Wildfires and Climate Change
QUICK FACTS FOR ANY STORY
Human-caused climate change is a significant contributor to the increasing size and intensity of, and damage from, western U.S. wildfires.
In the western United Statesi 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 1984.1
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.2
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.3
Earlier spring snowmelt and other factors related to human-caused climate change have contributed to a substantial increase in fire seasonii 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.4
Nights have warmed significantly—even more than days—during the U.S. fire season. That added heat has lowered the overnight relative humidity that once gave firefighters some of their best opportunities to gain control over wildfires.
Warming has been especially dramatic at higher latitudes.5 In Alaska, a state that already suffered severe wildfires early in 2019, the average overnight low temperature in July 2019 was the hottest since record-keeping began in 1925.6
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.7
A study of Rocky Mountain wildfire-prone areas found that moisture levels were lower in the 15 years after 2000 than in the 15 years before 2000—a “moisture deficit” that correlates with a post-2000 decline in tree seedling densities and forest regeneration.8 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.9
Wildfire “smoke waves” impose a significant health burden on millions of people in western U.S. states. During the 2004 to 2009 fire seasons, in the western U.S. counties where wildfires are frequent, wildfires were responsible for more than 70 percent of the air pollution on days when fine particulate matter in the air surpassed regulatory limits.10
Despite widespread improvements in air quality across the United States in recent decades, air quality during the summer has declined across much of the Northwest as a consequence of increased wildfire activity.11
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.12) 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?
Dr. A. LeRoy Westerling (University of California Merced) email@example.com
Dr. John Abatzoglou (University of Idaho) firstname.lastname@example.org
Dr. Loretta Mickley (Harvard University) email@example.com
i. For the studies cited, "western" includes Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming.
ii. For the study cited, “fire season” is defined as the time between the first large fire ignition and the last large fire control date in a given year.
1. Abatzoglou, J.T. and Williams, Park, (2016), Impact of anthropogenic climate change on wildfire across western U.S. forests, PNAS, 113, (42), 11770-11775 2. Westerling, A.L., et al., (2016) Warming and earlier spring increase western U.S. forest wildfire activity, Science, 313, 5789, 940-943 3. Williams, A.P., et al., (2019), Observed impacts of anthropogenic climate change on wildfire in California, Earth’s Future, 7 4. Westerling, 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 5. National Park Service, (2019), Alaska Nature and Science: High-latitude Climate Change 6. NOAA National Centers for Environmental Information, (2019), Climate at a Glance 7. Kelly, R., et al., (2013), Recent burning of boreal forests exceeds fire regime limits of the past 10,000 years, PNAS, 110, (32), 13055-13060 8. Stevens-Rumann, C.S., et al., (2017), Evidence for declining forest resilience to wildfires under climate change, Ecology Letters, 21, 243-252 9. Davis, K.T., et al., (2019), Wildfires and climate-change push low-elevation forests across a critical climate threshold for tree regeneration, PNAS, 116, (13), 6193-6198 10. Liu, J.C., et al., (2016), Particulate air pollution from wildfires in the western US under climate change, Climatic Change, 138, 655-666 11. McClure, C.D., and Jaffe, D.A., (2018), US particulate matter air quality improves except in wildfire-prone areas, PNAS, 115, (31), 7901-7906 12. Balch, J.K., et al., (2017), Human-started wildfires expand the fire niche across the United States, PNAS, 114, (11), 2946-2951
Last Updated October 11, 2019
This tip sheet for reporters is one in a series produced jointly by two philanthropically supported, editorially independent services for journalists: SciLine, based at the American Association for the Advancement of Science (AAAS), and Climate Communication.