We may be underestimating the true carbon cost of northern wildfires

(Press-News.org) Wildfires in the northern boreal forests of Alaska, Canada, Scandinavia and Russia may be more damaging to the climate than previously thought, a new UC Berkeley-led study suggests.

That’s because these fires don’t just burn through trees; they can also penetrate deep into the carbon-rich layers of soil underneath many boreal forests, releasing carbon that has been accumulating for hundreds or even thousands of years. These carbon-rich soils, also known as peat, are primarily found in the far north, where the cold, wet climate prevents vegetation from fully decomposing and leads to a buildup of partially decayed organic matter over time.

The study found that major models of wildfire carbon emissions — which are largely based on data from fires at lower latitudes, and use satellite images of visible flames to guide their estimates — are not properly accounting for the impact of fire on these underground carbon stores. 

“Many of the fires that matter most for the climate don’t look dramatic from space,” said study lead author Johan Eckdahl, a postdoctoral scholar in Berkeley’s Energy and Resources Group. “Peatlands and organic soils can smolder for weeks to years, releasing enormous amounts of ancient carbon.”

In the study, published today in the journal Science Advances, Eckdahl and his co-authors reconstructed the carbon emissions from 324 wildfires that burned in Sweden in 2018. By combining detailed national forest datasets with field measurements, they were able to create a high-resolution “map” of wildfire emissions, showing how variations in local climate and ground conditions can impact the amount of carbon that is stored in a forest and released by wildfire.

When they compared their detailed reconstructions with six of the most widely used global fire emissions models, the researchers found striking inaccuracies. Some regions showed large overestimates, while emissions from deep belowground carbon stores were dramatically underestimated.

For example, the models overestimated carbon emissions in the county of Gävleborg, a region that experienced large, high-intensity wildfires in drier forests that were clearly visible by satellite.

However, in the neighboring county of Dalarna, where low-intensity fires that were less noticeable by satellite burned into thick soil layers, the models underestimated carbon emissions by up to a factor of 14. 

“Sweden is a very large country, but it’s quite small compared to Siberia and Canada,” Eckdahl said. “We may be severely underestimating the impact of the recent extreme fire seasons in these regions.”

To measure the impact of wildfire on soil carbon, the team collected data at 50 of the sites that burned in 2018, 19 from high-intensity fires and 31 from low-intensity fires. At each site, they measured the depth of the organic-rich soil layer — which can vary from a few inches to many feet — and collected soil samples. By comparing the carbon content of the burned soil with soil from unburned forest land, they could calculate the amount of carbon released by the fire.

"Once you're out there, it's a simple task — just dig some holes — but the hard part is getting to the sites," Eckdahl said. "Sweden has a good network of forest roads, but in Siberia, I hear it's a real trek, which is one reason why we're severely missing measurements from that region."

As part of the Western Fire & Forest Collaborative, Eckdahl is now working with colleagues at UC Berkeley and across the nation to adapt these approaches to fire-prone forests in the Western U.S. While these forests may not have the same carbon-rich soil layers as boreal forests of the far north, there are still a variety of factors — including the local climate, the types of trees and vegetation present and the soil quality — that can have a dramatic impact on the wildfire emissions. Eckdahl’s focus will be on studying bacteria and fungi in the soil, and how they can help a forest recover after a wildfire.

“Forests in the Lower 48 and those far up north may look very different, but they share the common currency of carbon,” said Eckdahl. “By improving our understanding of how this element flows between the land and the atmosphere, we can better anticipate the impact of future fire regimes in a warming world and design smarter strategies to reduce climate risks on society.”

Lars Nieradzik of Lund University and Louise Rütting of the Brandenburg University of Technology are co-authors of the paper. 

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