EXCERPT
Observed Impacts of Anthropogenic Climate Change on Wildfire in California
Abstract
Recent fire seasons have fueled intense speculation regarding the effect of anthropogenic climate change on wildfire in western North America and especially in California. During 1972–2018, California experienced a fivefold increase in annual burned area, mainly due to more than an eightfold increase in summer forest‐fire extent. Increased summer forest‐fire area very likely occurred due to increased atmospheric aridity caused by warming. Since the early 1970s, warm‐season days warmed by approximately 1.4 °C as part of a centennial warming trend, significantly increasing the atmospheric vapor pressure deficit (VPD). These trends are consistent with anthropogenic trends simulated by climate models. The response of summer forest‐fire area to VPD is exponential, meaning that warming has grown increasingly impactful. Robust interannual relationships between VPD and summer forest‐fire area strongly suggest that nearly all of the increase in summer forest‐fire area during 1972–2018 was driven by increased VPD. Climate change effects on summer wildfire were less evident in nonforested lands. In fall, wind events and delayed onset of winter precipitation are the dominant promoters of wildfire. While these variables did not change much over the past century, background warming and consequent fuel drying is increasingly enhancing the potential for large fall wildfires. Among the many processes important to California's diverse fire regimes, warming‐driven fuel drying is the clearest link between anthropogenic climate change and increased California wildfire activity to date.
Plain Language Summary
Since the early 1970s, California's annual wildfire extent increased fivefold, punctuated by extremely large and destructive wildfires in 2017 and 2018. This trend was mainly due to an eightfold increase in summertime forest‐fire area and was very likely driven by drying of fuels promoted by human‐induced warming. Warming effects were also apparent in the fall by enhancing the odds that fuels are dry when strong fall wind events occur. The ability of dry fuels to promote large fires is nonlinear, which has allowed warming to become increasingly impactful. Human‐caused warming has already significantly enhanced wildfire activity in California, particularly in the forests of the Sierra Nevada and North Coast, and will likely continue to do so in the coming decades.
1 Introduction
In the western United States, annual area burned increased substantially in recent decades due to increased frequency and size of large wildfires (Abatzoglou & Williams, 2016; Balch et al., 2018; Dennison et al., 2014; Westerling, 2016). It is well established that this observed increase in wildfire activity was promoted in many areas by reduced fuel moisture due to warming‐induced increases in evaporative demand, reduced snowpack, and reduced warm‐season precipitation frequency (Abatzoglou & Williams, 2016; Holden et al., 2018; Kitzberger et al., 2017; Westerling, 2016). These recent climate trends are broadly consistent with those expected from anthropogenic climate change (Abatzoglou & Williams, 2016), but anthropogenic climate effects on wildfire can vary greatly across space and time due to confounding factors such as natural climate variations, land and fire management practices, ignitions from humans, spatial diversity in vegetation type, and the complex ways in which these processes interact (Williams & Abatzoglou, 2016). Therefore, location‐specific adaptation responses to wildfire require understanding how climate affects wildfire locally, how the key climate variables have changed over the past several decades, and whether these climate changes are likely to continue.
Perhaps nowhere on Earth has received more attention regarding recent wildfire trends and their causes than California. One reason for the attention is that increases in statewide burned area over the last several decades were dramatically punctuated in 2017 and 2018 by particularly extreme wildfire activity with substantial loss of life and property. In 2017, modern state records were set for the largest individual wildfire (Thomas Fire: 114,078 ha) and the most structures destroyed by an individual wildfire (Tubbs Fire: 5,636 structures), which led to 22 fatalities (CalFire, 2018). The total area burned in 2017 was also nearly a state record at the time (505,293 ha), behind 2007. In 2018, state records were set for total area burned (676,312 ha), largest individual wildfire (Mendocino Complex Fire: 185,800 ha), and most destructive wildfire (Camp Fire: 18,804 structures destroyed, 85 fatalities). In these 2 years, California spent over $1.5 billion on fire suppression, far more than any previous 2‐year period (CalFire, 2018).
California is a particularly difficult place to disentangle the drivers of changing wildfire activity. California's climate, vegetation cover, and human settlement patterns are highly diverse, causing the influences of these factors on fire activity to be spatially heterogeneous and complex (Jin et al., 2014; Jin et al., 2015; Keeley & Syphard, 2017; Swetnam & Baisan, 2003; Westerling & Bryant, 2008). Humans dominate the wildfire regime across much of the state by altering land cover (Sleeter et al., 2011; Syphard et al., 2018), supplying the vast majority of ignitions (Balch et al., 2017; Nagy et al., 2018), and attempting to suppress essentially all fires. Fire suppression over the past century allowed for artificial buildup of fuels in many regions that historically experienced frequent low‐intensity fires, reducing fuel limitation as a constraint on fire activity and putting many areas into a so‐called fire deficit (Higuera et al., 2015; Marlon et al., 2012; Minnich et al., 1995; Parks et al., 2015). Even under constant climate conditions, changes in California's fire activity over the past century would be expected as populations increased and cities expanded into surrounding wildlands (Radeloff et al., 2018), fire suppression strategies evolved (Stephens & Ruth, 2005), and frequency and type of human‐ignited wildfires changed (Balch et al., 2017; Keeley & Syphard, 2018). Changes in these nonclimatic factors may also promote nonstationarity in fire‐climate relationships, confounding efforts to isolate the influence of climate change on fire activity (Higuera et al., 2015; Hurteau et al., 2019; Littell, 2018; Mann et al., 2016; Marlon et al., 2012; Taylor et al., 2016).
The effect of climate on wildfire in California is highly seasonal and variable across vegetation gradients. In summer, when fires are most frequent in California, large burned areas are promoted by the cumulative drying effects of atmospheric aridity and precipitation deficits mainly in forest ecosystems where fuel availability is not a limiting factor (Abatzoglou & Kolden, 2013; Jin et al., 2014; Keeley & Syphard, 2016; Swetnam, 1993; Swetnam & Betancourt, 1998; Westerling et al., 2003; Williams et al., 2018). In fall, many of California's most destructive fires occur in coastal shrublands and are driven by often extreme offshore downslope wind events, where synoptic conditions advect dry air masses often originating from the continental interior high desert westward and southward across topographic barriers such as the Transverse, Peninsular, and Coastal Ranges (Conil & Hall, 2006; Guzman‐Morales et al., 2016; Hughes & Hall, 2010; Moritz et al., 2010; Nauslar et al., 2018). The most widely studied offshore wind events, termed Santa Ana winds in southern California, increase in frequency in the fall and peak in winter (Abatzoglou et al., 2013; Raphael, 2003). Strong offshore winds with very low relative humidity can quickly dry fuels and spread large wildfires when they occur prior to the onset of the winter precipitation season in California's Mediterranean climate (Billmire et al., 2014; Keeley, 2004; Moritz et al., 2010; Westerling et al., 2004).
The effects of anthropogenic climate change on California's fire regimes are likely to be diverse and complex, varying by region and season (Liang et al., 2017; Pierce et al., 2018; Syphard et al., 2019; Westerling, 2018). Climate model projections of warming and increased atmospheric aridity in California are strong and robust across models (Pierce et al., 2013). It is well established that warming promotes wildfire throughout the western United States, particularly in forested regions, by enhancing atmospheric moisture demand and reducing summer soil moisture as snowpack declines (Abatzoglou & Williams, 2016; Westerling et al., 2006). By contrast, model projections of precipitation in California are highly uncertain but with a tendency toward increased precipitation annual totals, particularly in northern California during winter (Maloney et al., 2013). However, many climate models have systematic biases in North Pacific storm tracks and tropical Pacific sea surface temperatures that should lead to strong skepticism regarding model simulations of future precipitation in California (Seager et al., 2019; Simpson et al., 2016). Climate models also project precipitation frequency declines in spring through fall that would partly offset winter increases, resulting in increased precipitation variability (AghaKouchak et al., 2018; Pierce et al., 2018; Polade et al., 2014; Polade et al., 2017; Swain et al., 2018). In fall, models project reduced frequency and intensity of Santa Ana wind events (Guzman‐Morales & Gershunov, 2019; Hughes et al., 2011). However, concurrent warming and decreased fall precipitation may, to some degree, counteract the effects of reduced offshore winds on fall fire risk in southwestern California (Hughes et al., 2011; Pierce et al., 2018), possibly extending the fire season towards the winter peak of the downslope wind season (Guzman‐Morales & Gershunov, 2019; Syphard et al., 2018).
While much has been published on projected changes in wildfire activity due to climate change (e.g., Barbero et al., 2015; Hurteau et al., 2019; Krawchuk & Moritz, 2012; Littell et al., 2018; Westerling, 2018; Westerling et al., 2011; Westerling & Bryant, 2008), less has been done to evaluate observed seasonal trends in fire‐relevant climate variables and whether these trends are consistent with those expected to arise from anthropogenic climate change. Here we provide a comprehensive empirical assessment of the observed effects of climate variability and change on California wildfire by season, region, and land cover. We first use wildfire and climate data within California to evaluate trends in seasonal burned area by region during 1972–2018, resolve the distinct seasonal and regional influences of climate and weather factors, and assess the stationarity of the dominant fire‐climate relationships over the past five decades. We then use climate model simulations to determine whether observed trends in the climate variables most pertinent to regional wildfire activity are consistent with expectations of anthropogenic climate change. A thorough and nuanced understanding of how, when, and where anthropogenic climate change has or has not affected wildfire in California over the past several decades is critical to guide sustainable societal decisions ranging from where to develop housing to how limited resources can be optimized for landscape management.
4 Conclusions
California has been the geographic focus of extensive speculation among scientists, politicians, and media as to the biophysical and societal factors that have contributed to recent exceptional wildfires and large increases in wildfire activity in recent decades (e.g., Krieger, 2018; Pierre‐Louis, 2018; Vore, 2018). Anthropogenic climate change is commonly debated as a driver of these recent wildfire changes, but there are many ways in which anthropogenic climate change could conceivably affect wildfire and many variables that wildfire in California is sensitive to. A nuanced understanding of how, when, where, and why California wildfire activity has increased in recent decades is critical for sustainable environmental and development decisions that specifically take into account how anthropogenic climate change is likely to proceed and affect wildfire across California's diverse landscapes. Our methods should be increasingly applicable to other regions globally, as governmental and satellite‐based records of wildfire activity are steadily alleviating observational duration as a major limiting factor for empirical studies of wildfire.
In this study we evaluated the various possible links between anthropogenic climate change and observed changes in California wildfire activity across seasons, regions, and land cover types since the early 1970s. The clearest link between California wildfire and anthropogenic climate change thus far has been via warming‐driven increases in atmospheric aridity, which works to dry fuels and promote summer forest fire, particularly in the North Coast and Sierra Nevada regions. Warming has been far less influential on summer wildfire in nonforest areas. In fall, the drivers of wildfire are particularly complex, but warming does appear to enhance the probability of large fall wildfires such as those in 2017 and 2018, and this effect is likely to grow in the coming decades.
Importantly, the effects of anthropogenic warming on California wildfire thus far have arisen from what may someday be viewed as a relatively small amount of warming. According to climate models, anthropogenic warming since the late 1800s has increased the atmospheric vapor‐pressure deficit by approximately 10%, and this increase is projected to double by the 2060s. Given the exponential response of California burned area to aridity, the influence of anthropogenic warming on wildfire activity over the next few decades will likely be larger than the observed influence thus far where fuel abundance is not limiting.
Below, we conclude with executive summaries of our primary findings for summer wildfire in forests, summer wildfire in nonforests, and fall wildfire.
4.1 Summer, Forest
Annual statewide burned area increased significantly during 1972–2018, largely due to an eightfold increase in annual summer forest‐fire extent, most of which occurred in the heavily forested North Coast and Sierra Nevada regions. Summer forest‐fire extent is strongly dictated by heat and atmospheric aridity, which reduce snowpack and dry out fuels. Warm‐season atmospheric aridity (vapor‐pressure deficit) increased significantly across California since the late 1800s, driven largely by daytime warming of approximately 1.8 °C (1.4 °C since the early 1970s). Based on a regression analysis, the vast majority of the observed increase in summer forest‐fire extent since 1972 is accounted for by observed significant increases in warm‐season vapor‐pressure deficit (caused by warming). Importantly, the sensitivity of burned area to aridity is modulated by background conditions such as fuel abundance and connectivity, ignition frequency, and resources dedicated toward suppression, all of which changed over the past century. However, the statistical relationship between vapor‐pressure deficit and forest fire area remained stable during 1972–2018, supporting the interpretation that increased aridity was the primary driver of the increase in summer forest‐fire area during this time. The observed rates of warming and increasing vapor‐pressure deficit are consistent with those simulated by climate models when forced by anthropogenic emissions, indicating that these trends are extremely likely to continue for decades to come. The large increase in California's annual forest‐fire area over the past several decades is very likely linked to anthropogenic warming.
4.2 Summer, Nonforest
Annual summer burned area did not increase in nonforest lands in the Central and South Coast regions, and increases in nonforest burned area were weak in the North Coast and Sierra Nevada. Summer nonforested burned area is most strongly promoted by high precipitation total in the year or two prior to the fire year, reflecting the necessity of precipitation for growth of fine fuels that can facilitate fire spread in the subsequent year. Fire‐year precipitation deficit and atmospheric aridity also appear to promote summer wildfire in these regions, but this effect is relatively weak. Over the past century, the frequency of wet years followed by dry years increased, which should have promoted nonforest summer wildfire. The lack of large increases in nonforest summer burned area may reflect the counteracting effects of the other factors such as human fire suppression, reduced ignitions, and reduced vegetation cover due to drought. Climate models do not represent the observed increase in interannual swings from wet to dry years as a robust result of anthropogenic climate change thus far. The link between anthropogenic climate change and summer wildfire in nonforest appears weak thus far.
4.3 Fall
Large fall wildfires became more frequent in California over the past several decades, mainly due to increases in the North Coast and Sierra Nevada regions. In all regions, large fall wildfires often occur when strong offshore wind events coincide with dry fuels. These conditions were extreme throughout California in 2017 and 2018, driving very large fall wildfires in all regions in one or both years. The character of offshore wind events did not change since records began in the mid‐1900s. Climate models project these wind events to decrease in frequency and intensity in the future. Fall fuel moisture is dictated largely by fall precipitation, but nonetheless is calculated to have declined significantly since the mid‐1900s due to warming. This warming‐induced drying was likely caused by a combination of anthropogenic forcing and natural multidecade variability and caused an 8‐day increase in the number of days per October–December with a high probability of large wildfires. This exemplifies an important secondary effect of background warming, which is projected to continue, on fall wildfires. In the South Coast, fall drying was also promoted by a small reduction in the frequency of fall precipitation, consistent with climate model projections. The link between anthropogenic climate change and fall wildfire appears weaker than in summer thus far but is likely to strengthen if continued warming and possibly delayed onset of winter precipitation counteract projected decreases in the intensity and frequency of offshore wind events.
Acknowledgments
All data sets used are publicly available, and the sources are listed in Table S1. A.P.W. was funded by Columbia University's Center for Climate and Life and the Zegar Family Foundation. We also acknowledge support from University of California Office of the President MRPI grant MRP‐17‐446315 (D.L., A.G., and J.G.M.), from NOAA via the CNAP RISA (D.L. and A.G.), from DOI via the Southwest Climate Adaptation Science Center grant G18AC00320 (A.G. and J.G.M.), the Visiting Scholar Program and Fire Centre Research Hub at the University of Tasmania (J.T.A.), Earth Lab through CIRES and the University of Colorado, Boulder's Grand Challenge Initiative (J.K.B.), and the USGS North Central Climate Adaptation Science Center (J.K.B.). LDEO publication 8332.
