Abstract Vapor pressure deficit (VPD) is a key fire weather indicator linked to increased burned areas in western US. Despite a strong increase in regional VPD due to climate change, we find no significant trend in VPD on fire ignition days (VPDF). This discrepancy is due to a decreasing climatological mean (VPDF‐m), driven by the expansion of fires into higher‐latitude and higher‐altitude regions with climatologically lower VPD, and a relatively stable anomaly (VPDF‐a). The weak trend in this VPDF‐a is, in turn, a result of two opposing trends: a significant increase in the thermodynamic contribution from background warming, offset by a weakening of the dynamic contribution from circulation patterns. This lowering of the circulation “bar” for ignition, where less extreme weather is now sufficient to start large fires, explains the observed spatial expansion of fire risk, meaning regions historically less prone to fires now face heightened risk.
Observations from the Little Bunker
Last updated: January 27, 2026 08:12 PM
Atmospheric CO₂
428.6 ppm
+17.4
ppm/year
Atmospheric CH₄
1940.59 ppb
+-0.24
ppb/year
Global Temperature
+1.48 °C
Peak: +1.73°C
World Population
8.1 billion
+67
million/year