Huge wildfire clouds affect stratosphere and climate
The Williams Flats Fire in northeastern Washington state generated a fire cloud, or pyroCb, that injected smoke into the stratosphere on August 8, 2019. A NASA plane flew up to investigate. (David Peterson, NRL)
Images of vast clouds of smoke from wildfires rising into the sky have become all too familiar over the past few years of record blazes in the western United States and elsewhere. Now, a team of atmospheric scientists have demonstrated that these plumes have major impacts on the stratosphere and climate. The results were right published in the journal Science.
Large forest fires can send plumes of smoke to immense heights. These towering clouds, called pyrocumulonimbus or pyroCbs, are generated when the intense heat of a wildfire ignites a massive thunderstorm that carries smoke into the stratosphere, five to seven miles above the surface.
In 2017, a US airborne mission to study the atmosphere over distant oceans interspersed with smoke from a huge pyroCb event in the Pacific Northwest. The smoke was so widespread that remote sensing instruments around the world monitored it for more than eight months. Measurements showed that it and several other pyroCb events in the Northern Hemisphere that year dominated black carbon and organic carbon contributions to the lower stratosphere, exceeding human emissions from vehicles, industry, heating, cooking and agricultural clearing. The net effect was to cool the planet.
“Fire-triggered thunderstorms are getting bigger and more frequent — witnessing record-breaking events in 2017, 2019 and 2020,” said study co-author Joshua Schwarz of the National Oceanic and Atmospheric. United States Administration, which conducted the research. “Their recent impacts on the stratosphere have been impressive.”
“We know that large fires emit a lot of aerosols. But we didn’t know that these aerosols could be injected so high into the atmosphere,” said the atmospheric chemist and co-author of the study. Roisin Commane from the Columbia Climate School Lamont-Doherty Earth Observatory. Commane participated in the long-range airborne missions over the oceans that captured aerosol data.
Scientists are interested in learning more about pyroCbs because their smoke lingers in the atmosphere longer than that of typical fires. The research should also provide information on the behavior of aerosols from volcanoes, aviation, or potential future solar geoengineering efforts.
Critical measurements of these huge clouds have so far been extremely limited due to their highly episodic nature and the logistical challenges of sending scientific instruments into the air and smoke on short notice. As a result, the smoke distribution and duration of these events is poorly known, as is the overall impact on climate and stratospheric aerosol chemistry, including the ozone layer.
A surprising finding from the study was the discovery of an extremely thick coating of other chemicals on solid particles of black carbon generated by forest fires. Scientists aren’t entirely sure how the grimy coating forms, but it appears to be made up of organic aerosols and gases that condense on the particles as the smoke plume rises and cools rapidly in the air. atmosphere, said study co-author Kara Lamb from the Data Science Institute at Columbia.
A more recent airborne mission provided direct sampling of hours-old pyroCb smoke from the 2019 Williams Flats Fire in northeast Washington State. Analysis of carbon black from this fire gave researchers more confidence in conclusions drawn from the 2017 smoke.
The thick coating on the black carbon particles, as well as their size and mass, was a remarkably stable feature of pyroCb smoke that the researchers realized could be used to “fingerprint” these particles in the bass. stratosphere. Using these fingerprints, they re-examined data from a total of 12 datasets from previous airborne missions dating back to 2006 to estimate the long-term influences of pyroCb in the lower stratosphere on recent climate.
They found that even in those years with relatively little pyroCbs before more active fire seasons beginning in 2020, the impact of smoke was significant, contributing about 20% of all stratospheric black carbon and carbon organic matter in the lower stratosphere during the previous decade.
“It gave us a reasonable estimate representing the period before things really started to lighten up,” Schwarz said. “We now recognize the longer term influence of pyroCbs on the stratosphere. It’s not just a big hit, but a steady state influence that needs to be taken into account.
Adapted from a National Oceanic and Atmospheric Administration press release.
