The severe, devastating wildfires that raged across southeastern Australia in late 2019 and early 2020 packed a powerful punch that extended far beyond the country, two new studies find.
The blazes injected at least twice as much carbon dioxide into the atmosphere as was previously thought, one team’s satellite-derived estimates revealed. The fires also sent up vast clouds of smoke and ash that wafted far to the east over the Southern Ocean, fertilizing the waters with nutrients and triggering widespread blooms of microscopic marine algae called phytoplankton, another team found. Both studies were published online September 15 in Nature.
Meteorologist Ivar van der Velde of the SRON Netherlands Institute for Space Research in Leiden and colleagues first examined carbon monoxide data collected over southeastern Australia by the satellite-based instrument TROPOMI from November 2019 to January 2020, during the worst of the fires. Then, to get new estimates of the carbon dioxide emissions attributable to the fires, the team used previously determined ratios of carbon monoxide to carbon dioxide emitted by the region’s eucalyptus forests — the predominant type of forest that was scorched in the blazes — during earlier wildfires and prescribed burns.
Van der Velde’s team estimates that the fires released from 517 trillion to 867 trillion grams of carbon dioxide to the atmosphere. “The sheer magnitude of CO2 that was emitted to the atmosphere … was much larger than what we initially thought it would be,” van der Velde says. The emissions “from this single event were significantly higher than what all Australians normally emit with the combustion of fossil fuels in an entire year.”
Previous assessments of CO2 emissions from the fires, based on estimations of burned area and biomass consumed by the blazes, calculated an average of about 275 trillion grams. Using the satellite-derived carbon monoxide data, the researchers say, dramatically improves the ability to distinguish actual emissions from the fires from other background sources of the gases, giving a more accurate assessment.
That finding has worrisome implications. The fires swiftly cut a swath through southeastern Australia’s eucalyptus forests, devastating the forests to a degree that made their rapid recovery more difficult — which in turn affects how much carbon the trees can sequester, van der Velde says (SN: 3/9/21). Fires in northern and central Australia’s dry, grassy savannas are seen as more climate neutral because the grasses can regrow more quickly, he says.
And severe fire seasons are likely to become more common in southeastern Australia with ongoing climate change. Climate change has already increased the likelihood of severe fire events such as the 2019–2020 fire season by at least 30 percent (SN: 3/4/20).
The smoke and ash from the fires also packed a powerful punch. Scientists watched in awe as the fires created a “super outbreak” of towering thunderclouds from December 29 to December 31 in 2019 (SN: 12/15/20). These clouds spewed tiny aerosol particles of ash and smoke high into the stratosphere.
Aerosols from the fires also traveled eastward through the lower atmosphere, ultimately reaching the Southern Ocean where they triggered blooms of phytoplankton in its iron-starved waters. Geochemist Weiyi Tang, now at Princeton University, and colleagues analyzed aerosols from the fires and found the particles to be rich in iron, an important nutrient for the algae. By tracing the atmospheric paths of the cloud of ash and smoke across the ocean, the team was able to link the observed blooms — huge patches of chlorophyll detected by satellite — to the fires.
Researchers have long thought that fires can trigger ocean blooms, particularly in the Southern Ocean, under the right conditions, says marine biogeochemist Joan Llort, now at the Barcelona Supercomputing Center and a coauthor on the study. But this research marks the most direct observation ever made of such an event — in part because it was such a massive one, Llort says.
Large ocean blooms are “yet another process which is potentially being modified by climate change,” says biogeochemist Nicolas Cassar of Duke University, also a coauthor on the study.
One of the big questions to emerge from the study, Cassar adds, is just how much carbon these phytoplankton may have ultimately removed from the atmosphere as they bloomed. Some of the carbon that the algae draw out of the air through photosynthesis sinks with them to the seafloor as they die. But some of it is quickly respired back to the atmosphere, muting any mitigating effect that the blooms might have on the wildfire emissions. To really assess what role the algae play, he says, would require a rapid-response team aboard an ocean vessel that could measure these chemical processes as they are happening.
The sheer size of this wildfire-triggered bloom — “larger than Australia itself” — shows that “wildfires have the potential to increase marine productivity by very large amounts,” says Douglas Hamilton, a climate scientist at Cornell University who was not connected with the study.
“The impact of fires on society is not straightforward,” Hamilton adds. The same smoke that can cause severe health impacts when inhaled “is also supplying nutrients to ecosystems and helping support marine food webs.” What this study demonstrates, he adds, is that to understand how future increases in fire activity might help shape the future of marine productivity “it is crucial that we monitor the impacts closely now.”
sciencenews.org, 15 September 2021