Title:  Understanding the observed increase in western US wildfire activity over the past 40 years

Abstract:  In this talk I will evaluate the satellite-based record of wildfire in the western US from 1984 through 2020, a period when annual burned area in the region increased by 300 percent. I will show that this increase in area burned is mostly due to a ten-fold increase in annual forest fire area, that the increase in forest area burned is mostly due to a growth in fire size rather than fire frequency, and that the growth in forest-fire size is largest for fires ignited by lightning. Comparing forest-fire data to climate data, yearly burned areas are strongly correlated with aridity. The increase in forest-fire area over the past four decades can be largely explained by climate trends toward warming and reduced warm-season precipitation frequency. Both of these trends are consistent with what we expect from anthropogenic climate change, implying that forest-fire sizes are likely to continue to grow in the coming decades. But fuel accumulation due to fire suppression has probably played a role as well. Recent wildfire trends are often discussed with the assumption of a false dichotomy between effects of climate trends and fuel accumulation. Instead, the effects of these two processes have very likely been intertwined; as forest fuels have accumulated, fuel availability has become less of a limiting factor for fire spread. As fuels have become less limited, fuel dryness can more often lead as the limiting factor, allowing the fire-promoting effects of heat and aridity to become increasingly potent. Continued work is needed to better understand the interacting effects of fuels and climate on western US wildfire in order to inform our expectations of future wildfire trends and decisions about how to better manage fire, fuels, and other related natural resources such as water.