It’s not solely an ignition event, whether a lightning strike or unattended campfire, that causes a wildfire. Fire ecology research over the past three decades has revealed that environmental drivers, such as snow pack, fuels availability, and temperature, also play a role. In particular, warming temperatures are implicated in the increasing number and intensity of wildfires occurring across the western United States. They cause earlier melting of the winter snowpack, resulting in longer fire seasons, and hotter summer temperatures that more thoroughly dry out vegetation and make it more likely to catch fire when ignited.
However, U.S. Forest Service scientists suspected another weather factor was being overlooked as a contributor to recent trends in wildfire: precipitation, specifically summer precipitation. Zachary Holden, an ecologist with the USFS Northern Region, Charles Luce, a research hydrologist, and Matt Jolly, a research ecologist, both with the Rocky Mountain Research Station anecdotally noticed low summer precipitation was associated with the 1988 Yellowstone fire and the major wildfire season of 2017 in the Pacific Northwest.
It intuitively makes sense why. "Summer dry periods are tightly coupled to how warm and dry the air is during the fire season," Holden said. "Longer windows without rain lead to more surface heating, which dries out woody fuels."