The economic and ecological costs of wildfire in the United States (U.S.) have risen substantially in recent decades. While climate change has likely enabled a portion of the increase in wildfire activity, the direct role of people in increasing wildfire has been largely overlooked. In order to understand the role of humans in wildfires in the U.S., we evaluated over 1.5 million government records of wildfires that were extinguished or managed by state or federal agencies from 1992-2012 using the U.S. Forest Service Fire Program Analysis Fire-Occurrence Database. We calculated the proportion of human- vs. lightning-caused wildfires within equal-area 50 x 50 km grid cells across the coterminous U.S. We calculated the length of the human- and lightning-caused fire seasons as the interquartile range of the Julian day of recorded fire ignition at the scale of the coterminous U.S. and for Level I ecoregions. We obtained monthly lightning strike density and 1000-hr dead fuel moisture data and aggregated these datasets to 50 x 50 km grid cells. We tested whether fire niche expansion (as determined by fuel moisture and lightning-strike density) due to human ignitions was significant using Mann-Whitney tests between human-started vs. lightning-started fires.
Humans have vastly expanded the spatial and seasonal fire niche in the coterminous U.S., accounting for 84% of all wildfires (n=245,526 lightning-started wildfires; n=1,273,138 human-started wildfires) and 44% of total area burned. During the 21 year time period (1992-2012), the human-caused fire season was about three times longer than the lightning-caused fire season (154 vs. 46 days for human- and lightning-caused fires, respectively) and added an average of 40,000 wildfires per year across the U.S. Human-started wildfires disproportionally occurred where fuel moisture was ~50% higher (p<0.0001) and lightning strike density was ~40% lower (p<0.0001) than for lightning-started fires, thereby expanding the geographic and seasonal niche of wildfire. Human-started wildfires were dominant (>80% of ignitions) in over 5.1 million km2, the vast majority of the U.S., while lightning-started fires were dominant in only 0.7 million km2, primarily in sparsely populated areas of the mountainous western U.S. Ignitions caused by human activities are a substantial driver of overall fire risk to ecosystems and economies. Actions to raise awareness and increase management in regions prone to human-started wildfires should be a focus of U.S. policy to reduce fire risk and associated hazards.