Handout (10.2 MB)
These fire index projections are part of a convergent science project looking to inform decision-making from national policy down to local scales by working with managers and policymakers to co-produce actionable knowledge. Changes in fire season length, severity, and peak as well as unprecedented conditions are of particular interest and may inform long term fire management planning and policies as well as reframe current decision-making in a changing climate.
As different fire indexes incorporate different fuel and soil conditions, and certain fire indexes are favored over others for various applications in different regions, we use 8 indexes, with 2 variations on 3 of those, for 11 measures of fire behavior. We examine changes in the number of days above the 80th, 90th, and 97th percentiles of these indexes, corresponding to thresholds used to assign descriptors of fire danger as high, very high, and severe. To define fire season length, we use the number of days above the 80th percentile for each given index as well as a supplementary definition of the number of days above the 50th percentile.
We employ thirteen simulations produced for the North American component of the Coordinated Regional climate Downscaling Experiment (NA-CORDEX), leveraging RCP8.5 emission scenario simulations for the future projections. From these simulations, we calculate the KBDI (Keetch-Byram Drought Index), mFFWI, (modified Fosberg Fire Weather Index), CFWI (Canadian Fire Weather Index), FM100 and FM1000 (100- and 1000-hour Fuel Moisture), ERC (Energy Release Component), BI (Burning Index), and SFDI (Severe Fire Danger Index). Two fuel scenarios, G and LAF, are input into ERC, BI, and SFDI.
By mid-century, most regions are projected to see an increase in the length of fire season, though the magnitude of this projected change varies considerably by region and fire index (and the latter’s sensitivity to precipitation). Agreement on a lengthening of the season by up to 50% is strongest across simulations and fire indexes over the Southern Plains and Southwest U.S. Changes in fire season severity are largest and most consistent across indexes for the U.S. west of the Mississippi River, excluding the intermountain West, where uncertainty is higher across the indexes and RCMs. An approximately 2- to 6-fold increase in the number of days that reach the severe fire danger threshold is projected by mid-century. Simulations for the far future are qualitatively similar, but the projected changes are quantitatively worse and more widespread.
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