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.

Santa Ana Winds of Southern California Impact PM2.5With and Without Smoke From Wildfires

“…These regional trends are consistent with a global increase in wildfire activity (Jolly et al, ). Recent work suggests that wildfire severity and risk specifically in SoCal will likely intensify in the warming future, while gradually shifting from fall to winter (Williams et al, ). This seasonal shift is associated with a projected weakening of SAW activity, particularly in the fall and spring (that is coincident with a projected decrease in fall precipitation; Pierce et al, ; Swain et al, ; and thus more likelihood of dry fuels persisting into winter) and a sharper seasonal SAW peak in December (Guzman‐Morales & Gershunov, ).…”

Indigenous Knowledge and Seasonal Calendar Inform Adaptive Savanna Burning in Northern Australia

“…Wildfire management is an escalating issue globally, with economic, environmental, social and cultural consequences [1][2][3][4]. In fire-prone regions such as the Americas, Australia and parts of Asia and Africa, wildfire management presents a formidable ongoing challenge that must be urgently addressed.…”

Attributing Increases in Fire Weather to Anthropogenic Climate Change Over France

“…2016; Williams et al, 2019). Figure 1 illustrates traces of 50year low-pass filtered signals of climate change for maximum temperature, precipitation, and minimum relative humidity for the month of July for a given GCM grid cell.…”

“…These trends are already evident globally in the observational record (Jolly et al, 2015), including across parts of France (Dupire et al, 2017;Fréjaville and Curt, 2017;Curt and Fréjaville, 2018). Increases in fire weather conditions have been attributed to anthropogenic global warming in portions of western North America (Yoon et al, 2015;Abatzoglou and Williams, 2016;Kirchmeier-Young et al, 2017Tan et al, 2018;Williams et al, 2019) but the degree to which global warming has contributed to changes in fire weather danger characteristics in France, and more generally across the Euro-Mediterranean basin, has not been quantified. The region is of particular interest as climate models project both a strong warming-the so-called Mediterranean amplification- (Brogli et al, 2019) and drier summers which are expected to collectively exacerbate fire weather conditions (Turco et al, 2018;Fargeon et al, 2020).…”