols, this time ones spread deliberately through solar geoengineering—the controversial idea of lofting sulfate particles into the stratosphere and creating a global, reflective haze. “It will be necessary to take temporary corrective measures,” he says, “almost surely including temporary purposeful use of aerosols to avoid catastrophic implications.”