Human-started wildfires expand the fire niche across the United States

Abstract

SignificanceFighting wildfires in the United States costs billions of dollars annually. Public dialog and ongoing research have focused on increasing wildfire risk because of climate warming, overlooking the direct role that people play in igniting wildfires and increasing fire activity. Our analysis of two decades of government agency wildfire records highlights the fundamental role of human ignitions. Human-started wildfires accounted for 84% of all wildfires, tripled the length of the fire season, dominated an area seven times greater than that affected by lightning fires, and were responsible for nearly half of all area burned. National and regional policy efforts to mitigate wildfire-related hazards would benefit from focusing on reducing the human expansion of the fire niche.

Spatial Distribution of Wildfires Ignited under Katabatic versus Non-Katabatic Winds in Mediterranean Southern California USA

“…The Santa Monica Mountains and greater Simi Valley experienced the most repeat fire driven by Santa Ana winds, and had little evidence of non-katabatic large fire growth, consistent with prior assessments of high fire danger during Santa Ana conditions [26,27]. As there is a high rate of fire ignitions in the study area due to high human population density [5], this may suggest that fires occurring under non-katabatic conditions are easily suppressed, due to factors such as relatively high fuel moisture associated with the marine layer [28]. Additionally, the spatial dichotomy of these fire regimes is likely also a byproduct of the seasonality of extreme fire danger, defined by days where the Burning Index calculated using the US National Fire Danger Rating System and a gridded surface meteorological dataset [29] from 1979-2017 exceeds the 95th percentile.…”

“…Prior to European settlement, fire-adapted vegetation and climatic conditions facilitated large, infrequent wildfires that occurred primarily when summer ignitions were held over into autumn and then exploded across the landscape during autumn katabatic wind events, known locally as 'Santa Ana' winds [3]. Today, the high density of anthropogenic ignitions has increased the frequency of large fires and expanded the fire season to year-round, fundamentally altering fire regimes [4,5]. Improved understanding of altered fire regimes and their impacts is crucial to both regional fire management agencies seeking to reduce catastrophic wildfires [6] and to those seeking to protect threatened ecosystems in the face of changing climate and ex-urban growth.…”

“…As dry lightning in the region is limited [19] and most wildfires are human-caused [5], one of the keys for preventing wildfire disasters is identifying and mitigating controllable human ignition sources. Such delineation helps to direct public education and outreach efforts, which have been shown to significantly reduce certain types of wildfire ignitions, particularly those that were accidental [20].…”

The 2017 North Bay and Southern California Fires: A Case Study

“…By contrast, trends in fire activity have been more subdued in Mediterranean California ecoregions and across broader southern California [14,16,17]. The reasons behind these diverging trends may be tied to a decrease in reported fire ignitions [17], which are nearly all human-caused [18], as well as anthropogenic activities including land-use, fire policies, and diligent fire suppression [19]. Nonetheless, interannual climate variability does exhibit significant relationships to burned area extent in these regions [3,17,20,21].…”

Brief Communication: Synoptic-scale differences between Sundowner and Santa Ana wind regimes in the Santa Ynez Mountains, California

“…The Mediterranean climate promotes the accumulation of fine fuel during mild wet winters that dry during extended warm and dry summers. In this region, humans are the primary source of ignitions (Balch et al, 2017), with notable Santa Ynez fires (Fig. 1a) resulting from accidental ignitions to arson.…”