Wildfires across northern Canada are “mining” carbon from the soil and turning the boreal forest into a carbon source after millennia of acting as a carbon sink, in a process that could accelerate global climate change, according to a new paper in the journal Nature.
“This study underscores why more frequent burning in the boreal forest due to wildfires is bad from a climate perspective,” said  lead author Xanthe Walker, post-doctoral researcher at the Center for Ecosystem Science and Society at Northern Arizona University.
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“We know that there is really old carbon in these soils—carbon that is hundreds to thousands of years old, carbon that is irreplaceable,” added senior author Michelle Mack.
“It’s making it much more difficult for us to target those reductions in human emissions because, all of a sudden, we have all these unaccounted-for sources,” added  co-author Merritt Turetsky, an ecologist at the University of Guelph. “Now those old forests are young forests, so when the next forest fire hits that area, those are going to be systems that are vulnerable to legacy carbon release. “We can have thousands of years of productivity stored and then released in a matter of minutes.”
The research “was launched in the aftermath of the severe 2014 fires in the Northwest Territories—the largest fire season in the region’s recorded history,” the University of Saskatchewan explains in a release. “Carbon is critical to soil function and productivity. Between forest fires, boreal soils accumulate carbon and are a globally significant carbon sink: boreal forests store about one-third of the world’s terrestrial carbon, primarily in soils. These pools of old carbon in the soil have been historically safe from combustion, since only some of this carbon is released when the forests undergo a fire.”
But “with warming of the forest climate and larger and more frequent wildfires, more of this sequestered carbon is being combusted and released—what the researchers describe as ‘mining’ the carbon from the soil,” USask adds.
“The combustion of this ‘legacy carbon’ in the soil has the potential to shift the global carbon cycle, as boreal forests that have acted as carbon sinks for millennia become sources of atmospheric carbon,” said USask adjunct researcher Jill Johnstone. “This could potentially accelerate climate warming.”
The researchers concluded that legacy carbon is protected in older stands of trees, but not in forests that are less than 60 years old. “In older stands that burn, this carbon is protected by thick organic soils,” said Walker. “But in younger stands that burn, the soil does not have time to re-accumulate after the previous fire, making legacy carbon vulnerable to burning. This pattern could shift boreal forests into a new domain of carbon cycling, where they become a carbon source instead of a sink.”
That’s a problem, because “the frequency of boreal forest fires is projected to increase even more with expected climate warming and drying, and as a result total burned area is expected to increase 130 to 350% by mid-century,” the paper states. USask says the increase in the burn area “would expand the proportion of young forests vulnerable to burning and loss of legacy carbon.”
“By defining and analyzing ‘legacy carbon,’ this paper offers a new way to think about long-sequestered carbon stocks in boreal forests and how vulnerable they are to being burned during increasingly frequent and severe wildfires,” said co-author Brendan Rogers, a scientist at the U.S. Woods Hole Research Center. The study’s approach to carbon dating “helps us understand when burning goes ‘outside the norm’ from a historical perspective and begins to combust carbon stocks that survived past fires.”
The ultimate question is the point at which boreal forests will tip over from being a “sink” that stores atmospheric carbon to a “source” that emits more than it sequesters. “I think we’re right on the tipping point now,” Turetsky said. “I think it’s happening in the western provinces already. I think it’s happening in Alaska.”