By Megan Sever
For the first time, scientists have seen exactly how towering clouds that rise from intense wildfires launch smoke high into the atmosphere, where it can linger for months and mess with the protective ozone layer.
Cooler air closer to Earth’s surface normally keeps smoke from rising too high. But as dozens of fires raged in western Canada and the U.S. Pacific Northwest in the summer of 2017, they created their own giant storm clouds called pyrocumulonimbus, or pyroCb, clouds. Within two months, these clouds had lofted smoke 12 to 23 kilometers up into the stratosphere, researchers report in the Aug. 9 Science. Solar radiation heating soot in the smoke helped it reach those soaring heights.
Using satellites, weather balloons and ground-based remote sensing, the team tracked the smoke over the Northern Hemisphere, measuring the levels of organics and black carbon, or soot. Smoke persisted in the stratosphere for about eight months, says Pengfei Yu, a climate scientist at Jinan University in Guangzhou, China.
Although smoke has been observed in the stratosphere before, this “mother of all pyroCbs” offered the first direct observation of a process called “self-lofting,” says coauthor Alan Robock, a climate scientist at Rutgers University in New Brunswick, N.J.
The observations confirmed what simulations had suggested would happen if large amounts of smoke were injected into the stratosphere via a nuclear war, the team says. “Nature did the experiment for us,” Robock says, confirming the “nuclear winter” scenario, in which smoke in the stratosphere from a city burning would have far-reaching and long-lasting climatic consequences, including blocking out sunlight and affecting ozone.