Australia’s recent fire season was so extreme that it altered large-scale wind patterns more than 10 miles overhead, in an upper layer of the atmosphere called the stratosphere, which normally isn’t affected by events on Earth’s surface, a new study has found.
These “pyrocumulonimbus” events, or pyroCbs for short, injected enormous plumes of smoke into the lower stratosphere, one of which circumnavigated the globe while rising to an unprecedented height of over 19 miles and spinning up its own winds, which circled the plume for more than two months. Two other plumes also triggered weaker, shorter-lived wind vortices in the stratosphere.
Scientists say the dramatic behavior of these fire-induced plumes helps confirm a key prediction about how fires generated by nuclear bomb blasts would impact the atmosphere.
Fire-induced thunderclouds only form in rare circumstances, when heat from intense wildfires creates a powerful updraft. Combined with atmospheric instability and ample moisture, these updrafts form towering thunderclouds can produce lightning, igniting new fires downwind. They also act like chimneys, shooting plumes of smoke filled with tiny particles called aerosols into the stratosphere in a manner similar to that of a volcanic eruption. Once in the stratosphere, that smoke can ride the jet stream around the world, potentially impacting Earth’s climate.
Southeastern Australia was already in the throes of an extreme fire season in late December when a passing weather front whipped up strong winds, causing fires to explosively intensify and triggering an outbreak of pyrocumulonimbus clouds. In total, at least 18 fire-induced thunderheads formed between Dec. 29 and Jan. 4, pouring smoke into the stratosphere where it was soon spotted by satellites.
“It was apparent within about the first week that this was an unprecedented event” in terms of the number of pyroCbs that occurred in a short time span and the amount of smoke these thunderclouds produced, said lead study author Pat Kablick, an atmospheric scientist at the U.S. Naval Research Laboratory.
Indeed, early estimates suggest that this single pyroCb outbreak injected up to 0.9 million tons of aerosols into the stratosphere. That’s roughly three times as much material as was put in the stratosphere by a fire-induced thundercloud that formed over British Columbia in 2017, which until recently served as scientists’ benchmark for extremely large pyroCb events.
Using a combination of satellite data and atmospheric models, Kablick and his co-authors tracked several distinct smoke plumes from the pyroCb outbreak, including one that was over 600 miles wide and three miles thick. As this plume streamed across the globe — first traveling east from Australia to South America, then reversing course and circumnavigating the southern hemisphere westward — it rose within the stratosphere by approximately 10 miles over the course of six weeks, climbing higher and faster than any pyroCb plume previously documented and peaking at an altitude of nearly 20 miles.
Jean-Paul Vernier, an atmospheric scientist at NASA who wasn’t involved with the study, noted that the last plume to reach such heights was the one produced by the cataclysmic 1991 volcanic eruption at Mount Pinatubo in the Philippines.
“The actual result is jaw dropping,” Kablick said.
Pengfei Yu, a researcher at Jinan University in China, said in an email that the Australian smoke plume’s epic atmospheric climb can be chalked up to tiny soot particles absorbing sunlight and releasing energy as heat, a process that warms the surrounding air and makes it more buoyant. Earlier research led by Yu shows that the 2017 British Columbia pyroCb plume rose through the stratosphere due to the same process. The Australian plume’s faster rate of rise and higher maximum height, he said, was likely the result of “more abundant absorbing materials” within the plume.
This self-lofting plume behavior is also something models have predicted would occur as the result of a nuclear-armed conflict that causes entire cities to burn, said Alan Robock, a climate scientist at Rutgers University who studies the climatological impacts of nuclear war. Nuclear warfare models show bomb-induced firestorms sending smoke up to the lower edge of the stratosphere where it would rise higher and higher, persisting for months and blocking sunlight from hitting Earth’s surface.
Between the 2017 pyroCb event and Australia’s recent pyroCb outbreak, “nature has done the experiment to validate our models” of a self-lofting plume, Robock said.
As it rose through the stratosphere, the Australian pyroCb plume also did something entirely unexpected: It created its own winds, which rotated around the plume in an anticyclonic, or counterclockwise, fashion at over 30 mph. This anticyclonic wind pattern, which Kablick described as “about the size of a large hurricane,” persisted in the stratosphere for roughly two months. During that time, it effectively created a small ozone hole by lofting a pocket of ozone-depleted air from the lower atmosphere into the stratosphere and preventing the two air masses from mixing.
This atmospheric disturbance was “remarkable in its duration and evolution,” said Neil Lareau, a fire weather researcher at the University of Nevada at Reno who wasn’t involved in the paper. The scale of the pyroCb outbreak, he said, “clearly translated up into the atmosphere and made an imprint … in a way we haven’t seen before.”
Kablick said the plume’s self-created winds were likely the result of a pressure gradient forming as the smoke heated up and rose. But he emphasized that there is still “some uncertainty” in this explanation and that more work will be needed to elucidate the exact mechanisms.
It is also unclear what, if any, impacts this smoke-fueled vortex had on broader circulation patterns in the stratosphere.
To further investigate these questions, Kablick and his colleagues are combing through historical meteorological records and modeled reconstructions of the atmosphere for other examples of pyroCb plumes altering stratospheric winds.
“We’re actually seeing that some of the [pyroCb] events that we’ve analyzed in the past do have this feature, and we just never noticed it because it was too small to see,” Kablick said. “This has kind of opened up a whole new vein of scientific research.”
Australia’s recent fire season was the worst on record for the country’s densely populated southeast, where it torched over 10 million acres of land. Extreme fire conditions were exacerbated by a combination of a multiyear drought, an unusually hot spring and summer weather, and human-caused climate change, which has warmed Australia by over a degree since the early 20th century and increased the frequency and severity of extreme heat events.
With continued warming, scientists expect more extreme fire years in Australia’s future. While it is uncertain whether that will translate to more pyroCb outbreaks, investigating the impacts of these violent weather events is becoming more urgent.
A separate study in review for publication found that the absorption of sunlight by smoke particles within the Australian pyroCb plumes reduced the amount of light hitting Earth’s surface, causing a temporary cooling effect similar to a volcanic eruption. That is another prediction of nuclear warfare models.