“Megafire” events, in which large high-intensity fires propagate over extended periods, can cause both immense damage to the local environment and catastrophic air quality impacts on cities and towns downwind. Increases in extreme events associated with climate change (e.g., droughts, heat waves) are projected to result in more frequent and extensive very large fires exhibiting extreme fire behavior (IPCC, 2007; Flannigan et al., 2009), especially when combined with fuel accumulation resulting from past fire suppression practices and an expanding wildland-urban interface. Maintaining current levels of fire suppression effectiveness is already proving challenging under these conditions, making more megafires a strong future possibility.

This project examines the weather and climate factors related to known megafires and very large wildfires that have occurred across the contiguous United States and projects the likelihood of megafires occurring during the 2046-2065 mid-century time period. A variety of statistical techniques and spatial scales are used in the analysis. The report ranks regions of future higher likelihood very large fire locations based on overall probability. In addition, the potential for large-scale smoke impact effects from very large fires is examined. This includes the overall potential for smoke emissions, as well as the potential for downwind transport to various kinds of sensitive receptors. Types of sensitive receptors examined include Class 1 airsheds, National Ambient Air Quality Standards non-attainment areas, and overall human population exposure. Smoke emissions and downwind transportation are combined to create an overall metric of Smoke Impact Potential (SIP). Combining future very large fire projections with site specific Smoke Impact Potentials allows for the ranking of locations based on the potential for large scale smoke impacts from very large fires.

While overall megafire risk is high in many parts of the western U.S. as well as in more limited areas along the east coast and the upper Midwest, the potential human population exposure from megafires is heavily concentrated in California, Minnesota, and along the eastern seaboard. A complete ranking of these locations is provided in the report.