Wildfires and climate change

Top Line

Human-caused climate change is a significant contributor to the increasing size, intensity, and damage of western U.S. wildfires, and to a lengthening of U.S. fire seasons.

Facts for Any Story

• U.S. wildfires are being fueled by southwestern North America’s driest 22-year period in at least 1,200 years, based on soil water content. Human-caused climate change was responsible for 42% of that soil dryness. ¹Williams, A. Park et. al., (2022), Rapid intensification of the emerging southwestern North American megadrought in 2020–2021, Nature Climate Change, 12, 232-234 View Source
• In the western United States human-caused climate change caused more than half the increase in forest fuel aridity (how dry and flammable vegetation is) since the 1970s and has approximately doubled the cumulative area burned in forest fires since 1984.²Abatzoglou, J.T. and Williams, Park, (2016), Impact of anthropogenic climate change on wildfire across western U.S. forests, PNAS, 113, (42), 11770-11775 View Source
• Climate change-related declines in western spring snowpack, and increased evaporation from higher temperatures in spring, summer, and fall, have in the decades since the early 1980s reduced moisture and contributed to a marked increase in the frequency of large fires and the total area burned by western wildfires.³Westerling, A.L., et al., (2017) Warming and earlier spring increase western U.S. forest wildfire activity, Science, 313, 5789, 940-943 View Source A study of western U.S. ponderosa pine and Douglas fir forests concluded that climate change-related moisture deficits are undermining post-wildfire forest regeneration and recovery there.
• Fires in 2020 alone contributed 44% of the total area burned in the central Rocky Mountains from 1984 to 2020. Since 2000, high-elevation forests in this area have been burning at rates higher than at any point in the past 2,000 years.⁴Higuera, Philip E., et al., (2021) Rocky Mountain subalpine forests now burning more than any time in recent millennia, PNAS, 118 (25) View Source
• From 1972 to 2018, there was an eight-fold increase in the annual area burned in California’s summertime forest fires. Evidence suggests that this dramatic increase was caused primarily by unusually dry air linked to human-caused climate change.⁵Williams, A. Park, et al., (2019), Observed impacts of anthropogenic climate change on wildfire in California, Earth’s Future, 7 View Source
• Earlier spring snowmelt and other factors related to human-caused climate change have contributed to a substantial increase in fire season length in forests throughout the western United States. Western U.S. fire seasons in 2003–2012 averaged more than 84 days longer than in 1973–1982. The average burn time of the largest wildfires also increased during this period, from nearly six days to more than 50 days.⁶Westerling, A.L., (2016), Increasing western US forest wildfire activity: sensitivity to changes in the timing of spring, Philosophical Transactions of the Royal Society B: Biological Sciences, 371, (1696), 20150178 View Source
• Many factors contribute to wildfire risk, including some that may be unexpected and far flung. For example, Arctic sea ice loss—due primarily to human-caused warming—and resulting shifts of warm air in the atmosphere may be contributing to wildfire-conducive weather in the western United States.⁷Zou, Yufei, (2021), Increasing large wildfires over the western United States linked to diminishing sea ice in the Arctic, Nature Communications, 12, 6048 View Source
• Similarly, some of the effects of wildfires can be far flung as well. One computer modeling study found evidence that heat and smoke released from wildfires in the western United States can lead to more intense rainfall and more large hailstones in U.S. central states.⁸Zhang, Yuwei, (2022), Notable impact of wildfires in the western United States on weather hazards in the central United States, PNAS, 119 (44) View Source
• Nights have warmed significantly—even more than days—during the U.S. fire season.⁹(2021), Climate Change Indicators: High and Low Temperatures, United States Environmental Protection Agency View Source That added heat has lowered the overnight relative humidity that once gave firefighters some of their best opportunities to gain control over wildfires.¹⁰Balch, Jennifer K., et. al., (2022), Warming weakens the night-time barrier to global fire, Nature, 602, 442-448 View Source
• The frequency and intensity of wildfires in Alaska have been greater in the past few decades than for any period in the past 10,000 years.¹¹Kelly, R., et al., (2013), Recent burning of boreal forests exceeds fire regime limits of the past 10,000 years, PNAS, 110, (32), 13055-13060 View Source
• Some populations are disproportionately at risk from wildfires. U.S. census tracts populated primarily by Black, Hispanic, or Native American people have roughly 50% more wildfire risk than do other census tracts.¹²Davies, Ian P., et al., (2018), The unequal vulnerability of communities of color to wildfire, Plos One, 13 (11) View Source
• More and bigger wildfires means more toxic smoke exposures. Computer models have estimated that, over the past decade, the number of people experiencing at least one day of wildfire smoke levels deemed unhealthy for all age groups increased 27-fold, with nearly 25 million Americans so exposed in 2020 alone.¹³Childs, Marissa L., et al., (2022), Daily Local-Level Estimates of Ambient Wildfire Smoke PM2.5 for the Contiguous US, Environmental Science & Technology, 56 (19), 13607-13621 View Source Other analyses have suggested that wildfire smoke may have a bigger impact on health and lead to more respiratory hospitalizations than smoke with similar particle sizes from other sources.¹⁴Aguilera, Rosana, et al., (2021), Wildfire smoke impacts respiratory health more than fine particles from other sources: observational evidence from Southern California, Nature Communications, 12, 1493 View Source

Pitfalls to Avoid

Many factors contribute to wildfire occurrences, and human activities are by far the leading source of wildfire ignitions even as climate change has contributed significantly to wildfire size and intensity. (From 1992 to 2012 in the United States, humans ignited 84 percent of wildfires.¹⁵Balch, J.K., et al., (2017), Human-started wildfires expand the fire niche across the United States, PNAS, 114, (11), 2946-2951 View Source) Instead of asking whether climate change “caused” a wildfire, it’s better to ask:

• How is climate change influencing the likelihood of wildfires such as these?
• To what extent was this wildfire larger and/or more intense because of climate change?
• How has climate change made the U.S. more vulnerable to large fires like this one?