New research on the Australian bushfires in 2019 and 2020 has shown that almost one million metric tons of smoke rose into the stratosphere, causing it to warm by about 1°C for six months.
The research also revealed that the bushfires likely contributed to the large and persistent ozone hole that formed over Antarctica during the Southern Hemisphere’s spring. The December 2019 to January 2020 Australian wildfires were the most devastating in Australian history.
Led by Pengfei Yu, a former CIRES scientist at NOAA’s Chemical Sciences Laboratory, a team of researchers used a climate model to study the transport, microphysics, chemistry, and climate impacts of smoke from the bushfires. The study explored how extreme volumes of wildfire smoke can cause persistent impacts to the dynamics and chemistry of the stratosphere.
“Understanding stratospheric aerosols is crucial to understanding our climate,” said Yu, now at the Institute for Environment and Climate Research at Jinan University in Guangzhou, China. “These massive high-altitude clouds of smoke serve as excellent opportunities to constrain and test our climate model for various purposes: understanding past, present and future climate associated with aerosols, as well as simulating the efficacy and climate implications of solar management strategies.”
Wildfire smoke particles are mainly composed of organic carbon and black carbon. When black carbon particles absorb sunlight, the particles and the surrounding air heat become buoyant, forcing that heated air parcel to loft to higher altitudes. The higher an air parcel rises, the longer it stays in the stratosphere, allowing the smoke aerosols to persist for months.
Observations made by instruments on the International Space Station and on the NOAA/NASA JPSS satellite show that large amounts of smoke were lifted to 22km in altitude within the first three months following the smoke injection.
Injecting particles in the stratosphere has been proposed as a way to reflect sunlight high in the atmosphere to reduce solar heating at the surface.
Particles similar to those emitted by large volcanoes are good reflectors of sunlight, and do not absorb a lot of solar energy. Smoke particles reflect sunlight, but they also absorb solar energy so they warm up the air very efficiently. In this case, the smoke reflected incoming sunlight, but did not produce a measurable cooling effect on the troposphere. It would take much more smoke to cool the troposphere than these fires injected, and that amount of smoke would have very large impacts on the stratosphere and the ozone layer, researchers said.
With this in mind, the modelling study estimated that chemical reactions initiated by the injected smoke caused a 4-6% loss in the protective ozone layer over Antarctica from August to December. The 2020 ozone hole was the third largest in the past decade and 18% larger than the 2010-2019 average.
Co-author Karen Rosenlof, part of a team of researchers from NOAA’s Chemical Sciences and Global Monitoring Laboratories, CIRES, NCAR and the University of Colorado who worked on the study, said that it demonstrates that scientists are now able to model how smoke gets high into the stratosphere, where it can reside for a long time.
“What we don’t understand well are possible impacts on surface weather and climate,” said Rosenlof. “More work is needed to understand these feedbacks, because this is somewhat of a natural analog for impacts that could be caused by smoke-like aerosols that are emitted in the stratosphere from rockets, aviation, or by potential future geoengineering projects.”
To read the full study, click here.