Challenges in assessing Fire Weather changes in a warming climate

Abstract

The Canadian Fire Weather Index (FWI), widely used to assess wildfire danger, typically relies on noonspecific meteorological data. However, climate models often provide only daily aggregated values, posing a challenge for accurate FWI calculations.Weevaluated daily approximations for FWI95d-the annual count of extreme fire-weather days-against the standard noon-based method (1980-2023). Our findings reveal that noon-based FWI95d show a global increase of ~65% (11.66 days over 44 years). In contrast, daily approximations tend to overestimate these trends by 5-10%, with combinations involving minimum relative humidity showing the largest divergences. Globally, up to 15 million km²-particularly in the western United States, southern Africa, and parts of Asia-exhibit significant overestimations. We recommend (i) prioritizing the inclusion of sub-daily meteorological data in future climatemodel intercomparison projects to enhanceFWI accuracy, and (ii) adopting daily mean approximations as the least-biased alternative if noon-specific data are unavailable.