Climate scientists may need to revise their predictions: instead of acting as carbon sinks, Earth’s northern boreal forests could start releasing carbon at a faster rate than they can capture it.

Most global climate models treat these forests as storing about one-third of all terrestrial carbon in trees and, especially, in thick layers of peaty soil. Forest fires disrupt this storage temporarily, releasing carbon to the atmosphere until it can be recaptured by regrowth.

The net effect of this cycle depends critically on the scale and frequency of fires. Yet we only have about a half-century of good data on boreal forest fires, so climate modellers have had to assume – knowing they were probably wrong – that past fires were like those of the present.


That assumption is certainly wrong in the Yukon Flats of central Alaska, which has burned extensively in recent decades.

Ryan Kelly, an ecosystem modeller at the environmental consulting firm Neptune and Company, and his colleagues had previously analysed charcoal deposited in sediments to show that the modern wildfires there are more severe than at any time since the end of the last ice age.

That means the forest today is younger than it used to be, and therefore stores less carbon.

Altered conclusion

When Kelly’s team reran carbon-storage models based on this information, the results were dramatically different. Instead of storing carbon steadily, the Yukon Flats has probably been a net exporter of carbon into the atmosphere since 1950, they found.

“It’s not just a little problem,” says Kelly. “It totally changes the conclusion from the model.”

The forest as a whole is probably still storing carbon, says Kelly, because few places have been as affected by fires as the Yukon Flats. However, most experts expect warming climates to worsen boreal fires over coming decades.

Indeed, this year was Alaska’s second-worst for fires on record, and the three worst years have all happened since 2004. If the trend continues, then more and more of the forest will come to resemble the Yukon Flats and begin adding carbon to the atmosphere.

This potentially creates positive feedback – warming causes fires, releasing carbon and raising temperatures further. “That would add a bit of urgency to addressing current climate warming,” says Philip Higuera at the University of Montana in Missoula.

Image credit: Philip Higuera/University of Montana

Journal reference: Nature Climate Change, DOI: 10.1038/NCLIMATE2832