Extreme heat often brings extreme wildfires, and this year is no exception, with unprecedented heat waves fueling enormous outbreaks of fires in the western United States and Canada, as well as across the Mediterranean and in Siberia. But paradoxically, the most intense wildfires can have the opposite effect on temperatures, cooling Earth’s surface both regionally and globally.
Dense wildfire smoke can temporarily block sunlight near the ground, causing regional temperatures to drop by several degrees. Wildfire smoke can also have global cooling effects by making clouds in the lower atmosphere more reflective or blocking sunlight in the upper atmosphere, similar to what a volcanic eruption does.
Scientists have only recently begun studying these effects, with Australia’s record 2019-2020 bushfire season marking the first time researchers detected wildfire smoke-induced global cooling. The effect is far too small to counteract human-caused global warming, but beyond that, researchers say it’s too early to predict what it means for the broader climate system.
But with fire seasons growing more intense around the world and this summer triggering a spate of extreme fire weather in North America and elsewhere, the search for answers is growing increasingly urgent.
“It’s absolutely clear that research related to the effect of wildfires on climate is very topical,” says Sergey Khaykin, a fire weather expert at Sorbonne University in France.
Dimmer skies, brighter clouds
As wildfires burn across the land, they emit a cocktail of tiny particles, water vapor, and gases into the air. Swept around by winds, this wildfire smoke can pollute the air hundreds to thousands of miles away. Residents of the northeastern U.S. experienced this firsthand in mid-July, when a pulse of smoke from a wildfire outbreak in southern Canada wafted over New York City, Philadelphia, and Washington D.C., reddening sunsets and triggering air quality alerts. In late July, smoke from Canadian wildfires spread across Minnesota, leading to “unprecedented” levels of air pollution. This week, Athens is choking on smoke as wildfires rage across nearby forests. The Dixie Fire is currently California's largest blaze, destroying over 100 homes and leveling a historic town.
In addition to creating a health hazard, dense wildfire smoke near the ground sometimes blocks enough light to reduce surface temperatures. As far back as 1950, scientists have measured this effect by comparing temperatures on heavy smoke days with temperatures forecast to have occurred in its absence.
“The effect ranges, depending how far you are from the source, how big the fire was and how much smoke it injected,” says Robert Field, a research scientist at NASA’s Goddard Institute for Space Studies. But when wildfire smoke is thick enough, Field says, “you could get a brief 5 degree Celsius [9°F] cooling effect.”
Field describes these cooling effects as “episodic” and “almost an academic curiosity” compared with the public health impacts of wildfire smoke. But new research on Australia’s 2019-2020 bushfire season points to another, potentially far more significant, way that wildfire smoke can cool Earth’s surface.
Writing in the journal Geophysical Research Letters last week, scientists at the National Center for Atmospheric Research reported that those Australian fires poured so much smoke into the Southern Hemisphere’s atmosphere it triggered a “strong and rapid” global cooling effect of about 0.06°C ( 0.01°F). According to lead study author John Fasullo, this was largely due to how smoke particles interact with clouds in the lower atmosphere, or troposphere.
Smoke particles act as seeds for water vapor to form droplets, resulting in clouds with additional droplets of water that are smaller and reflect more sunlight. While smoke tends to rain out of clouds fairly quickly, the Australian bushfires raged for months, pumping more and more cloud-brightening smoke into the atmosphere.
“It’s not a big effect, but when you integrate over the entire Southern Ocean, it adds up,” Fasullo says.
Indeed, the impact seems to have rippled across the climate system. The researchers’ models showed that smoke-induced cooling in the Southern Hemisphere shifted a critical belt of tropical thunderstorms, the Intertropical Convergence Zone, further north. Fasullo says that this shift might have contributed to the emergence of La Niña conditions last year, which cooled ocean surface temperatures in the eastern equatorial Pacific, although more research is needed to confirm that hypothesis.
But according to Fasullo, there’s no longer any question that “wildfires can create their own climate, or instigate a climate response.”
While Fasullo’s research highlights the cooling effects of smoke in the lower atmosphere, on occasion, wildfire smoke punches through the troposphere and into the stratosphere, the atmospheric layer that starts about 10 miles up. There, it can have additional effects on the climate.
Smoke reaches the stratosphere when heat from a powerful wildfire creates an updraft that combines with moisture in the atmosphere to generate towering thunderclouds. Known as pyrocumulonimbus clouds, or pyroCbs, these fire-fueled thunderclouds can act like chimneys, funneling smoke into the upper atmosphere where it can circle the globe and linger for months.
This happened in late 2019 and early 2020 in Australia, when an unprecedented “super outbreak” of 38 pyroCbs blew roughly a million metric tons of smoke into the lower stratosphere. Research led by Khaykin and published last September showed that the smoke shaded Earth’s surface for several months, likely producing an additional, small global cooling effect on top of the effect of cloud brightening.
Something similar could happen this year.
Western North America’s fire season is off to a raging start, with June and July bringing exceptionally hot weather and numerous wildfires to California, the Pacific Northwest and British Columbia. Alongside the fires, scientists have witnessed a rash of pyroCbs, beginning with a massive fire-fueled thundercloud in British Columbia on June 30 that blew smoke about ten miles overhead. That event kicked off “what became a multi-day pyroCB wave,” says David Peterson, a scientist at the Naval Research Laboratory who studies pyroCbs.
Between late May and August 2, Canada has seen 49 pyroCb events, including an outbreak of 10 pyroCbs on July 15 that ranks second to the Australian super-outbreak in terms of the number of firestorms that occurred in a single outbreak. Another 21 pyroCbs have formed over the continental U.S. this summer. Taken together, Peterson says, this is the largest number of pyroCbs observed in North America since scientists began keeping detailed records of the phenomenon in 2013.
Khaykin, who is tracking these pyroCbs, as well as others in Siberia, says that so far, they have pumped considerably less smoke into the stratosphere than last year’s Australian outbreak did. But with several months left in the wildfire season and hot, dry conditions persisting across much of the West, that could change.
Khaykin adds that while the Australian pyroCb outbreak blasted smoke to unusually high altitudes, this year, smoke is being injected into a lower part of the stratosphere near where cirrus clouds form. This, he says, might lead to “enhanced aerosol cloud interaction” in the stratosphere, “which is something that is really poorly known.”
“We might still be in the infancy of understanding how large and how important the effects of these wildfires are,” he says.