Exacerbating risk in human-ignited large fires over western United States due to lower flammability thresholds and greenhouse gas emissions.

Large fires in the western United States become highly probable when dry conditions surpass critical thresholds of vapor pressure deficit (VPD_t). VPD_t likely differs between human- and lightning-ignited fires, potentially leading to ignition-type varied responses of fire weather risk to natural variability and various anthropogenic forcings, yet a comprehensive quantification remains lacking. Here, through fire observations with ignition types and a machine learning method, we found that human-ignited large fires had consistently lower thresholds (VPD_t) across western US ecoregions. Consequently, the annual number of flammable days (when VPD > VPD_t) for human-caused large fires was 93% higher on average and increased 21% more rapidly than those caused by lightning during 1979-2020. Through robust statistical detection and attribution of Earth System Models, we found that the anthropogenic greenhouse gas (GHG) emissions predominantly (81%) controlled the human-related flammable day increases, which was 18% greater than the effect of GHGs on the increases in lightning-related flammable days. Such ignition-type varied fire weather risk indicates more large fire-prone conditions for human-regulated fire regimes when GHG emissions are enhancing and ignitions are not limited by fuels.