Evolving fire frequency in the western US and its links to human influence

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Burned area and damages from wildfires in the western United States have increased in recent decades. Contrary to the common belief that the frequency of wildfires has also increased, trends in the number of fires have been less clear than trends in area burned, and they vary by data set, region, and fire size. Using a comprehensive fire occurrence data set, we find a significant decline in the annual number of wildfires in the western United States over the past three decades. Decreases in human-caused ignitions are most evident in California and Arizona, while human-caused ignitions increased in Wyoming. When examining how the number of human-caused fires relates to population patterns, we find that the relationship varies. In areas with very low population, fires increase as population increases. However, after the population reaches a certain level, further population growth is associated with fewer fires. Considering the influences of human populations and demographics on wildfire could improve both global and western United States estimates of future wildfire activity compared to estimates based on climate change alone.