As human-made climate change brings more heat and drought, wildfires are burning longer, hotter and more often in forests from Canada to Greece and Australia.

Beyond the devastating loss of biodiversity, these unprecedented infernos are releasing carbon, a powerful, planet-heating greenhouse gas.

Forests store, or sequester, a lot of carbon. When trees and plants photosynthesize, they draw carbon from the atmosphere into their leaves, roots and down into the soil. The older forests get, the more carbon they store.

Though these forests burn naturally, much more sequestered carbon is released back into the atmosphere during extreme fires. This creates a "feedback loop," meaning climate change fuels wildfires that in turn fuel climate change.

This is a problem since 30% of the human carbon emissions driving global heating are absorbed by land-based ecosystems that are mostly forests, noted Kristina Dahl, principal climate scientist for the Climate & Energy program at the Union of Concerned Scientists. 

Recovering forests do sequester carbon again, but full regeneration of these carbon stores is less likely after "high-severity" fires, she told DW.  

Increasing wildfire pollution puts squeeze on carbon budget  

The 2023 Canadian wildfires have already emitted twice as much carbon as the previous record, according to Europe's Copernicus Atmosphere Monitoring Service (CAMS) — which has been monitoring wildfire emissions since 2003. Meanwhile, the burned area now stretching from Canada's east to west coast and deep into the north, is around double the previous record, which was set in 1989.

The area of forest burned globally has actually declined in recent decades, due in part to a shift away from open burning agricultural practices in tropical areas in Africa, notes CAMS senior scientist Mark Parrington.

"But once we get outside of the tropics, there has been increasing 'extreme fire,'" he said. As fires burn more intensively in hotter, drier conditions, Parrington describes a rising incidence of "pyroconvention" events whereby self-generating fire storms and lightning compound the inferno. 

A 2022 reportby the UN Environment Programme (UNEP)  predicted that, due to climate change, extreme fires would rise 30% by 2050 and 50% by century's end. 

While the world's forests sequestered about twice as much carbon as they emitted between 2001 and 2019, wildfire emissions are set to put pressure on a fast-shrinking carbon budget.

This refers to the quantity of greenhouse gases humanity can burn and still have a 50% chance to limit warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit), as agreed by world leaders in Paris in 2015. Climate think tank Carbon Brief predicts that at current emissions rates, the CO2 budget of 380 billion tons will be blown in just nine years.

Extreme fire years in 2017 and 2018 in the western Canadian province of British Columbia emitted three times more greenhouse gases in the region than all other sectors combined, including energy, noted Carly Phillips, a research scientist with the Science Hub for Climate Litigation at the US-based Union of Concerned Scientists.

Meanwhile, the 2020 Californian wildfires were so intense that carbon dioxide equivalent emissions were roughly double California's total greenhouse gas reductions since 2003, according to one study.

Discounting future vegetation regrowth, CO2 emissions from the 2020 Californian wildfires were the second biggest source of carbon in the state, coming in above both industry and electric power generation, according the researchers.

Vital carbon stores becoming sources of carbon

The world's boreal forest found across the northern hemisphere — nearly 30% of which is in Canada — holds around 11% of terrestrial or above-ground carbon, making it the planet's biggest carbon sink of its kind. 

But researchers have shown how some of these forests had, by 2016, already become a carbon source due to wildfires that burn the trees, peat and soil where carbon is stored. This includes the carbon produced when we burn fossil fuels, which accounts for over 90% of total carbon dioxide emissions in the US, for example.

Significant uncertainty remains about how forest ecosystems will store carbon as temperatures rise, said Kristina Dahl of the Union of Concerned Scientists.

She told DW that more CO2 in the atmosphere through fossil fuel emissions, for example, can stimulate more plant growth and encourage carbon sequestration.

But a drier and hotter climate that increases wildfire intensity will cancel out any such benefit and risks turning land into "a net carbon source," she said.

This will, in turn, reduce the so-called sink strength of forests, meaning they can reduce a forest's capacity to pull carbon out of the atmosphere.

"Severe fires can inhibit forest regrowth and can change the species composition of the forest," wrote Carly Phillips regarding boreal forest fires. "Altogether, wildfires increase the amount of carbon leaving forests and can decrease the amount coming in."  

The best way to tackle rising wildfire carbon emissions?

Rapidly cutting planet-heating fossil fuel emissions is the ultimate way to reduce wildfires, and their climate impacts, says Kristina Dahl.

The researcher also advocates for better forest management, including reducing fuel loads through thinning and controlled burning depending on local conditions and with a view to limiting carbon emissions and further biodiversity loss. 

"This would help to reduce severe wildfires in which the carbon-rich layers of soil are burned," Dahl added.

Amid declining forest resilience to wildfires, including a decrease in forest regeneration since 2000, Dahl also suggests encouraging more climate-resilient species that "will thrive under the current and future climate."

Edited by: Jennifer Collins