Due to climate change, forest fires are not only increasing in our latitudes and further south – fires are also burning more and more frequently in the Arctic. But these taiga and tundra fires could release far more greenhouse gases than previously thought, according to a new analysis. Unlike in southern climes, these Arctic fires also burn parts of the organic soil layers. This allows these peat fires to continue burning for weeks, converting thousands of years of carbon and releasing it as CO2. The problem: Because these fires often smolder in secret, they can hardly be seen in satellite images. But these images form the basis for models to estimate the global greenhouse effects caused by fires.
Every year, vast areas of forest go up in flames almost all over the world. One reason for this increase in fires is climate change and the extreme weather associated with it: heat waves and drought make forests and bushland more vulnerable to fire and make it easier for them to spread. These fires then release large amounts of climate-impacting gases such as carbon dioxide and thus further fuel global warming – a self-reinforcing feedback. Climate researchers use satellite data to record how many greenhouse gases are released globally by such fires. Based on the intensity and area of the fires detected from orbit, they determine their likely impact on the climate. These fire climate models have so far also been used for fires that occur at high latitudes – in the taigas and tundras of the Arctic.
Tundra fires often smolder in secret
In the Arctic, extensive and long-lasting fires are also occurring more and more frequently due to climate change. The permafrost regions characterized by moors, marshes and other wetlands are becoming increasingly drier and more susceptible to fire. In addition, the fires find particularly rich food there: gigantic amounts of organic material are stored in the permanently frozen soils of the Arctic – millennia-old plant remains that have hardly been broken down by bacteria due to the prevailing cold. When a fire ignites on these soils, it releases this millennia-old carbon as CO2. The expanding and more frequent Arctic fires are contributing significantly to the global greenhouse effects caused by fire. “The boreal regions store more carbon than the atmosphere and do so largely in thick, combustible organic soil layers,” explain Johan Eckdahl from Lund University in Sweden and his colleagues. “They are one of the fastest growing sources of greenhouse gases due to climate change and increasing fire frequency.”
But how much CO2 is released by the Arctic fires may have been greatly underestimated, as the researchers explain. “Many of these fires with a strong climate impact don’t look particularly dramatic from orbit,” says Eckdahl. Unlike the forest fires that are visible from afar with flames and burning treetops, the Arctic fires can eat deep into the organic layers of the permafrost and feed on the peat and other undecomposed organic materials in the permafrost soil. Barely visible from orbit, these fires smolder for weeks or even months. Studies have shown that even under the winter snow cover, peat fires can continue to burn. The following spring, these “zombie fires” flare up again and set other areas on fire.
Up to 14 times more CO2
To find out how well common fire models capture emissions from Arctic fires and how much carbon is actually released in such Arctic fires, Eckdahl and his team examined 324 fires that occurred in different regions of Sweden in 2018. They evaluated satellite data for these fires and also conducted on-site analyzes of burned and unburned soil for 50 fires to determine the amount of carbon converted into CO2. The result of their analyzes is a detailed map of fire emissions that shows how local soil conditions, climate and vegetation influence the amount of CO2 released. The researchers then compared these values with the data from six common fire models.
This comparison revealed striking discrepancies. For example, the models based on satellite data often overestimated CO2 emissions from intense, large forest fires in dry forests in southern Sweden, as Eckdahl and his team found. Because the flames of these fires produce a strong signal in orbit and the models are not accurate enough, they result in higher emissions than actually occur. The opposite was seen with the less visible peat fires: a fire in a region characterized by thick organic soil layers produced 14 times more CO2 than the current models calculated. According to Eckdahl and his colleagues, this suggests that the true extent of greenhouse gas emissions from such Arctic, less visible fires has been greatly underestimated. “Sweden is a big country, but compared to Siberia and Canada it is quite small,” says Eckdahl. The true climate effect of the fires in the vast tundra and taiga areas of these countries could be correspondingly large.
Source: Johan Eckdahl (Lund University, Sweden) et al., Science Advances, doi: 10.1126/sciadv.adw5226