During 2000–2018, repeated severe precipitation shortfalls were superposed on a generally drier- and warmer-than-average background state to cause extended drought conditions across much of western North America. The consequences included fresh-water shortages for humans and riparian ecosystems, forest die offs, and giant wildfires that affected air quality over thousands of kilometers. The severity, duration, and consequences of 21st century drought have been unprecedented across much of western North America in the context of the past century of observations, motivating valid questions about the possible role of anthropogenic climate change. However, paleo proxies for past climate remind us that western North America is capable of undergoing massive hydroclimatic variations without assistance from humans, as evidenced by the infamous Medieval megadroughts in the western United States and northern Mexico that occurred repeatedly and lasted sometimes for several decades.
In this talk I will use a new tree-ring reconstruction of summer soil moisture across western North America to place the 21st century drought into a 1200-year context. I will then use hydrological modeling to evaluate the effect of anthropogenic climate trends and the degree to which these trends have affected how drought conditions during 2000–2018 compare to those of the Medieval megadroughts. Given that the 21st century drought coincided with marked increases in wildfire activity, and this audience’s likely interest in the effects of wildfire on atmospheric composition, the talk will conclude with a specific examination of the case for anthropogenic climate change as an important factor promoting wildfire in the western United States.