Wildfires ravaging vast boreal forests in Alaska, Canada, Scandinavia and Russia may be having a bigger impact on the climate than scientists once thought. A new study led by researchers at the University of California, Berkeley suggests that these northern fires could release far more carbon into the atmosphere than current estimates.
That’s because these fires don’t just burn wood. In many boreal regions, flames can spread downward into the thick layer of carbon-rich soil beneath the forest floor. These soils, known as peat, contain partially decomposed plant matter that has accumulated over hundreds or even thousands of years. The cold, wet conditions of the far north slow the decomposition of organic matter, so these landforms store vast amounts of carbon underground.
Satellite data may miss underground peat fires
Research shows that many of the models widely used to estimate the carbon footprint of wildfires do not fully capture these underground fires. Most of these models rely heavily on satellite observations of visible flames and are primarily based on fires that occur at low latitudes. As a result, slow, hard-to-see fires that smolder deep in peat and organic soils can be overlooked.
“Many of the most climate-critical fires are not dramatically visible from space,” says study lead author Johan Ekdahl, a postdoctoral fellow in Berkeley’s Energy and Resources Group. “Peatlands and organic soils can smolder for weeks to years, releasing huge amounts of ancient carbon.”
Reconstruction of Swedish wildfire emissions
Research published in journals scientific progressanalyzed 324 wildfires that occurred across Sweden in 2018. Ekdahl and colleagues combined detailed national forest records with direct measurements in the field to reconstruct the amount of carbon released from each fire.
Using these data, the team created a detailed map of wildfire emissions. Their analysis showed that local conditions such as climate, vegetation, and soil characteristics have a significant impact on the amount of carbon stored in forests and the amount of carbon released during wildfires.
Key differences from the Global Fire model
When the researchers compared their reconstructed emissions with six widely used global wildfire models, they found significant discrepancies. In some locations, the model overestimated carbon emissions. In other regions, especially where fires burned deep into the soil layer, emissions were significantly underestimated.
For example, the model predicted an increase in emissions in Gävleborg County. There, intense fires burned through dry forests and were clearly visible from satellites.
However, the situation was very different in neighboring Dalarna County. There, low-intensity fires burned quietly in a thick layer of organic soil and were less noticeable from space. In this region, the model underestimated carbon emissions by a factor of 14.
“Sweden is a very large country, but it’s quite small compared to Siberia and Canada,” Ekdahl said. “We may be significantly underestimating the impact of recent extreme fire seasons in these regions.”
Field measurements reveal soil carbon loss
To measure how much carbon wildfires release from the soil, the researchers collected data from 50 locations affected by fires in 2018. There were 19 high-intensity fires and 31 low-intensity burns.
The researchers measured the thickness of the organic-rich soil layer at each site (which varied from a few inches to several feet) and collected soil samples. The researchers calculated how much carbon was released by comparing carbon levels in the burnt soil to samples from nearby unburned forest.
“Once you’re out there, it’s as simple as digging a hole, but the hard part is getting there,” Ekdahl said. “Sweden has a good network of forest roads, but I’ve heard that in Siberia it’s serious mountaineering, which is one reason why measurements for this region are sorely missing.”
Expanding research to fire-prone forests in the United States
Eckdahl is currently working with colleagues at the University of California, Berkeley and other institutions as part of the Western Fire and Forest Collaboration to apply similar research methods to forests across the western United States.
Although forests in the western United States typically do not contain the thick peat soils found in northern boreal regions, several other factors still influence wildfire emissions. These include local climate patterns, the types of trees and vegetation present, and soil conditions. Ekdahl will study the role of soil microorganisms, such as bacteria and fungi, and how they contribute to forest recovery after wildfires.
“The forests of the lower 48 Rivers and the forests of the far north may look very different, but they share a common currency: carbon,” Ekdahl said. “By improving our understanding of how this element flows between land and the atmosphere, we can better predict the impacts of future fire regimes in a warming world and develop smarter strategies to reduce climate risks to society.”
Lars Nieradzik from Lund University and Louise Rütting from Brandenburg University of Technology are co-authors of the paper.
