Spatial and temporal patterns of wildfire drivers across Europe

Wildfires are becoming increasingly more frequent and devastating across Europe. In recent years, wildfires consistently set new records, and have occurred in regions that are historically less fire-prone. It is still unclear how the drivers of wildfires vary within space and time across Europe, though understanding their composition is highly relevant for mitigating fire risk and exposure, especially with regards to climate change. 

Here, we study the spatial and temporal patterns of wildfire drivers in eight distinct European climate regions by leveraging daily FireCCI burned area observations together with CERRA reanalysis data for hydro-climatic variables and MODIS gross primary productivity for fuel availability between 2001 and 2020. We develop random forest models for each region and season to identify the most important drivers of wildfire occurrence. To identify the time scales over which wildfire-favoring conditions develop, we analyzed the persistence of standardized anomalies before a fire event by using incrementally increasing temporal windows.

We find strong anomalies of all drivers on fire days in comparison to non-fire conditions across all subregions and seasons - but the combination and strength of these drivers varies in time and space. Overall, drought conditions are the most important modulator of wildfire activity. Vegetation deficits are most relevant for wildfire occurrence in spring and summer, while long-term drought indicators, such as soil moisture deficits and the Standardized Precipitation Evapotranspiration Index, are most important in fall and winter. The seasonal cycle of gross primary productivity (GPP) before wildfire occurrence underlines the dynamic interactions between vegetation, drought, and fire. During spring and summer, wildfire events occur under seasonal GPP deficits, whereas in fall and winter fires occur under seasonal GPP surpluses. The persistence analysis highlights the time scales over which hot and dry conditions reduce GPP and increase fuel availability: In summer, dry conditions lead to less GPP and higher fuel loads on fire days in comparison to non-fire days, whereas in the fall high fuel loads originate from GPP surpluses of the previous spring that dry out during hot and dry summer weather. 

Our findings illustrate the  complex interdependencies of factors contributing to wildfire events across different climate regions in Europe and time scales, underscoring the need for targeted wildfire mitigation and adaptation strategies, especially in the context of climate change.