Perspective: Climate crisisIncreasing Wildfires Threaten to Turn Northern Hemisphere’s Boreal Forests from Vital Carbon Stores into Climate Heaters

In 2014, Stefan H. Doerr, Chuanyu Gao, and Cristina Santin travelled to the northern boreal forests of Canada to set experimental fires that would help us understand the effect of wildfires on the global carbon cycle. Sadly, theynever got the chance to set those fires, because the firefighters enlisted to help us were busy dealing with an area the size of Belgium that was already burning.

That wildfire season was the most severe on record in the region, which itself forms part of the wider boreal ecosystem that engulfs much of the Northern Hemisphere’s subarctic lands with coniferous forests shaped by fire.

Doerr, Gao, and Santin write that thanks in part to the carbon-hungry soils and peatlands they contain, they punch well above their weight as carbon sinks, covering 10 percent of the world’s land, but storing one-third of the land’s carbon.

According to a new study examining the impacts of Canada’s 2014 wildfires, that stored carbon is under threat. Wildfires are becoming so frequent and intense that they are already turning some boreal forest areas from carbon sinks into net emitters.

Most of the carbon in these ecosystems isn’t stored in the trees, but in the soils below. In the cold and often waterlogged boreal landscapes, organisms living in the soil aren’t able to “eat” dead organic matter that falls onto the forest floor as quickly as in warmer, dryer climates. This allows the soils to accumulate carbon over millennia, making boreal ecosystems some of the most important carbon sinks in the world.

Wildfires, mostly started naturally by lightning, interrupt this accumulation process by burning the trees and the top layer of this organic soil – the latter accounting on average for three times as much of the CO2 released during burning as from the trees themselves.