Global warming and the Pacific Decadal Oscillation drive seasonally varying increases in extreme fire weather over the southwestern US

Wildfire activity has increased across the western United States (US), with evidence linking these trends to anthropogenic climate forcing. Here, we analyze the frequency, intensity, and drivers of fire weather extremes in the southwestern US over the past 83 years. These extremes exhibit strong seasonality: wintertime changes align with long-term warming trends, while summertime variations are dominated by decadal fluctuations modulated by the Pacific Decadal Oscillation (PDO). Fire risk has increased steadily during December–May, outside the traditional fire season, primarily due to rising temperatures and declining relative humidity (RH). In contrast, summertime fire risk—largely governed by humidity—follows a roughly V-shaped trajectory, decreasing from 1940–1980 and then rising from 1981–2022, in close alignment with decadal shifts in the PDO. Although Earth system models (ESMs) reliably capture temperature-related changes, they underestimate interdecadal variability in humidity, manifested by a weak representation of the observed PDO–RH relationship and low-RH extremes in summer.