'Atmosphere has a memory': Pollution spikes can heat the sky for 48 hours before cooling

For about two days after pollution from people and wildfires fills the air, the atmosphere may warm instead of cool, upending prior assumptions about how these airborne particles act in our skies.

The study, currently available as a preprint from Nature Communications, challenges a common assumption that such aerosols start lowering temperatures immediately and suggests a brief warming phase can come first.

What's happening?

Researchers took a closer look at aerosols — the microscopic particles tied to wildfire smoke, pollution, dust, and sea spray — and found that their role in the climate may be more complicated than previously thought.

As summarized by Earth.com, the new study says abrupt increases in aerosol concentrations can produce an initial rise in atmospheric warmth, despite their reflective properties that help move sunlight away from Earth's surface.

The study's simulations, led by Hebrew University of Jerusalem professor Guy Dagan, tracked conditions in the days after particle amounts increased. They indicated that heat remained trapped for roughly the first 48 hours, with the strongest warming — about 20 watts per square meter — showing up during the first day, Earth.com explained.

The short-term heating appears to be tied to cloud changes. Rising particle levels can reduce light rain, push more water higher into the atmosphere, and promote additional ice and high-altitude clouds. 

Those upper clouds hold in heat that might otherwise escape into space. Once that cloud response starts to fade, the particles' better-known ability to reflect sunlight becomes stronger, and the overall effect of the aerosols shifts back toward cooling.

Why does it matter?

Aerosols are already one of the biggest uncertainties in climate forecasting, especially because of the way they interact with clouds. If pollution can warm the atmosphere before it cools it, a single look at the sky may miss part of the story.

Dagan summarized the findings simply. "Our results show that the atmosphere has a memory," he said in a university news release, per Phys.org.

That timing issue could matter for both climate models and the interpretation of satellite observations. Many approaches are based on measurements taken at one moment, but Dagan's study suggests the atmosphere may need days rather than minutes to fully respond. Whether a burst of pollution is warming or cooling may depend not just on how much is in the air, but also on when it is observed.

Because aerosol concentrations are always changing, the atmosphere may be responding to one shift before the next one arrives. The study suggests that if those ups and downs happen every few days, the system may never fully settle into a cooling phase, potentially changing how researchers estimate the climate impact of pollution events, as human-caused pollution more broadly continues to contribute to rising global temperatures. 

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