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August 21st, 2019

Accelerating boreal wildfires releasing old carbon to the atmosphere, study reveals

An Arctic wildfire burns during the record fire season of 2014. Photo courtesy of Merritt Turetsky.

More frequent and larger wildfires are releasing old carbon stored in soils that in the past was able to escape burning, according to a new study involving incoming INSTAAR director Merritt Turetsky.

As wildfires this year ravage northern areas across the globe, a research team investigated how extreme burning affects these historically preserved carbon stores in soil and vegetation across northwest Canada. They found that larger fires can unlock the carbon and release it into the atmosphere, accelerating global warming. Their study was published today in the journal Nature.

“Northern fires are happening more often and their impacts are changing,” said Turetsky, who worked on the study with lead authors Xanthe Walker and Michelle Mack from the Center for Ecosystem Science and Society (Ecoss) at Northern Arizona University as well as a team of Canadian scientists.

In 2014, the Northwest Territories suffered the largest fire season in recorded history. The series of megafires created the ideal environment to study whether carbon stores are being combusted by these types of fires.

“Between fires, boreal soils accumulate carbon, and in most cases only some of this carbon is released when the forests experience the next fire,” said Jennifer Baltzer, Wilfrid Laurier professor and coauthor on the study. “Over time, this explains why the boreal forest is a globally significant carbon sink. We wanted to see whether the extreme 2014 fires tapped into these old legacy carbon layers or whether they were still preserved in the ground.”

Near Kakisa, Northwest Territories, Merritt Turetsky collects a sample of the thick organic soil layers quantified in the study.

The research team collected samples from more than 200 forest and wetland plots across the Northwest Territories. They then applied a novel radiocarbon dating approach to estimate the age of the carbon in the soil samples.

“Carbon accumulates in these soils like tree rings, with the newest carbon at the surface and the oldest carbon at the bottom,” said Mack, one of the lead authors. “We thought we could use this layering to see how far back in time, in the history of the forest, fires were burning.”

The researchers found the combustion of legacy carbon in nearly half of the samples taken from young forests or forests less than 60 years old. This legacy carbon had escaped burning during the previous fire cycle but not during the record-setting fire season of 2014.

“In older stands that burn, this carbon is protected by thick organic soils,” said NAU’s 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.”

Climate warming is linked to extreme events, such as heatwaves and extreme wildfires. This means that the release of old carbon to the atmosphere could be increasingly common in the future.

“Understanding the fate of this stockpile of boreal carbon is really important in the context of atmosphere greenhouse gases and the Earth’s climate,” Turetsky said. “This is carbon the atmosphere lost hundreds or sometimes even thousands of years ago. Fire is one mechanism that can release that old carbon back to the atmosphere quickly where it can contribute to the greenhouse gas effect.”

Wildfire regimes around the world are changing in response to global warming and human alteration of the landscape. Because humans have deep and complex relationships with fire, Turetsky says, it is more important than ever for scientists to work with those affected by burning. As incoming Director of INSTAAR, she has already initiated dialogue with INSTAAR researchers and CU partners at NCAR and the US Geological Survey on how to best leverage research strengths in extreme climate events with stakeholder engagement.

Grad students Kirsten Reid and Jean Holloway take a sample by drilling into permafrost soil in one of the burned forest peatlands included in the study. Photo courtesy of Merritt Turetsky.