Overestimating Fire Weather Trends: Challenges in Using Daily Climate Data

The Fire Weather Index (FWI) is a widely used metric for assessing wildfire danger, relying on sub-daily meteorological data, typically recorded at local noon. However, most climate models and observational datasets only provide daily-aggregated variables, which can introduce biases in fire weather assessments under climate change. This study evaluates how approximating noon-specific calculations impacts the trends of extreme fire weather days (FWI95d), defined as the annual number of days exceeding the 95th percentile of daily FWI values (FWI95d).

Using global data from ERA5 for 1980–2023, we find that FWI95d have increased by 65% over 44 years, corresponding to an average of 11.66 additional extreme fire weather days per year. Daily approximations consistently overestimate this trend by 5–10%, with the largest differences observed in fire-prone regions such as the western United States, southern Africa, and parts of Asia. Among the tested proxies, the combination of daily mean values for air temperature, relative humidity, precipitation, and wind speed exhibits the lower biases, while proxies involving minimum relative humidity tend to overestimate trends more significantly.

Our findings emphasize the importance of sub-daily meteorological data for accurate wildfire risk projections. In its absence, we recommend prioritizing daily mean approximations over other proxies as the least-biased alternative in the absence of noon-specific data. These results underscore the potential for misrepresentation of future fire weather risks in climate models, particularly if systematic biases introduced by daily approximations are not addressed. Future climate model intercomparison projects should prioritize the inclusion of sub-daily meteorological outputs to enhance the reliability of fire weather assessments globally.

Acknowledgements
M.T. acknowledges funding by the Spanish Ministry of Science, Innovation and Universities through the Ramón y Cajal Grant Reference RYC2019-027115-I and through the project ONFIRE, Grant PID2021-123193OB-I00, funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”. This work was supported by the project ‘Climate and Wildfire Interface Study for Europe (CHASE)’ under the 6th Seed Funding Call by the European University for Well-Being (EUniWell).