Future Climate Change Projections of Wildfire Risk in Western U.S.

Wildfires are an annual threat to life and property in many countries of the world and especially in the U.S. Some of the biggest fires (in terms of fuel or biomass burned) and especially costliest fires (in terms of property damage) in the entire world occur in California. Several decades worth of data indicate the risk has been increasing in many states. Thus, the impact that climate change will have on wildfire in the future is a growing concern to all especially to the insurance industry.

Future projections of wildfire risk for the western U.S. have been recently developed at Verisk (Extreme Event Solutions formerly AIR Worldwide) for four different IPCC climate scenarios: SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5; and for two different time horizons: 2030 and 2050. The scenarios indicate where wildfires will burn more area and how property losses will change – including other loss metrics such as 1-in-100-year events, by zip code. The projections are in the form of a catalog of events that Verisk clients can use to analyze their portfolios to address regulatory measures and to make future business decisions.

The projections are developed by first defining statistical relationships (Generalized Linear Models or GLMs) from historical climate and wildfire data for each EPA ecoregion (Level III) within the 13 westernmost states of the continental U.S. The climate variables include seasonal vapor pressure deficit and prior year precipitation. Downscaled data from a suite of CMIP6 GCMs is then used to determine how the climate variables will change and then in conjunction with the GLMs to determine how corresponding annual average burn areas will change by ecoregion. Importantly, the process accounts for the negative feedbacks that occur when vegetation (fuel) burns. Namely, those areas cannot burn again until regrowth occurs. Additionally, burned regions can cause fire breaks to prevent larger areas of existing vegetation to burn.

An existing 10,000-year catalog of Verisk wildfire events that corresponds to the current climate is then resampled by ecoregion according to the future burned areas (e.g., the targets). The process is repeated to account for differences that inherently occur owing to the resampling process.

Results indicate that from a hazard standpoint (i.e., where more area will burn) will be across the inter-mountain region and coastal Pacific Northwest. California can expect modest increases in annual burned area although property damages will still be largest for all states modeled.

The technique used to create the projections can be applied to other regions where wildfire risk is significant but to fully capture the financial impacts it is best to use with an existing catalog of events that represents the current climate risk.