As wildfires have sent palls of smoke streaming across large swaths of North America, some have spawned fire-breathing thunderstorm clouds.
As is evident from the image above and others that follow, these pyrocumulonimbus clouds, or pyroCbs, offer a visually dramatic reminder of just how extreme wildfire behavior can get. But they are significant for other reasons as well.
PyroCbs can hurl barrages of lightning bolts to the ground, triggering even more wildfires. They also can help spread harmful particulate pollution far and wide, and even drive smoke into the stratosphere, five to seven miles above Earth's surface. Here, the smoke can actually influence the global climate, recent research has shown.
In the photo above, a pyroCb is seen erupting from Oregon's Falls Fire on July 15, 2024. The blaze has produced a number of these events, including one on July 13 that up until that point was the biggest one of the summer in the western United States.
The Falls Fire ignited on July 10, 2024 in the Malheur National Forest, about 20 miles northwest of Burns, Oregon. As of today, the blaze was just 15 percent contained and had scorched 120,919 acres — an area equivalent to about two thirds the size of New York City. The fire was human-caused.
I spotted a pyroCb erupting above the blaze in GOES-18 satellite imagery acquired on July 15. In the following animation of that imagery, look for a sudden spurt of ashen material roughly in the middle of the frame:
Here's compelling video footage showing what the fire looked like from the ground on July 18:
And for a sense of scale, check out this photo captured the same day showing a single-engine air tanker flying near the blaze:
A pyroCb cloud can form as a wildfire (or volcanic eruption) triggers vigorous updrafts of hot air, carrying ashy particulate matter high into the atmosphere. As the air cools and expands, water vapor condenses onto the particles, forming a grey or brown cloud.
If the updrafts are vigorous enough, and there's sufficient water vapor, a large volume of ice particles can form high in the cloud, helping to create lightning. This, in turn, can increase the risk of generating more and larger fires.
"It takes a very large and very hot wildfire to be able to produce an updraft strong enough that it leads to pyroCb development," says Scott Bachmeier, recently retired from the University of Wisconsin's Cooperative Institute for Meteorological Satellite Studies.
And as Bachmeier's CIMSS colleague, Scott Lindstrom, notes, "From my point of view, a pyroCb is most interesting because it is injecting into the upper troposphere/lower stratosphere a whole lot of particulate matter that can then be transported long distances."
A Rise in Fire-Breathing Smoke Storm Activity
It has long been known that pyrocumulonimbus clouds rise up from volcanic eruptions and nuclear explosions. But the first scientific confirmation of a pyroCb erupting from a wildfire didn't come until the year 2000.
As the climate has warmed, and wildfire activity has intensified, pyroCbs have grown larger and more frequent, with record-breaking events in 2017, 2019, 2020 and 2021. In that latter year, an early and unusually warm fire season produced particularly scary pyroCb outbreaks.
On July 16 of that year, an astonishing 10 pyroCbs blew up along the Saskatchewan-Manitoba border in Canada. Up until that point, this was a greater number than scientists had ever observed in North America on a single day since satellite tracking began in 2013, according to NASA.
The outbreak came just two weeks after a monster pyroCb astonished scientists. It happened as a storm cell grew above a wildfire in British Columbia and spread across more than 62,000 square miles. That's an area slightly larger than the state of Georgia. This gargantuan cloud propelled a chimney of smoke into the stratosphere, as high as 10 miles up.
It turned out to be the largest pyroCb cloud scientists had ever observed in North America.
"The North American Lightning Detection Network recorded nearly 113,000 cloud-to-ground lightning strokes during the event, a large amount for a storm in Canada," according to NASA. "One meteorologist calculated that this one pyroCb event produced about 5 percent of Canada’s total annual lightning all at once."
Climate Impacts
Research published last year in the journal Science showed that black carbon particulate matter rising in the updrafts of pyroCbs are having a much more significant and longer-term impact on the stratosphere — and the climate, too — than previously believed.
As part of the study, scientists analyzed 13 years of airborne observations to determine what makes up pyrocumulonimbus smoke and to quantify its impact on the stratosphere. The researchers found that pyroCbs are responsible for as much as 25 percent of the black carbon and organic aerosols in the lower stratosphere. Although the climatic impact of this material is complex, by acting like an umbrella, it is believed to exert a cooling effect.
As the frequency and severity of wildfires increase with continuing climate change, the impact of pyroCbs is likely to rise as well. But it would be a mistake to think this will help save us from the ravages of a warming climate.
That's because the warming impact of the carbon dioxide and other greenhouse gases we continue to pour into the atmosphere swamps the shading influence of particulates and aerosols. Moreover, CO2 that streams into the atmosphere from wildfires boosts warming. In fact, scientists estimate that overall, it makes up 5 to 10 percent of annual global CO2 emissions.
