Environmental sciences: ‘Zombie’ forest fires that survive the cold

Forest fires in the Northern Hemisphere that smoulder through the winter then reignite in the spring are associated with warmer summers and can account for up to one third of the total burn area, suggests a paper published in Nature. The findings may have implications for fire management policies.

Fires that can survive the cold and wet winters of boreal (Northern Hemisphere) forests are described as overwintering (or zombie) fires. Boreal forests, despite their high latitudes, present favourable conditions for overwintering fires with deep organic soil and a warming climate, but the extent of overwintering fires in these forests and their drivers remains unclear.

Rebecca Scholten, Sander Veraverbeke and colleagues developed an algorithm to identify overwintering fires in Alaska, USA and the Northwest Territories, Canada from 2002–2018 using field and remote sensing data. They identified that overwintering fires were responsible for 0.8% of the total burned area over the study period, but this area varied in individual years and in one year was 38%. The authors found that overwintering fires were more likely following warmer summers. They suggest that warmer temperatures allow fires to burn deeper into the organic soil, which may help to sustain them over the winter.

The authors note that although overwintering fires are relatively rare in boreal forests, as climate change increases temperatures they may become more common. They suggest that early detection and suppression of these fires could contribute to cost savings for fire management agencies.