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Decreasing number of rainy days in summer has increased western wildfire

Date: September 06, 2018

New research shows that significant declines in summer precipitation, and lengthening summer dry spells, are major drivers of the increase in fire activity.


The number and size of large wildfires have increased dramatically in the western United States during the past three decades. Prior understanding was that the increase in fires was mainly attributable to warming temperatures and earlier snowmelt. In this study, a research team contrasted the three, main hypothesized climatic drivers of recent increases in western US wildfire activity: decreased snowpack, increased temperature, and decreased precipitation. Data was pulled from satellite-derived maps of forest wildfire area burned from eight western US ecoregions, and daily gridded temperature, humidity, and snow water equivalent datasets, to examine the sensitivity of wildfire area burned to standardized hydrologic and precipitation indices. Scientists then quantified trends from 1979 to 2016.

A fire burns through the understory.
A fire burns through the understory.

Key Findings

  • Summer precipitation totals and the duration of dry spells were the strongest controls on forest area burned by wildfire.
  • Declining summer rain rather than declining winter snow or increasing wildfire season length has been a primary driver of increasing wildfire in the western U.S.

  • Less summer rain exacerbates increasing temperatures as well because less water is available for evapotranspiration.

  • We can use an understanding of where future summer rain declines are likely to occur to guide adaptation planning.

Featured Publications

Holden, Zachary A. ; Swanson, Alan ; Luce, Charles H. ; Jolly, William M. ; Maneta, Marco ; Oyler, Jared W. ; Warren, Dyer A. ; Parsons, Russell A. ; Affleck, David , 2018

Principal Investigators - External: 
Zachary A. Holden (Principal Investigator) - United States Forest Service (Region 1)
Alan Swanson - University of Montana
Marco Maneta - University of Montana
Jared W. Oyler - Pennsylvania State University
Dyer A. Warren - University of Montana
David Affleck - University of Montana