Northern Rockies Forest Wildfires
The Northern Rockies region includes western portions of Montana and Colorado, northern and central Idaho, and eastern Washington and Oregon. This entire area shares generally similar climate and geography; combining data across the region makes patterns stand out more clearly than they would if you analyzed individual states.
The data for this graphic come from Tony Westerling at the University of California-Merced. They’re from a 2006 paper (Abstract) in the journal Science written by Westerling and colleagues. The data actually come only from National Forests (USDA Forest Service) and National Parks (U.S. Department of Interior’s National Park Service). But while that may seem restrictive, these areas represent most of the forested lands in the Northern Rocky Mountains. In part because fire records are much more reliable for large fires compared to small fires, and also because large fires are more destructive, Westerling and colleagues focused only on large fires (at least 1000 acres in size); by doing this they eliminated over 99% of the total reported fires, yet could still include 73% of the total area burned.
The graphic shows an increase in area burned in recent decades. The difference is that in recent decades (only), there are years with very high fire activity. These tend to be years with earlier onset of spring, specifically earlier snowmelt. This results in a longer and more severe fire season.