Summary

The Verisk Wildfire Model for the United States is designed to capture the current-day risk of wildfire to property in the Western U.S. The purpose of this document is to help clients understand future wildfire risk by describing a set of climate change projections for the Verisk model.

Verisk scientists developed the projections using a science and data-based approach. The model domain, which includes 13 Western states, was partitioned into the 49 Environmental Protection Agency level III ecoregions that span the region; each ecoregion is defined by its climate, ecology and vegetation features. The Verisk team created ecoregion-specific relationships between climate variables and wildfire burned area, and then estimated how the changes to those ecoregion-specific climate variables would impact future burned area. The team used these future estimates to generate climate adjusted catalogs for the Verisk Wildfire Model, created by resampling events from the existing 10K catalog.

The climate-burn area relationships were developed from historic climate data, including precipitation and a derived atmospheric moisture variable known as vapor pressure deficit (VPD), and historic wildfire data which details burn area from many previous wildfire seasons. The relationships were tailored to the vegetation and conditions in each ecoregion and considered a feedback process through which previous wildfires in an area affect future wildfire hazard in those same areas. Climate model output from the Coupled Model Intercomparison Project Phase 6 (CMIP6) was used to estimate VPD and precipitation through mid-century under four different climate change scenarios: SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5.

The results of the historical climate-fire analyses show that especially in forested ecoregions, the burned area is highly sensitive to VPD. Coupled with CMIP6 output of VPD, it was found that the largest changes in projected burn area will likely occur in the forested regions of the Northwest (Washington and Oregon) and Intermountain region (Montana, Utah, Colorado and northern California). Results of the burn area projections are consistent with those from other studies.

Climate change impacts to loss were evaluated using the Verisk industry exposure data for the modeled states. Loss changes at 2030 show little variation across the climate scenarios, with overall industry average annual losses increasing by approximately 25%. At 2050, there is more variation across the scenarios; for SSP2-4.5 average annual loss is expected to increase by 60% overall. Greater variation will be expected geographically. In California, which currently contributes two-thirds of the industry annual loss, increases of approximately 20% and 50% are anticipated at 2030 and 2050, respectively, under SSP2-4.5, with higher changes in Montana, Oregon, Washington and Colorado and smaller changes in Arizona, Texas and Oklahoma. In future years, California will remain the largest contributor to countrywide industry loss, owing largely to exposure and already high risk. However, Arizona and Texas, today's second and third highest loss producing states, will likely be replaced by Colorado and Washington, respectively. Additional details, including county level changes, are presented in more detail.

The results are subject to multiple sources of uncertainty, including uncertainty associated with the climate scenarios, variability in the output across multiple CMIP6 climate models, changes in ecoregion vegetation in the future climate, and limitations in the historical data used to create the climate-burned area relationships. The results also do not contemplate exposure change or adaptation measures, for example improvements in building codes, better fuel management and a more resilient power grid. These and other sources of uncertainty are detailed in the report.

The document is organized as follows: Section 2 provides an overview of factors influencing wildfire and a discussion of modeling the climate change impact to these factors, as well as a brief summary of climate change scenarios. Section 3 describes the methodology for developing the hazard projections, including the fire-climate relationships, future VPD and precipitation estimates, and the resulting projections for future burn area. Section 3 also describes the approach for developing the projected catalogs. Section 4 presents the results of the projections for the climate variables, burn area, and modeled losses, with a detailed discussion of the loss metrics and variation by geography. Section 5 includes closing remarks and a discussion of sources of uncertainty.