THE DRIVERS AND EXTENT OF WILDFIRE REGIME CHANGE IN WASHINGTON STATE, USA

Wildfires have become increasingly devastating, with the seven largest ever fire seasons in the US occurring between 2005 and 2017. This trend holds true across essentially every western state. For example, Alaska’s worst wildfire season occurred in 2004 and second worst in 2015, Arizona’s worst in 2011, California’s worst in 2017, and Montana’s worst in 2017. This increase has been attributed to a number of causes, including the spread of invasive species, increased human populations and activity, forest management policy, and climate change. However, the interactions between these causal factors and their relationship with wildfires is complex and highly variable in space and time. Here, we present an analysis of the Washington State’s natural wildfire regimes over a 30-year period, and examine the connections between wildfires, dryness, air temperature, and global climate phenomena. We found that lightning-ignited wildfires in Washington have become more frequent and more devastating during a period from 1986 through 2015, and that this increase was strongly correlated with increased dryness, which was affect in part by rising temperatures, and by the interaction of the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Our results illustrate the variability and magnitude of change in wildfire regimes across ecoregions even when human ignitions are not considered. Furthermore, this study highlights the relationship between the global climate and local conditions that can have large impacts on wildfire severity. Our findings suggest that wildfires seasons will be increasingly severe in the future, as air temperatures and thus also dryness, are predicted to rise.