COVID-19 cut pollution in 2020, warming the atmosphere

Researchers trace that uptick in temps to a drop in planet-cooling aerosol pollution

The haze seen here hanging over Los Angeles, Calif., is due to tiny aerosols hanging in the air. Early last year, COVID-19 lockdowns briefly reduced air pollution in industrial cities. That, in turn, cut the light-colored, planet-cooling airborne particles. This briefly warmed some regions by as much as 0.37 degree Celsius (0.67 degree Fahrenheit).

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Since COVID-19 burst on the scene last year, the virus that causes it has killed more than 2.7 million people worldwide. It has sickened at least 122 million more. And, weird but true, the economic lockdowns triggered by the pandemic warmed parts of the planet for a few months. That’s the finding of a new study.

That warming was small and came from brief but sizable regional drops in pollution.

When the pandemic really took hold early last year, some countries imposed so-called lockdowns. Workers and school children stayed home for weeks to months. Traffic on highways dropped sharply. Many businesses shut down too, notes Andrew Gettelman. He works at the National Center for Atmospheric Research in Boulder, Colo. Keeping people home led to big drops in global releases of planet-warming greenhouse gases, the atmospheric scientist notes. The shut downs also triggered declines in tiny particles known as aerosols.

Aerosols float in the air. Some are liquid droplets. These include the water in low clouds and sulfuric acid spewed by volcanic eruptions. Solid aerosols include dust, pollen, combustion pollutants and particles of salt left behind when droplets of sea spray evaporate. They also include the ice crystals in high-altitude clouds.

Some aerosols are dark, like black soot. Light-colored ones, however, help cool the climate, says Gettelman. For one thing, they reflect the sun’s warming light back into space. A drop in light-colored aerosols allows more solar energy in to warm the land, seas and atmosphere. But aerosols also have another effect, he notes. They make rain clouds brighter. The more aerosols there are, the more drops that form in clouds and the smaller those droplets are. Smaller droplets make clouds brighter. That also reflects more light back to space, which in turn cools Earth.

Last year’s lockdowns cut the release of pollutant aerosols. Rain clouds also developed larger — and therefore darker — droplets. They also warmed more than normal.

Gettelman and his teammates used computer models to simulate how much warming all of this likely caused. For input, the researchers plugged pollution estimates reported by various countries. The computer then compared actual temperatures measured at weather stations with the simulation results that didn’t include the effects of having fewer aerosols.

Some areas heated little. The Southern Hemisphere is one such region. That’s because there’s not much industrial activity there to emit aerosol pollution. But in industrial parts of the Northern Hemisphere, some areas warmed notably. In Russia and the United States, for instance, some regions warmed by about 0.37 degree Celsius (0.67 degree Fahrenheit).

Gettleman and his team shared these findings February 16 in Geophysical Research Letters.

Air temps often swing several degrees over the course of a day. But from one year to another, average temperatures change far less. The warming found by the new study is “not super-important, but it is noticeable,” says Gavin Schmidt. He’s a climate scientist at NASA’s Goddard Institute for Space Studies in New York City. Still, this small heating might have affected how 2020 stacked up against other years, he says.

According to a NASA analysis, 2020’s global average temperature tied with 2016’s. That earlier year had been the hottest in modern history, says Schmidt. Without the slight bump in global temperatures due to COVID-19, 2020 might have come in second place.

Another analysis by European researchers helps support that claim. It found that Europe, a heavily industrialized region, had its hottest year ever in 2020. That continent’s record heat might have been tipped over the edge by its months-long drop in planet-cooling aerosols during lockdowns.

About Sid Perkins

Sid Perkins is an award-winning science writer who lives in Crossville, Tenn., with his wife, two dogs and three cats. He enjoys cooking and woodworking, and he really, really wants to get better at golf.

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