Permafrost thaw has enlarged the burned area proportion from naturally-ignited wildfires in Arctic-boreal regions.

Recent studies revealed a simultaneous increase in Arctic-boreal wildfires and permafrost thaw since 2000. Despite the agreement that wildfires accelerate permafrost thaw, it remains unclear whether permafrost thaw makes the landscapes more prone to wildfires. Our study developed a model-based approach to disentangle wildfires ignited naturally and anthropogenically. The historical simulation demonstrates an abrupt increase in the proportion of burned area by naturally-ignited wildfires that occurred around 2010 in the Arctic-boreal regions. The increase in burned area proportion results from the shallow-layer soil drying due to permafrost thaw. Model results and observations verified rapid permafrost thaw in the early 2000s, followed by soil drying lagged by 4.29 ± 1.52 years. The shallow soil drying that can enhance fuel combustibility is more pronounced in permafrost regions dominated by natural wildfires. Our study demonstrated that permafrost thaw has become a critical driver of Arctic-boreal wildfires, especially for the naturally-ignited fires that have a larger burned area on average per event in permafrost regions. The resulting positive feedback, namely, permafrost thaw promoting more fires, and fires further accelerating thaw, could foreshadow profound climate repercussions.