The increasing frequency of wildfires across the Arctic is having a more substantial impact on global warming than previously understood. While initial assumptions suggested that primarily recent vegetation was burning, soil core studies reveal that these fires are igniting ancient carbon deposits accumulated for over 5,000 years.

“Soil combustion has the potential to release long-term stored carbon from soil, which was previously considered a carbon sink,” explains Meri Rappel from the Finnish Meteorological Institute in Helsinki. Current climate models neglect the release of this ancient carbon.

In the cold conditions of the Arctic, plant growth is slow, leading to the accumulation of organic matter in the soil as peat and other forms over centuries or even millennia. This factor positions Arctic and adjacent boreal soils as significant carbon sinks, which effectively remove carbon dioxide from the atmosphere.

However, increasing wildfire incidents are changing this dynamic. Rappel’s research team has collected soil cores from recently burned areas to study the impact of these fires.

Their findings indicate that while surface vegetation may burn quickly, the underlying organic material smolders for a longer duration, releasing considerable amounts of soot and carbon dioxide into the atmosphere.

Black carbon, a byproduct of these fires, absorbs sunlight, contributing directly to atmospheric warming. Moreover, in colder regions, black carbon can accumulate on ice and snow, accelerating melting processes that would otherwise not occur.

“We discovered that the age of the carbon released during fires varied significantly depending on soil depth and burn intensity,” Rappel stated during the European Geosciences Union Conference in Vienna.

The risk of releasing ancient carbon is particularly high as it tends to increase toward the North Pole, where organic matter accumulates close to the surface. For instance, in Canada’s Northwest Territories, fires are penetrating soil just a few centimeters deep, unleashing carbon stored for up to 400 years.

In Greenland, fires can consume up to 10 centimeters of soil, releasing carbon that is over 560 years old, with some areas experiencing burns of up to 15 centimeters, releasing carbon that has been stored for 1,000 years.

Remarkably, a boreal forest site in Quebec, Canada, has been identified where fires released carbon dating back 5,000 years. “However, this occurrence is not widespread,” said Ruppel at the conference.

The critical question remains: how much ancient carbon is currently being released by wildfires? Rappel emphasizes that this study is merely the starting point, indicating the need for further research to quantify the released carbon.

“Rappel’s work is vital and underscores the urgency,” noted Sandy Harrison from the University of Reading. “It’s evident that substantial old carbon exists in high-latitude soils and peat. As new fire regimes evolve, destroying topsoil layers and peatlands, this ancient carbon will be released into the atmosphere.”