Increasing exposure of humans to wildfires in the western US

Abstract

Wildfire impacts to society have increased in the US and parts of the globe in recent decades. This talk provides a synthesis of three recent studies that quantify population exposure to wildfires, characteristics of populations directly impacted by wildfires, and typologies of wildfires that bear the brunt of the impacts. Over 600 thousand people have resided in the line of large fires during the past two decades – with California accounting for three-fourths of the exposures. Population exposure to large fires has more than doubled across the US during the past two decades, with largest relative increases among populations with high social vulnerability. We find that population dynamics since 2000 accounted for less than a quarter of the increase in overall population exposure. This suggests that the principal factors responsible for the heightened exposure are the expanded extent of wildfires and their proximity to human populations. A large fraction of these exposures in portions of the western US occurred coincident with downslope mountain winds that allow fires to rapidly spread downhill and often toward human settlements. We find that a majority of the fatalities and structure losses in wildfires in the western US since 1999 were linked to wildfires that ignited during downslope winds. The results underscore the various direct impacts that the surge in wildfire extremes has imposed on society, emphasizing the urgent need for measures to help communities effectively cope with the escalating wildfire threat.