A team of US scientists has used modelling to find that Australia’s 2019–20 Black Summer bushfires had a more dramatic short-term effect on the Earth’s climate than the 2020 lockdowns caused by the COVID-19 pandemic.
Published in Geophysical Research Letters, the research suggests that while the COVID-19 lockdowns had a slight warming effect that will be experienced up to 2022, the bushfires had a planetary cooling effect that was apparent in months.
In the long term, CO2 emissions from both industrial sources and bushfires warm the planet’s atmosphere, but over months or a few years, excess particles from pollution and smoke can cool the Earth’s surface by seeding clouds, which reflect energy from the sun back into the atmosphere.
This meant that the lockdowns associated with the COVID-19 pandemic had a short-term warming effect, as industry and transport slowed and fewer particulates went into the atmosphere. Conversely, smoke from the bushfires had a cooling effect across the Southern Hemisphere.
“We’ve theorised that the climate system responds this way to major volcanic eruptions,” says John Fasullo, a scientist at the National Center for Atmospheric Research (NCAR) in the US and lead author on the paper.
“But those tend to happen every 30 years or so. In contrast, major wildfires can occur every couple of years and therefore have more recurring impacts. We clearly need to learn more about how they affect global climate.”
The researchers estimated the change in emissions from both the pandemic and the bushfires. They then used a supercomputer to run simulations on an earth systems model that had been developed at NCAR, examining temperature differences over a decade-long period (2015 to 2024, thus seeing how events in 2019–20 would change things).
They found that the lockdowns were likely to produce a slight warming effect, creating an average rise in global temperature of 0.05°C by late 2022. Conversely, the bushfires cooled the planet by roughly 0.06°C by mid-2020.
“The main climate forcing of 2020 wasn’t COVID-19 at all,” says Fasullo. “It was the explosion of wildfires in Australia.”
“In climate sciences, there’s been a lot of interest in simulating the impact of COVID lockdowns on emissions,” says Dr Jatin Kala, a senior lecturer in climate science at Murdoch University, who was not involved with the study.
“The reduction in emissions from COVID lockdowns is just a small blip.”
He adds that the paper also contributes to the body of literature showing that the Black Summer fires had a profound impact on the atmosphere.
“What this paper is showing is that if you inject aerosols into the atmosphere, it creates an almost instant cooling effect. But it’s a very short and sharp cooling. You can see from the paper the radiative forcing goes down and goes back up straight away.”
This has interesting implications for geoengineering: specifically, the idea that particles like sulfur dioxide could be sprayed into the atmosphere to seed clouds, reflect sunlight and cool the earth.
“What this paper indirectly shows is that if you want to do that, you will need to put a lot of sulfur in the atmosphere over a long period of time to keep sustaining that bit of cooling,” says Kala.
“It would have consequences that would be very difficult to plan for or predict.”
Ellen Phiddian is a science journalist at Cosmos. She has a BSc (Honours) in chemistry and science communication, and an MSc in science communication, both from the Australian National University.
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