The summer of 2025 marked the second largest fire season on record in Canada, after the recent record of 2023. A new study now reveals that these fires have consequences that go far beyond the smoke and CO2 released into the atmosphere. For example, fires also change how the land surface reflects sunlight for many decades after the fire. This can also regionally cool the climate. Researchers Max van Gerrevink, Sander Veraverbeke and Nick Schutgens (Vrije Universiteit Amsterdam) and international colleagues now show that the long-term climate impacts of boreal fires are shifting towards fires with a net climate warming effect.
The study shows that post-fire greenhouse gas emissions from permanently frozen ground, known as permafrost, and reduced cooling from post-fire snow cover are tipping the balance, underscoring the growing role of northern fires in amplifying climate warming.
Regional cooling
Northern fires emit large amounts of CO2 into the atmosphere and their smoke plumes can travel long distances, even reaching as far as Europe. We know that the greenhouse gas emissions contribute to climate warming, yet these fires also affect climate in other ways. For example, when fires burn in regions covered by snow during winter and spring, they can actually result in regional cooling. This happens because freshly fallen snow is highly reflective, and burned areas covered by snow reflect even more sunlight back into space. The longer the snow remains on the ground, the more time this increased reflectivity has to cool the regional climate by reducing the amount of sunlight absorbed by the surface.
By combining historical fire records, satellite and climate data with machine learning, researchers at Vrije Universiteit Amsterdam quantified and mapped the net long-term climate impacts from multiple sources for fires across Alaska and western Canada between 2001 and 2019. The results indicate that, on average, Alaskan fires contribute to climate warming, whereas fires in Canada generally have a climate-cooling effect. The findings were published in Nature Geoscience.
Northen forests continue to warm
With extreme fire seasons occurring more frequently across high-latitude forests, scientific attention has increasingly turned to how fires interact with ecosystems and the climate system. “What many people may not realize is that fires are not always harmful to ecosystems and the climate,” says lead author van Gerrevink. The research revealed that nearly half of the fires in Alaska were linked to a net warming influence, compared to only one in ten in Canada.
Van Gerrevink: “While the majority of northern forest fires in North America are currently exerting a climate-cooling influence, this is likely to change as northern forests continue to warm. Continued warming is expected to reduce snow cover and shorten its duration, a shift that may substantially alter the net climate impacts of future fires as it reduces the dominant cooling source from increased reflectivity from snow.”
Carbon and ice-rich landscapes are most vulnerable
The study shows that fires were more likely to contribute to climate warming in regions with dense fuels and extensive permafrost. “Climate-warming fires burn deeper into organic soils, and thus release more greenhouse gases into the atmosphere. By removing parts of the insulating soil layer, heat can reach deeper soil layers, causing more permafrost to thaw each summer and releasing additional carbon”, says co-author Veraverbeke. In contrast, fires in regions without permafrost and with fewer trees are more likely to have a climate-cooling effect. This work provides the first continental-scale assessment of how northern high-latitude forests fires affect climate at an unmatched resolution of 500 meters.
“This detailed information is very useful for fire and forest managers as it indicates which regions are most likely to experience warming vs. cooling fires, and hence this can steer fire management operations in ways that mitigate climate warming effects”, concludes co-author Brendan Rogers (Woodwell Climate Research Center).
The work was funded by the European Research Council through a Consolidator grant under the European Union’s Horizon 2020 research and innovation program.