Ancient carbon released in Arctic-boreal wildfires

Wildfires are rapidly increasing in boreal forests and are extending to Arctic environments at an unforeseen scale. Above-ground biomass burning may be compensated by regrowth in the years following a wildfire, impacting atmospheric CO₂ levels only temporarily.  However, high-latitude wildfires characteristically combust deep into carbon-rich soils accumulated over centuries to millennia and thereby risk transforming these long-term carbon sinks into net sources into the atmosphere. Hitherto, research on Arctic-boreal fires has largely focused on their surface impacts, including the burned area, severity, and forest recovery, while many of their underground characteristics are poorly understood. For instance, observations of the age of carbon released in the fires remain scarce, resulting in incomplete understanding of the climate impact of high-latitude fires.

To determine the age of carbon released in recent Arctic-boreal fires, we collected charred organic material for radiocarbon dating from a tundra fire in Greenland, and two boreal forest and one tundra fire site in northwestern Canada. Our results indicate that, contrary to previous observations, up to centennial to millennial-aged carbon was released in these arctic and boreal wildfires. Moreover, laboratory combustion experiments of Arctic-boreal biomass collected from fire-susceptible surface layers (0-30 cm depth) from Svalbard, Russia, Norway and Finland, demonstrate that the combustion mode, and thus the phase of the emitted carbon, depend on the age of the combusted material. Above-ground modern vegetation combusts flamingly emitting mainly gases, while below-surface older and partly decomposed organic material smoulders, producing increasing carbonaceous particle/gas ratios with increasing age of the combusted material. Similar to the studied Greenland and Canadian wildfires, the laboratory combustion of the Arctic-boreal biomasses show up to millennia-aged carbon emissions.

Our results indicate that centennial to millennial-aged carbon is released in Arctic-boreal wildfires, thereby causing long-lasting feedback to the global climate system. Currently, climate models do not consider the potential release of ancient carbon from wildfires. Thus, our results indicate that increasing Arctic-boreal wildfires may exacerbate global warming more than previously estimated.