Last summer saw unprecedented wildfire activity in Siberia and the Arctic Circle. In June alone, the fires were estimated to have released 50 megatonnes of carbon dioxide into the atmosphere – an amount equivalent to Sweden’s total annual emissions. Yet after this devastating season, embers of the extinguished fires may have continued to smolder underground beneath the winter snow cover.
Following the joint hottest April on record, scientists believe that these “zombie fires” or “overwintering fires” may have reemerged from the snowmelt and ignited new wildfires throughout the dried boreal forests in Arctic regions.
“We have seen satellite observations of active fires that hint that “zombie” fires might have reignited, yet it has not been confirmed by ground measurements,” Mark Parrington, Copernicus Atmosphere Monitoring Service (CAMS) Senior Scientist and wildfire expert, said in a statement. “The anomalies are quite widespread in areas that were burning last summer. If this is the case, then under certain environmental conditions, we may see a cumulative effect of last year’s fire season in the Arctic, which will feed into the upcoming season and could lead to large-scale and long-term fires across the same region once again.”
“Zombie fires” are not completely unknown; indeed there is already evidence to suggest that they occur in Alaska. “Looking at satellite imagery from the end of the summer fire seasons, we can see what appear to be “dead fire” scars left on the landscape from the burns," Dr Thomas Smith, Assistant Professor in Environmental Geography at the London School of Economics and Political Science, told IFLScience. "However, when you return to look at images in the following year, we can see new fires emerging from the edges of what we thought were extinguished “dead” fires.”
Researchers looking into these reported overwintering fires suggest that they “occur more frequently after large fire years in combination with subsequent mild winters and springs leading to an early snowmelt,” and tend to re-emerge 50 days after this regional snowmelt.
However, the only way to confirm a “zombie fire” is by investigating the fires on the ground – something that hasn’t yet been carried out for the present bout. Therefore, despite the overlap of burn scars with current wildfires in the Siberian Arctic circle, scientists cannot verify their origins. “This region is… known for having lots of human ignitions for various traditional agricultural practices, so it’s very difficult to confirm overwintering fires just from looking at the satellite images,” Smith explained. “With wildfires, a good rule of thumb is that they are most likely to have had a human ignition!”
Regardless of the cause, the risk of these wildfires has only been aggravated by the record temperatures and dry conditions of 2020.
“We know from the climate data provided by C3S (Copernicus Climate Change Service) that the Arctic Circle regions most affected by fires in 2019 were experiencing warmer and drier surface conditions, providing the ideal environment for fires to burn and persist,” Parrington said.