This story was originally published by Grist. Sign up for Grist’s weekly newsletter here.
With 600 square miles burned so far, the Park Fire is already one of California’s largest wildfires ever — and it’s far from contained. Driven by strong winds, the fire has chewed through uprooted vegetation, sending smoke high into the atmosphere. So much smoke and rising hot air, in fact, that it was creating firestorms and one of the world’s strangest natural phenomena: the pyrocumulonimbus cloud, or pyroCb.
It’s the smoke cloud that makes wildfires as dangerous as the Park Fire, burning in the northern part of the country, even more unpredictable. PyroCbs can produce lightning that continues to light many fires around the same flame that created the cloud. And as the planet warms, pyroCbs appear to be becoming more common, as they are caused by larger, more aggressive wildfires, which are themselves getting worse. “PyroCbs are very large, almost like a volcano,” said Rajan Chakrabarty, an aerosol scientist who studies clouds at Washington University in St. Louis. “These pyroCbs create their own fire atmosphere.”
The Park Fire has largely grown on a diet of very dry fuel. This part of California hasn’t gotten hot in decades, so a lot of plant life has been created and spent under the summer sun. The very low humidity helped absorb what little moisture was left in the plant, turning the surrounding area into a pile of tinder.
Such a large and powerful fire is a breeding ground for pyroCbs – the wonders of fire physics. As a blaze like the Park fire burns – and burns hotter because of climate change that produces higher temperatures and drier fuels – the flames’ heat rises, sending plumes of smoke tens of thousands of feet into the atmosphere. As air rises, it cools and expands. The water then combines with the smoke particles, and a cloud forms.
Masses of rising air in pyroCb form a kind of void at the ground level, which absorbs more air, producing winds that encourage the spread of flames. The most intense wildfires consume so much oxygen that they are able to extinguish themselves, but pyroCb winds add more gas to the storm. “It’s an ongoing process,” said Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research. “Because the more intense you are, the more oxygen enters, which means the more intense you are, the more oxygen enters. So you can see how that goes.”
At the same time, higher pyroCb can create a downdraft, making the upper winds even more invisible. “Convection creates a lot of turbulence, so it’s very difficult to predict where the air is coming from and where it’s moving,” said Payton Beeler, an atmospheric scientist who studies pyroCbs at the Pacific Northwest National Laboratory in Richland, Washington. That, in turn, leads to chaotic fire behavior, as those winds push the flames around the world at different speeds and in different directions.
PyroCb smoke travels above the fire you created. “Some aerosols that are deposited in the upper troposphere or lower stratosphere tend to stick around for up to six to eight months,” Beeler said. “And they can be transported across hemispheres, basically.”
The dark carbon from the pyroCb cloud doesn’t behave well there, either. In a paper published last week in the journal Nature Communications, Beeler found that black carbon from pyroCb absorbs twice as much sunlight as black carbon from small fires or urban sources, such as coal burning. “PyroCb plume particles tend to have really thick organic matter,” Beeler said, “and that’s different from black carbon in other sources.”
That increases the absorption of light and raises the temperature in the atmosphere. “It’s like a black sweater — it absorbs all the sun and warms the environment around it,” said Chakrabarty, who co-authored the paper with Beeler.
Why this happens in pyroCb clouds, however, scientists do not fully understand. It could be that there is something different about the way wildfires produce pyroCb, or that there is a secondary process going on within the cloud to coat the particles with more organic material. (Organisms in this case come from the burning of plants.)
Another unanswered question is whether pyroCb clouds are becoming more common due to wildfires fueled by climate change, or whether scientists are getting better at detecting them with satellites, or a combination of the two. PryoCb pipelines have been popping up around the world, from Australia to Siberia, as fuel dries up and temperatures rise. The burning monster in Canada this summer is also breeding. “It seems to happen more often,” Beeler said. “Whether that’s a function of climate warming and better detection, I think both are possible. But the effects seem to be long-lasting and take a long time.”
This article first appeared on Grist at Grist is a non-profit, independent media organization dedicated to telling stories about climate solutions and a just future. Learn more at Grist.org
Source link
