Australian wildfires caused twice as much CO2 as previously estimated

Vast clouds of dust and ash emitted by fires fuelled enormous algal blooms in Southern Ocean

A satellite image shows smoke from the 2019-20 Australian wildfires covering parts of the Southern Ocean (Japan’s National Institute of Information and Communication Technology)

Australia’s 2019-2020 bushfire season was unprecedented, even in a country used to seasonal wildfires.

Over 46 million acres of land was burnt, 36 people were killed, and another 445 died indirectly, following smoke inhalation. An estimated 3 billion animals were killed or displaced, and the fires destroyed almost six thousand buildings.

They were also known to have released massive amounts of carbon dioxide into the atmosphere, but estimates of the total emissions have been uncertain.

However, a new study using high-resolution satellite measurements atmospheric gases, suggests as many as five previous estimates of the CO2 output from the fires only accounted for less than half of the total emissions.

It is now believed the bush fires, known as the “Black Summer” fires”, released 750 terragrams (750 million tonnes) of CO2 into the atmosphere between November 2019 and January 2020.

This figure surpasses Australia’s normal annual fire and fossil fuel emissions by 80% per cent, the scientists said.

The research team said the Australian wildfires were so large, they still overshadow recent fires in Europe and California.

Meteorologist Dr Ivar van der Velde from the Netherlands Institute for Space Research told The Independent: “A unique feature of the Black Summer fires is that they raged in eastern Australian eucalyptus forests where we usually don’t see these kinds of large fires.

“In terms of size, these fires were of a different order of magnitude than other fires in temperate regions. The magnitude of wildfires in California, and the Mediterranean is really a lot smaller. For these kinds of big events you normally have to go to the boreal region and even there it would be a big fire year.”

Fires regularly occur in the savannahs of Northern Australia, where over the course of the year, the CO2 emissions are largely balanced out by vegetation regrowth. But the scale and location of the Black Summer fires means the vast amount of CO2 emitted will not be counteracted by regrowth on a short time scale, and effectively adds to the CO2 burden in the atmosphere worsening the climate crisis.

Dr van de Velde said: “The question that now arises is what will happen to this CO2 in the long term. The fires were so large that rapid recovery of the affected forests is more difficult, therefore it is likely that part of the emitted CO2 won’t be compensated quickly by CO2 uptake during regrowth.

“Given current trends in global warming, we believe it is quite possible that we will see more of these types of large wildfires in Australia, and possibly elsewhere. This will likely contribute to even more CO2 in the atmosphere than expected.”

This process could lead to a damaging positive feedback loop, Dr van der Velde said.

“The spike of very high CO2 emissions in a short time window might take many years before it is compensated by vegetation regrowth. Leaving excess CO2 in the atmosphere, contributing to global warming, droughts and thus more fires.”

In addition to carbon emissions, the expansive clouds of smoke and ash from the fires also triggered widespread algal blooms in the Southern Ocean, thousands of miles downwind to the east, a sister study, led by Duke University in Australia has revealed.

The international research team said their study shows that tiny aerosol particles of iron in the windborne smoke and ash fertilised the water as they fell into it, providing nutrients to fuel blooms at a scale unprecedented in that region.

They said this discovery raises “intriguing new questions about the role wildfires may play” in spurring rapid growth of microscopic marine algae known as phytoplankton.

The algae absorb large quantities of climate-warming carbon dioxide from Earth’s atmosphere through photosynthesis and are the foundation of the oceanic food web.

“Our results provide strong evidence that pyrogenic iron from wildfires can fertilise the oceans, potentially leading to a significant increase in carbon uptake by phytoplankton,” said Nicolas Cassar, professor of biogeochemistry at Duke University.

The huge algal blooms triggered by the fires were so extensive that the subsequent increase in photosynthesis may have temporarily offset a substantial fraction of the fires’ CO2 emissions, Professor Cassar suggested.

But the scientists said it remains unclear how much of the carbon absorbed by that event, or by algal blooms triggered by other wildfires, remains safely stored away in the ocean and how much is released back into the atmosphere.

Gaining a stronger understanding of the atmospheric burden of CO2 caused by these fires, and what they will cause in the future, is critical for constructing future scenarios of the global carbon balance, the authors said.

The studies are both published in the journal Nature.