Exceptionally high summer mean temperatures predict tundra fire activity, with stronger responses at greater previous-year leaf area index​

The Arctic tundra is a vast, treeless landscape where the ground remains frozen for much of the year, supporting a unique community of low-growing plants like mosses, lichens, and shrubs. For centuries, this region was considered too cold and wet to sustain frequent fires, but as the planet warms, the climate there is changing faster than almost anywhere else on Earth. When temperatures rise, the air can hold more moisture, drying out the vegetation and making it more likely to burn if a spark occurs. Scientists have long suspected that these warming trends could turn the tundra into a fire-prone environment, but the exact recipe for these blazes has remained unclear. It is difficult to know whether the heat itself is the primary driver, or if the amount of plant growth from the previous year, the frequency of lightning strikes, or the amount of rain plays a more critical role in deciding when and where a fire will start and how large it will become.

To untangle these complex relationships, researchers examined two decades of data, from 2001 to 2020, looking at fire activity across the entire tundra biome. They divided the region into small grid cells and tracked how much carbon was released by fires and how much of the land was burned each year. By comparing these fire records with nine different environmental factors, including temperature, rainfall, wind, and the density of plant leaves, the team sought to find the strongest signals. They used a method that grouped years into categories based on how often certain conditions occurred, allowing them to see how fire behavior changed as conditions shifted from cool and wet to hot and dry. This approach helped them separate the influence of immediate weather from the lasting effects of the landscape itself.

The analysis revealed that the most powerful predictor of tundra fire activity was simply how hot the summer was. When summer temperatures reached the highest levels, fire activity surged, particularly in regions like Alaska and Russia. Closely following temperature was the dryness of the air, measured by how much moisture the atmosphere was pulling from the plants. The study found that these two factors—heat and atmospheric dryness—had the strongest connection to both the amount of land burned and the carbon released. While a standard weather index used to predict fire danger in forests did help explain the size of the burned areas, it was the raw summer temperature that drove the overall trend across the entire biome.

Crucially, the research showed that the landscape's history matters just as much as the current weather. The amount of green leafy vegetation present in the year before a fire was a key factor. When the previous year had abundant plant growth, the tundra became much more sensitive to heat. In these areas, a hot summer led to a disproportionately large increase in fire activity. This suggests that a warm year followed by a year of lush growth creates a perfect storm: the extra plants provide more fuel, and the heat dries that fuel out, making it ready to ignite. The study also found that lightning, which provides the spark needed to start a fire in these remote areas, strengthened this relationship. However, heavy rainfall acted as a brake, weakening the link between heat and fire, while wind speed did not show a consistent, reproducible effect on the outcome.

The researchers observed that the hottest summers, which are becoming more common, tend to overlap spatially with the areas where fires are most intense. This means that as the Arctic continues to warm, the conditions that trigger these large-scale blazes are not just becoming slightly more likely, but are appearing with increasing frequency in the same places. The findings indicate that the tundra is not just reacting to warmth in a steady, linear way. Instead, the combination of record-breaking summer temperatures and the fuel provided by previous years' plant growth creates a feedback loop where fire activity could jump sharply. As the climate shifts toward these upper limits of temperature, the tundra may experience fire events that are far more severe than what has been seen in the past, driven by the simple, powerful interaction of heat, dryness, and the vegetation left behind.