Anthropogenic and lightning‐started fires are becoming larger and more frequent over a longer season length in the U.S.A.

Abstract

Aim Over the past several decades, wildfires have become larger, more frequent, and/or more severe in many areas. Simultaneously, anthropogenic ignitions are steadily growing. We have little understanding of how increasing anthropogenic ignitions are changing modern fire regimes. Location Conterminous United States. Time period 1984–2016. Major taxa studied Vegetation. Methods We aggregated fire radiative power (FRP)‐based fire intensity, event size, burned area, frequency, season length, and ignition type data from > 1.8 million government records and remote sensing data at a 50‐km resolution. We evaluated the relationship between fire physical characteristics and ignition type to determine if and how modern U.S.A. fire regimes are changing sensu stricto given increased anthropogenic ignitions, and how those patterns vary over space and time. Results At a national scale, wildfires occur over longer fire seasons (17% increase) and have become larger (78%) and more frequent (12%), but not necessarily more intense. Further, human ignitions have increased 9% proportionally. The proportion of human ignitions has a negative relationship with fire size and FRP and a positive relationship with fire frequency and season length. Areas dominated by lightning ignitions experience fires that are 2.4 times more intense and 9.2 times larger. Areas dominated by human ignitions experience fires that are twice as frequent and have a fire season that is 2.4 times longer. The effect of human ignitions on fire characteristics varies regionally. Ecoregions in the eastern U.S.A. and in some parts of the coastal western U.S.A. have no areas dominated by lightning ignitions. For the remaining ecoregions, more intense and larger fires are associated with lightning ignitions, and longer season lengths are associated with human ignitions. Main conclusions Increasing anthropogenic ignitions – in tandem with climate and land cover change – are contributing to a ‘new normal’ of fire activity across continental scales.

Wildfire Trend Analysis over the Contiguous United States Using Remote Sensing Observations

“…Previous studies have used regression analyses, both bivariate and multiple, to determine relationships between anthropogenic-wildfire relationships, and climate-wildfire relationships at different levels and types of aggregation (e.g., state, national, ecoregions) [21,25,26,36,48]. Linear regression analyses have also been used to evaluate the wildfire frequency and burned area by vegetation type to determine changes in fire regimes over time [49].…”

“…Anthropogenic climate change has also influenced fuel aridity (increasing flammability) in the Western US through increases in temperature and vapor pressure deficit trends [25]. Additionally, areas dominated by human ignitions exemplify fires that are two times more frequent and occur over fire season lengths that are 2.4 times longer [26]. However, areas dominated by lighting ignitions experience average fires that are increasing 23 times faster in size over time than areas dominated by anthropogenic activities [26].…”

“…Additionally, areas dominated by human ignitions exemplify fires that are two times more frequent and occur over fire season lengths that are 2.4 times longer [26]. However, areas dominated by lighting ignitions experience average fires that are increasing 23 times faster in size over time than areas dominated by anthropogenic activities [26]. In addition, the highest absolute gains in the wildland-urban interface (a region where housing and wildland vegetation interact) area occurred in the east; whereas, high gains in houses and people in the wildland-urban interface were most common in the South and Southwest portion of the US [27].…”

“…The MTBS database is a well-established source for wildfire research. Various studies have implemented MTBS data with varying periods and study regions, to understand climate-wildfire relationships [7,17,24,[34][35][36][37], analyze human influences on wildfire regimes [23,26,27,38], large wildfire trend identification within ecoregions [21,39], analyze the repercussions US wildfire management had on wildfire regimes [40], understand the role of how fuel treatments affect wildfire size and risk [41], and for the development of wildfire models to better predict large wildfire occurrences across the US [42]. However, some notable limitations of the MTBS database are that there is a~2-year lag in the inclusion of data (limiting this study to 2017), islands of no detectable change are included in the derived fire perimeters, phenology offsets are not automatically applied to some spectral indices, and the existence of highly variable classification thresholds for mapping burn severity [43].…”

More than one way to kill a spruce forest: The role of fire and climate in the late‐glacial termination of spruce woodlands across the southern Great Lakes

“…Anthropogenic global warming is creating novel disturbance regimes, and the combination of warming and novel disturbance is altering the structure and function of boreal forest and taiga ecosystems in North America and Eurasia, with the potential to cause vegetation‐type conversions (Girardin et al., 2009; Johnstone et al., 2016; Keeley et al, 2019; Noce et al., 2019; Turner, 2010). In North American boreal forests, temperatures are expected to rise by 4–11°C over this century (IPCC, 2014), causing fires to increase in size, severity, and frequency (Balshi et al., 2009; Cattau et al., 2020; Flannigan et al., 2000, 2005; Kasischke & Turetsky, 2006). The combination of warming and intensified fire regimes in boreal ecosystems can trigger abrupt transitions to new vegetation states, as boreal species are replaced by species that are well‐suited to new climatic conditions (Chapin et al., 2004; Johnstone, et al., 2010; Weber & Flannigan, 1997).…”