“Cloud flyers” influence our climate

NASA once referred to pyrocumulonimbus (PyroCb) clouds as “fire-breathing cloud fliers.” Even a ‘cumulonimbus’ cloud is impressive, with its massive, anvil-shaped thunderstorm tower, often more than ten kilometers high, the US space agency writes in an article. Add “pyro” to that and you have an explosive thundercloud created by the smoke and heat of the fire that can blow the smoke into the stratosphere like a chimney.

Soot particles in the atmosphere are not broken down
The soot particles and organic aerosols thrown up by the “cloud-flyer” are not broken down in the upper troposphere and lower stratosphere for a long time. They make up between 10 and 25 percent of the aerosols there, writes the international research team led by Joshua Schwarz of the US climate research agency NOAA, which also includes Bernadett Weinzierl, Maximilian Dollner and Agnieszka Kupc of the Aerosol Physics and Environmental Physics research group. at the University of Vienna in the journal Science.

In the upper atmosphere, aerosol particles can have different effects. They can contribute to the warming of these air layers, but they can also reflect sunlight. PyroCb has therefore been observed for years, but the effects on climate are still unclear.

In August 2017, as part of NASA’s Atmospheric Tomography Mission (ATom) to study the atmosphere over the oceans, a research aircraft flew through a PyroCb cloud caused by multiple wildfires in Washington State and British Columbia (Canada). Using the recorded data, the researchers showed “that this was the largest PyroCb event observed to date in the satellite era,” as they write in their work.

Dubbed the “Pacific Northwest Event,” this event accounted for only a small portion of the area devastated by wildfires in the Northern Hemisphere in 2017 and was responsible for only about five percent of the resulting CO2 emissions. A year later, however, about 40 percent of the soot particles in the lower stratosphere of the Northern Hemisphere were still due to this fire cloud.

Clouds of fire with fingerprints
To the scientists, the “extremely thick layers” that had accumulated on the soot particles were surprising and remarkable. This result was “so unexpected” that they conducted further tests to confirm it. This “coating”, along with the size and mass of the soot particles, is a stable feature of PyroCb smoke and can be used as a “fingerprint” of the particles emanating from this source in the lower stratosphere.

Using these “fingerprints,” they analyzed data from twelve airborne missions in the northern and southern hemispheres since 2006 to estimate PyroCb’s long-term impact on the lower stratosphere. They showed that even in years with only a few “fire-breathing cloud flyers”, the influence of the smoke was very large.

Effects still poorly understood
The researchers advocate mapping major PyroCb events into atmospheric models “to capture both the annual stratospheric variations at this level and the larger short-term radiation effects.” Because PyroCb “contributes more to the composition of the stratosphere than we thought, acts in a different way than we thought and lasts longer than we thought,” the researchers said.

Not only is this important given the expectation that the frequency, size and geographic distribution of bushfires will increase, the researchers also point to Australia’s 2019 and 2020 PyroCb “super outbreak” which sent three times more smoke into the stratosphere than the ” Pacific Northwest Events”. A better understanding also helps to assess the long-term effects of a possible use of aerosol geoengineering to combat climate change.