Two years ago, the crew of the Polarstern, a German icebreaker frozen into Arctic sea ice, shot a green laser up into the night. The beam’s reflected light was meant to help researchers study icy winter clouds. Instead, the beam encountered something unexpected: a kilometers-thick layer of particles in the stratosphere, more than 7 kilometers up. The haze, the researchers later concluded, was smoke from enormous wildfires that had ripped through Siberia that summer.
The smoke was more than a curiosity. By March 2020, as the Siberian smoke lingered, satellite measurements of ozone levels in the Arctic hit a record low—not quite a “hole,” by Antarctic standards, but worryingly low. Although the case is far from closed, it seems likely the smoke helped deplete the ozone, says Kevin Ohneiser, a graduate student at the Leibniz Institute for Tropospheric Research (TROPOS). Similar dips have occurred the past 2 years in Antarctica following Australia’s record-breaking “Black Summer” fires, which injected more than 1 million tons of smoke into the stratosphere. “We cannot prove this,” he says. “But these [results] seem to be a hint.”
The findings, which Ohneiser and his colleagues published last month in Atmospheric Chemistry and Physics, suggest climate change may have an unexpected impact on atmospheric chemistry, as smoke from increasingly severe wildfires invades the stratosphere and potentially erodes the ozone layer that screens out damaging ultraviolet (UV) radiation. “Until recently, smoke was really discounted in terms of a global impact,” says Catherine Wilka, a stratospheric chemist at Stanford University. Now, she adds, it’s “shaping up to be one of the new frontiers.”
“This is really new,” adds Omar Torres, a remote sensing scientist at NASA’s Goddard Space Flight Center. Since the late 1970s, satellites have been capable of tracking smoke particles, easily visible from space because they are strong absorbers of UV light. Until 2017, however, the satellites saw no sign of smoke penetrating the stratosphere in any appreciable amount, Torres says.
The Arctic smoke event is particularly worrisome because it had no business being there. “Everyone thought the Arctic would be really clean,” Ohneiser says, because it lacks the thunderstorms that can propel pollutants into the stratosphere, a calm, isolated layer above the troposphere. Today’s fiercest wildfires, such as those in Australia, can generate their own towering storm systems, capable of pumping material into the stratosphere like volcanoes. But while Siberia burned, it was trapped in a heat wave and a high-pressure system that smothered the convective updrafts that form large storms. The smoke must have had another route to the stratosphere.
In a model not yet published, the TROPOS group attempts to explain how the region could feed smoke so high, invoking a decade-old theory called “self-lifting.” Their model suggests the dark smoke particles absorbed sunlight so effectively that they rapidly heated the air around them, causing the smoke to rise. After only a few days, the process could have lofted smoke 10 kilometers above the ground, where winds could then usher it into the low Arctic stratosphere. And indeed, on passes over the Siberian fires, NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) laser satellite captured plumes of what seemed to be smoke rising from 4 to 10 kilometers, Ohneiser says.
The self-lifting idea, never documented in the troposphere, is controversial. In the small world of fire storm, or pyrocumulonimbus (pyroCB), research, “Somehow the idea has been advanced that the only way smoke aerosol can get to the stratosphere is due to direct injection,” says Torres, who identified self-lifting as part of the way that smoke from 2017 fires in British Columbia reached the stratosphere. “But the observations are showing it is still happening when we have no pyroCBs.”
Others are not convinced. Michael Fromm, a pyroCB researcher at the U.S. Naval Research Laboratory, calls it an “extraordinary claim,” requiring more robust evidence. He thinks that without the extra boost from a firestorm, smoke is unlikely to penetrate the tropopause, a boundary that helps isolate the stratosphere. Instead of smoke, Fromm believes most of the Arctic particles are lingering sulfate aerosols from Raikoke, a volcano southwest of Russia’s Kamchatka Peninsula that in 2019 heaved gas and ash into the stratosphere. He points out that CALIPSO can’t distinguish between smoke and sulfates.
But Ohneiser and his colleagues are standing firm. Their advanced lidar measures light absorption and reflection at two different wavelengths, and observations of the Australian fires using the same instrument showed smoke particles have a distinctive signature. These are “unambiguous optical fingerprints of wildfire smoke,” Ohneiser says. “There is no room for other interpretations.” In the paper, the TROPOS team does see sulfate particles from Raikoke, but they form a thin layer even higher up in the stratosphere.
Once smoke is in the stratosphere, “the potential is certainly there” for it to deplete ozone, says Jessica Smith, an atmospheric chemist at Harvard University. Polar ozone loss depends on chlorine, still lingering in the stratosphere from chlorofluorocarbons and other pollutants even though they were banned decades ago. The chlorine attacks in winter, when thin iridescent clouds form in the stratosphere. Their droplets provide a surface for chemical reactions that result in free radicals of chlorine, which chew through ozone. Smith says smoke particles might boost ozone loss by seeding the formation of these clouds and endowing them with smaller, more abundant droplets.
Smoke particles might also be coated in chemicals such as sulfates that could reduce ozone by directly reacting with chlorine. Or the smoke could somehow strengthen a collar of stratospheric winds called the polar vortex, further chilling the poles and boosting depletion. The loss mechanisms are speculative, Smith says, but they “could take a strong year and tip into an extreme year.”
The influence of stratospheric smoke isn’t necessarily limited to the poles. At midlatitudes, the stratosphere is much higher, and in theory more insulated from pollution. But as wildfires worsen, Wilka says, smoke might even have a shot at reducing ozone above the midlatitudes, home to most of the world’s population, much as the 1991 volcanic eruption from Mount Pinatubo did. Throw enough smoke and other particles up there, she says, and “you can absolutely start driving this chemistry.”
Source: Science Mag