Australian wildfire smoke had dramatic impact on global climate in 2020 - The Washington Post

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The 2019 to 2020 Australian wildfire season was historic. More than 42 million acres burned in an unprecedented outbreak of extreme fires, which produced lightning, launched smoky aerosols into the stratosphere and turned New Zealand’s glaciers brown with ash. The suffocating smoke was blamed for hundreds of deaths.

Now a new study publishedin Geophysical Research Letters suggests that the fires’ impact may have spread around the globe. Researchers with the National Center for Atmospheric Research (NCAR) in Boulder, Colo., found that smoke produced by the fires cooled the global climate and had a greater impact than the change in emissions stemming from covid-19 lockdowns, which had a small warming influence.

“Beyond their effect on local weather, wildfires are becoming large enough, and intense enough, to have a material effect on climate,” said John Fasullo, the lead author of the study. “In this work, we demonstrate their potential to influence climate variability. We are still in the process of understanding other aspects.”

Fasullo and his colleagues concluded that the 2019-2020 Australian wildfire season resulted in 0.1 degrees Fahrenheit of cooling by mid-2020. The cooling, however, was tacked atop a continued net warming of the climate and had a negligible effect on slowing the pace of human-induced climate change from fossil fuel burning.

The team’s assessment stemmed from the outputs of a computer model ensemble, which simulated global temperatures in the 2015 to 2024 time frame under the same background conditions but with and without emissions from the biomass burned in Australia’s wildfires. A similar 2019 to 2024 model with 50 ensemble members was run to handle covid-era emission data. An ensemble member is an individual model run with subtly-tweaked inputs; together, ensembles help capture a range of possibilities.

Wildfires are a common occurrence in Australia, known there as bush fires. They develop during the summertime and are most common in dry inland areas.

Amid the 2019 to 2020 wildfire season, however, blazes lined the eastern periphery of the country and, in some areas, burned their way all the way toward the coast.

Wildfires developed and grew in areal coverage and intensity in September 2019. By December, nearly 100 fires were burning across New South Wales alone, charring an area 1.5 times the size of the state of Connecticut. An “out of control,” “mega fire” complex less than 40 miles west of Sydney shrouded the city of 5 million in a toxic veil of smoke. Air quality indexes plummeted as skies turned orange. Beaches turned black as ash washed ashore.

The most intense wildfires produced pyrocumulonimbus clouds, or enormous plumes of smoke, ash and other aerosols, like sulfur dioxide, that towered to heights of up to 19 miles. Those fire-induced mushroom clouds ejected aerosols into the stratosphere, which surfed the jet stream eastward and gradually dispersed. The result was a volcano-like cooling of the global climate.

Smoke from wildfires has a range of effects on the Earth’s climate. Black carbon actually absorbs solar radiation and heats the atmosphere, while other organic materials reflect or scatter light, yielding cooling. Smoke occasionally helps seed cloud development, blocking incoming sunlight but also retains outgoing heat. Sulphur dioxide aerosols can be converted into sulfuric acid, blocking incoming sunlight and also contributing to the destruction of ozone. Researchers noticed a “localized stratospheric ozone-hole.”

The NCAR team used complex computer modeling to estimate the wildfires reduced solar input by about 3 Watts per meter squared over the midlatitudes of the southern hemisphere, or one Watt worldwide. Picture a lightbulb over every square meter of Earth, and then dim the bulb by about 1 Watt. That, they state, corresponds to a temperature drop of 0.1 degrees.

Quantifying global temperature response due to fallout of covid-19 was a bit trickier due to the range of pollutants and feedback mechanisms influenced by the pandemic.

Covid-19 sparked a marked drop-off in travel, manufacturing and commerce that peaked in March of 2020. While that meant a lesser emission of greenhouse gases, which warm the climate, cleaner air in places like Asia yielded a reduction in areal coverage of smog and haze. That comparative absence of smog, which ordinarily reflects some sunlight back into space, may have produced a warming influence.

Experts were able to attribute about a third of East Asia’s observed uptick in sunlight to reductions in pollution.

Overall, the NCAR team thinks that 0.09 degrees of warming may have resulted globally from the covid-19 shutdowns. The effect should stretch into the end of 2022.

Among the collateral impacts of the fires may also have been a northward shift of the Intertropical Convergence Zone (ITCZ), or a meandering belt of thunderstorm activity anchored over the tropics that follows the sun north and south of the equator. The ITCZ generates tropical waves that often grow into named storms or hurricanes.

The northward shift occurred because the fires disproportionately cooled the southern hemisphere, prompting the storms to saunter in the direction of comparative warmth. The 2020 Atlantic hurricane season featured a record 30 named storms.

The findings offer insight that could help atmospheric scientists better understand the role wildfires play in the Earth’s climate system. Meanwhile, an outbreak of wildfires continues to burn in the western United States as North America inches closer to peak wildfire season.

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Australian wildfire smoke had dramatic impact on global climate in 2020 - The Washington Post
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