Summer wildfires in the Northern Hemisphere's boreal forests have been expanding over the past two decades and produced record high carbon dioxide emissions in 2021, according to a study published on Thursday.
"Boreal forests could be a time bomb of carbon, and the recent increases in wildfire emissions we see make me worry the clock is ticking," said Steven Davis, one of the authors of the study published in the journal Science.
Davis, a professor of Earth system science at the University of California, Irvine, said the boreal forests have attracted less attention than tropical forests.
Boreal forests cover parts of the high-latitudes of eight countries including Canada, Russia and the United States and constitute one of the most extensive biomes on Earth. They are generally made up of pine, spruce and fir trees.
The growth in wildfires in boreal forests is linked to warming in the region, which is the fastest on Earth.
The greenhouse gases released into the atmosphere by the boreal forest fires contribute to the warming of the planet and have alarmed climate scientists.
The researchers said boreal fires typically account for 10 percent of global fire carbon dioxide emissions.
But they contributed 23 per cent in 2021, or 480 million metric tons of carbon release, equivalent to 1.76 billion tons of CO2.
That was the most recorded by the researchers in a single year since 2000 and about double the total emissions from aviation in 2021.
It was approximately equivalent to the fossil fuel emissions from Japan, said the main author of the study, Bo Zheng of Tsinghua Shenzhen University in China.
The researchers said 2021 was an "abnormal" year with boreal forests in North America and Eurasia both experiencing severe drought.
Philippe Ciais, a climate researcher at Paris-Saclay University, said there is a risk of a sort of vicious circle.
"If the fires become more frequent the forest won't have time to recover carbon," he said. "We will have an amplification effect on climate warming."
Normally, 80 percent of the carbon released by wildfires is absorbed by vegetation in subsequent seasons while the remaining 20 percent of CO2 stays in the atmosphere.
"The increase in fire emissions poses a widening threat to climate, given that part of the emissions might not return to vegetation and soils because of postfire regrowth failure in a warming climate," the study said.
Davis said this is "really worrisome."
"If this ecosystem stops being such an effective carbon sink and becomes a larger source of CO2...that makes our job of stabilizing the climate much harder," he said.
Davis said some studies have suggested using firefighters to put the fires out instead of just letting them burn in the remote, unpopulated areas where they are happening.
"It could be cost effective in terms of dollars per ton of CO2 emissions avoided to go and send firefighters to try to stop these fires," he said.
Whatever the solution, Ciais said "we cannot afford not looking at these natural areas. We need to know what's happening over them."
For the study, the researchers used satellite data to detect carbon monoxide concentrations from boreal wildfire emissions and then converted them to CO2 emissions based on wildfire combustion conditions.