Wildfires don’t just scorch landscapes. Some are so intense that they create their own weather systems, such as pyrocumulonimbus thunderstorms that loft smoke up to 10 miles (16 kilometers) into theatmosphere. While it’s long been known that this high-altitude smoke can persist in the atmosphere for weeks or months, their effect on climate has been hard to measure, due to the difficulties in collecting samples. That is, until now.

Atmospheric scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences report the first direct measurements of five-day-old wildfire smoke in the upper troposphere, about nine miles (14.5 kilometers) above Earth’s surface. They discovered large smoke particles that aren’t represented in current climate models, and these particles appear to actually cool the atmosphere.

To capture fresh smoke directly, the team flew a NASA ER-2 high-altitude aircraft into a plume created by a New Mexico wildfire in June 2022, just five days after the fire ignited. Onboard instruments measured particle size, concentration and chemical composition.

Inside the smoke cloud, researchers detected aerosols roughly 500 nanometers wide — about twice the size of typical wildfire aerosols at lower altitudes. The team suggests the large size can be attributed to efficient coagulation.

“Particles can coagulate at any place in the atmosphere,” Yaowei Li, the lead author of a study on the research, said in astatement. “But in that specific region, the air mixes very slowly. That allows wildfire smoke particles to remain concentrated and collide often, making coagulation much efficient.”

Such aerosols play a role in changing the amount of radiation that gets to theEarth’s surfacewhether by absorbing sunlight or reflecting back toward space. In this study, the larger particles had a striking effect: They increased outgoing radiation by 30% to 36 compared to lower-altitude particles, producing a measurable cooling effect that current climate models don’t account for.

research is needed to determine further effects of such high-altitude wildfire smoke on both weather and climate. Study co-author and project scientist John Dykema suggests that the large coagulated smoke particles could affect atmospheric circulation through local heating, potentially shifting jet streams. “I think all of these things are possible, and we don’t currently have enough information to say which way they could go,” he said.

The study was published on Dec. 10 in the journalScience Advances.

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