Coniferous forests make up around half of all forest areas worldwide. North America alone is home to a third of these boreal forests. Fires have been much more frequent there in recent decades. In 2023, all records were broken: 140,000 km2 of Canadian forest were on fire, which corresponds to 1.4 percent of Canada's area or around 40 percent of the area of Germany. The fires not only release immense amounts of CO2 that was previously trapped in the wood. The impact on plant physiology can still be detected long afterwards. In many regions, the otherwise permanently frozen ground (permafrost) thaws after a forest fire. Depending on the topography, this can lead to waterlogging of the soil and increased emissions of methane, a gas that has a particularly strong impact on the climate, into the atmosphere. Fires change landscapes permanently and the vast barren areas may have an impact on the climate for decades. Studies that examine the possible long-term effects in more detail are therefore extremely important.
A team led by Dr. Manuel Helbig, a scientist in Section 1.4 "Remote Sensing and Geoinformatics" at the GFZ Helmholtz Centre for Geosciences, has analyzed the climatic effects of fires in North American coniferous forests dating back to 1928. Based on data from satellite and ground-based measurements, it examined surface temperatures and the leaf area index as well as the heat exchange between the forest floor and the atmosphere. Among other things, the researchers were able to show that burnt coniferous forest areas result in higher surface temperatures during the day for up to five decades in the cool summers of northern latitudes. The study has been published in the journal AGU Advances and was carried out in collaboration with researchers from Dalhousie University in Halifax, Canada, Shinshu University in Matsumoto, Japan, and the Graduate School of Agriculture at Osaka Metropolitan University in Sakai, Japan.
Researchers use satellite data in combination with ground measurements
In intact forests with lots of vegetation and growth-related height differences in the treetops, there is a good exchange of air and therefore also a good heat exchange with the atmosphere. After a forest fire, the air exchange can be less, as tree crowns are no longer present or are not yet fully developed again. As a result, the so-called surface roughness, i.e. the height difference of the vegetation, is lower, which leads to less air turbulence above the forests. As a result, the earth's surface heats up more. When it comes to heat exchange between the forest and the atmosphere, the height and complexity of the canopy is therefore an important factor. To determine this, the scientific team used satellite-based observations of over 100 burnt forest fire areas in Canada and Alaska. The international team also analyzed satellite data as well as direct measurements on the ground of surface temperatures, the "surface albedo", i.e. the reflected radiation, which quantifies the ratio of reflected to incident light, and the so-called "leaf area index", which indicates the leaf density in forests. The scientists assessed how all these parameters contribute to temperature changes on the Earth's surface and to climate change in the long term. They also looked at past fire events and compared the changes that had actually occurred with the changes predicted in various studies at the time.
The results: Another five decades of higher summer temperatures
In the summer months of July to September 2024, the entire Canadian boreal habitat, which covers large parts of the country, was on average 0.27 °C warmer than if the area had not been affected by forest fires due to past forest fires. In addition, after an initial reduction following a forest fire, evaporation increases over three decades as the leaf density increases with the regrowing forest as a result of the fire.
In the late winter months of February to April, snow can cover the small-growing, regenerating vegetation better than a mature forest, on whose tops the snow does not remain for long. As a result, the warming sunlight is better reflected, leading to an overall slight average cooling of around 0.02 degrees Celsius.
The study also examined a whole range of forest combustion rates. However, it was not possible to conclusively clarify the exact effect of fire intensity on the subsequent temperature development. The researchers assume that the variability in surface temperatures after a fire is only partly due to differences in the severity of the fires.
Scenarios up to 2050
If climate change leads to more frequent and larger-scale fires in boreal forests, this could have a significant additional impact on warming there. The researchers have calculated various scenarios for possible developments up to the year 2050:
For a scenario with a high increase in burned forest area (by 150 percent between 2020 and 2050), the increase in annual mean temperature caused by forest fires alone would rise by 30 percent in this period, from 0.12°C in 2020 to 0.16 ± 0.04°C in 2050. In contrast, a scenario with a low increase in burned area (by 36 percent between 2020 and 2050) would not lead to any additional amplification effect in warming by 2050.
Further conclusions
The results illustrate the climate consequences of a change in forest fire dynamics in the boreal forests of North America. Rising surface temperatures in fire-damaged coniferous forests could not only affect the climate of these regions, but also jeopardize important ecosystem services such as carbon storage in the soil. With the expected increase in forest fires, the need for further research is growing: "The consequences for ecology and climate are profound and require increased attention in climate research," explains the lead author of the study, Dr. Manuel Helbig. And he adds: "Our investigations also make it clear how important it is to reduce greenhouse gas emissions globally. Because by accelerating global warming, they also increase the risk of forest fires and thus of permafrost thawing and the release of more carbon dioxide and methane from the soil."
Original publication:
Helbig, M., Daw, L., Iwata, H., Rudaitis, L., Ueyama, M., & Živković, T. (2024). Boreal forest fire causes daytime surface warming during summer to exceed surface cooling during winter in North America. AGU Advances, 5, e2024AV001327.
https://doi.org/10.1029/2024AV001327