title: Human-started wildfires expand the fire niche across the United States.
abstract: The economic and ecological costs of wildfire in the United States have risen substantially in recent decades. Although climate change has likely enabled a portion of the increase in wildfire activity, the direct role of people in increasing wildfire activity has been largely overlooked. We evaluate over 1.5 million government records of wildfires that had to be extinguished or managed by state or federal agencies from 1992 to 2012, and examined geographic and seasonal extents of human-ignited wildfires relative to lightning-ignited wildfires. Humans have vastly expanded the spatial and seasonal "fire niche" in the coterminous United States, accounting for 84% of all wildfires and 44% of total area burned. During the 21-y time period, the human-caused fire season was three times longer than the lightning-caused fire season and added an average of 40,000 wildfires per year across the United States. Human-started wildfires disproportionally occurred where fuel moisture was higher than lightning-started fires, thereby helping expand 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 United States, whereas lightning-started fires were dominant in only 0.7 million km2, primarily in sparsely populated areas of the mountainous western United States. 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 United States policy to reduce fire risk and associated hazards.
authors.affiliation: Earth Lab, University of Colorado, Boulder, CO 80309; jennifer.balch@colorado.edu bbradley@eco.umass.edu.|Department of Geography, University of Colorado, Boulder, CO 80309.
authors.forename: Jennifer K
authors.initials: JK
authors.lastname: Balch
authors.affiliation: Department of Environmental Conservation, University of Massachusetts, Amherst, MA 01003; jennifer.balch@colorado.edu bbradley@eco.umass.edu.|Organismic and Evolutionary Biology Program, University of Massachusetts, Amherst, MA 01003.
authors.forename: Bethany A
authors.initials: BA
authors.lastname: Bradley
authors.affiliation: Department of Geography, University of Idaho, Moscow, ID 83844.
authors.forename: John T
authors.identifier: 0000-0001-7599-9750
authors.initials: JT
authors.lastname: Abatzoglou
authors.affiliation: Earth Lab, University of Colorado, Boulder, CO 80309.
authors.forename: R Chelsea
authors.initials: RC
authors.lastname: Nagy
authors.affiliation: Organismic and Evolutionary Biology Program, University of Massachusetts, Amherst, MA 01003.
authors.forename: Emily J
authors.initials: EJ
authors.lastname: Fusco
authors.affiliation: Earth Lab, University of Colorado, Boulder, CO 80309.|Department of Geography, University of Colorado, Boulder, CO 80309.
authors.forename: Adam L
authors.initials: AL
authors.lastname: Mahood
mesh_terms: D005843:Geography; D006802:Human Activities; D006801:Humans; D012621:Seasons; D062211:Spatio-Temporal Analysis; D014481:United States; D000075923:Wildfires
keywords: anthropogenic wildfires; fire starts; ignitions; modern fire regimes; wildfire causes