Two years ago the crew of the research vessel shot Pole Star a green laser into the night. The reflected light from the beam should help researchers study icy winter clouds. Instead, the beam hit something unexpected: a kilometer-thick layer of particles in the stratosphere, at a height of more than seven kilometers. As the researchers later discovered, it was smoke from huge forest fires that raged in Siberia that summer.
The smoke was more than a curiosity. In March 2020, when the Siberian smoke was still there, the ozone levels in the Arctic hit a record low – not a “hole” by Antarctic standards, but a worryingly low level. Although the investigation is far from over, it seems likely that the smoke contributed to the ozone depletion, says Kevin Ohneiser, a doctoral student at the Leibniz Institute for Tropospheric Research (Tropos). Similar developments occurred in Antarctica in the past two years after the record Australian “Black Summer” fires when more than a million tons of smoke entered the stratosphere.
The results, the Ohneiser and his colleagues last month in the trade journal Atmospheric Chemistry and Physics have published, suggest that climate change could have unexpected effects on atmospheric chemistry: accordingly, smoke from ever larger forest fires penetrates the stratosphere, a calm, isolated layer above the troposphere. Once there, it may break down the ozone layer, which shields harmful UV radiation. “Until recently, the global impact of smoke was considered to be minor,” said Catherine Wilka, a stratospheric chemist at Stanford University. Now it is developing into a new frontier area in climate research.
The “self-ascent” of smoke into the stratosphere is controversial
“This is really new,” says Omar Torres, a remote sensing scientist at NASA’s Goddard Space Flight Center. Satellites have been able to track smoke particles since the late 1970s. These are easy to see from space because they absorb UV light strongly. However, until 2017 satellites provided no evidence that smoke was entering the stratosphere in any significant amount, says Torres.
The smoke in the Arctic is particularly worrying because it appeared there unexpectedly. “Everyone thought the Arctic was really clean,” says Ohneiser, because there are no thunderstorms there that could drive pollutants into the stratosphere. The most violent forest fires on earth, like those in Australia, can create huge storm systems that, like volcanoes, pump material into the stratosphere. But while Siberia burned, it was trapped in a heat wave and high pressure system that choked off the convective updrafts emanating from large storms. The smoke must have reached the stratosphere by a different route.
In a model that has not yet been published, the Tropos group tries to explain how the smoke rose so high in the region. In doing so, the researchers refer to a decades-old theory known as “self-ascent”. The model states that the dark smoke particles absorb sunlight so effectively that they quickly heat the air around them, causing the smoke to rise. After just a few days, this process could have lifted the smoke ten kilometers above the ground, where it could then be carried by winds into the low arctic stratosphere. The NASA laser satellite Calipso I noticed plumes of smoke when flying over the Siberian fires, which seemed to rise from four to ten kilometers, according to Ohneiser.
The idea of self-ascension, which has never been documented in the troposphere, is controversial. In the small world of firestorm or “pyrocumulonimbus” (pyroCB) research, “the idea has somehow gained acceptance that smoke aerosols can only get into the stratosphere by direct injection,” says Torres. He had identified self-buoyancy as the way smoke from the British Columbia fires reached the stratosphere in 2017. “But the observations show that this also happens when we don’t have PyroCBs.”
Others are not convinced of it. Michael Fromm, a PyroCB researcher at the US Naval Research Laboratory, calls this an “extraordinary claim” that requires more solid evidence. In his opinion, the smoke is unlikely to penetrate the tropopause, a boundary that helps isolate the stratosphere, without the added boost of a firestorm. Fromm believes that most of the arctic particles are not smoke, but sulfate aerosols from the Raikoke. This volcano, southwest of Russia’s Kamchatka Peninsula, hurled gas and ash into the stratosphere in 2019. The scientist points out that Calipso cannot distinguish between smoke and sulphates.
But Ohneiser and his colleagues stick to their point of view. Their advanced measuring instrument, Lidar, measures light absorption and reflection at two different wavelengths. Observations of the Australian fires with the same instrument showed that smoke particles have a distinctive signature. These are “clear optical fingerprints from forest fire smoke,” says Ohneiser. “There is no room for other interpretations.” In the study, the Tropos team sees sulfate particles from the Raikoke, but they form a thin layer higher up in the stratosphere.
Once the smoke is in the stratosphere, “there is a very good chance” it depletes ozone, says Jessica Smith, an atmospheric chemist at Harvard University. The polar ozone depletion is due to chlorine, which is still in the stratosphere and originates from chlorofluorocarbons and other pollutants, although these 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 create free chlorine radicals that eat their way through the ozone. According to Smith, smoke particles could promote ozone depletion by encouraging the formation of these clouds and supplying them with smaller droplets. The smoke particles could also be coated with chemicals such as sulfates, which could break down ozone as a result of a reaction with chlorine. Or the smoke could somehow increase the winds of what is known as the polar vortex, thereby cooling the poles and increasing the degradation.
The influence of the stratospheric smoke is not necessarily limited to the poles. In the middle latitudes, the stratosphere is much higher and is theoretically better protected from pollution. But if the forest fires worsen, Wilka said, the smoke could even have a chance to deplete the ozone layer over mid-latitudes, where most of the world’s population lives – much like the Pinatubo volcanic eruption in 1991. If you send enough smoke and other particles up there, says Wilka, “you can definitely get this chemistry going”.
The original of this article is in the science magazine Science published by the AAAS. German version: cvei