The boreal forest is burning more and more, and it's a problem for the climate

A total of 196 913 hectares burned in Yukon in 2022. On average, this total is just over 189,000 hectares. (File photo)

The number of fires in the boreal forest, the ring of green that encircles the Arctic, has been on the rise for two decades and the year 2021, in particular, has been exceptional in releasing a record amount of CO2 into the atmosphere, according to a study published Thursday.

These fires are fueled by drier and hotter conditions, caused by climate change. By releasing greenhouse gases into the atmosphere, these fires in turn contribute to global warming in a vicious circle.

This finding undermines efforts to tackle climate change, say researchers.

The boreal forest, found notably in Siberia, northern Canada and Alaska, is the largest wilderness in the world. However, the study regrets that it has so far not received the same attention as the damage to the rainforest.

Or , it releases 10 to 20 times more carbon per unit area burned than other ecosystems.

In 2021, boreal fires emitted some 480 million tonnes of carbon , a far larger amount than any other year studied by researchers, between 2000 and 2020.

This amount of emissions is about double the emissions from aviation in 2021, or the fossil fuel emissions of Japan, the fifth highest emitting country.

In the Northwest Territories, nearly 600 000 hectares burned last year. Lightning remains the main source of these fires.

These fires increase the concentration of CO2 in the atmosphere and contribute to global warming, which in turn increases the likelihood of devastating fires in the future, explained to AFP Bo Zheng, lead author of this study, published in the journal Science.

In 2021, boreal forest fires accounted for 23% of global fire-related emissions, rather than the usual 10%.

The researchers explain this anomaly by droughts that occurred simultaneously in the north of the American continent and in Eurasia, that year.

For their estimates, the scientists used a new method. They did not rely, as usual, on satellite data visually assessing the burned areas, the resolution of which is not precise enough, according to them.

This technique also forces to evaluate the CO2 emitted by making assumptions about the amount of vegetation burned in each area, or even the degree of combustion, as explained by Philippe Ciais, co-author of the study and researcher at Paris-Saclay University, during a press conference.

Clouds of smoke caused by wildfires can be seen from satellites, here in 2019.

Instead, they used data from a directly observing satellite carbon monoxide (CO) in the atmosphere. This has a shorter lifespan (a few weeks or months) than CO2. Carbon monoxide emitted from fires has spatiotemporal distributions distinct from other sources, allowing it to be identified.

They thus determined a significant upward trend in emissions over the boreal region, mainly in July and August, the study describes.

L' The Arctic is warming up much faster than the rest of the planet, which increases the soil water deficit, with evaporation and therefore increased air humidity, causing an increase in lightning, and all this leads to a greater fire hazard, says Bo Zheng.

Typically, about 80% of the carbon released by forest fires is then reabsorbed by vegetation that regrows the following season. Some 20% of the carbon emitted remains in the atmosphere, however, contributing to the build-up of CO2.

In addition, the more fires there are, the less the vegetation has time to grow back and an increasing part of these emissions may not be reabsorbed.

Burnt trees about 50 kilometers outside of Wrigley, Northwest Territories, the July 12, 2022.

This study contributes to the growing body of evidence indicating that forest and tundra fires are becoming larger and more frequent in the higher latitudes of the northern hemisphere, said for AFP David Gaveau, who studies forest fires, but was not involved in this work. This situation is worrying for the future.

So what to do? Already, monitoring the situation very closely in these regions, said Steve Davis, also a co-author of the study.

Other studies have shown that it could be worthwhile, in terms of dollars per ton of CO2 avoided, to send firefighters to put out these fires instead of letting them burn as is the case. current case, he said.

We cannot afford not to worry about these natural areas, concludes for his part Philippe Ciais

A text by Lucie Aubourg