Wildfires have seemed inescapable in the Northwest the past few summers after record-setting conflagrations erupted in the region’s prized forests. These blazes have proved so dangerous and persistent that some rural residents have become accustomed to waiting for evacuation warnings while city dwellers hundreds of miles away can watch ash descend onto public parks and beaches. Wildfire smoke was so thick in some Cascadian locales that the late-summer air quality ranked among the most hazardous in the world.
Is human-caused climate change to blame? A number of media accounts have broadly linked wildfires to global warming, but understanding the real connection requires careful attention to the science. So, to shed light on the topic, we examined all the major peer-reviewed research literature—16 published studies in all—on the relationship between global warming and wildfire, particularly as it pertains to Cascadia.
At the highest level, there is a general scientific understanding that climate influences the frequency and severity of wildfires. But there isn’t yet much scholarship on the links between human-caused climate change and wildfire, and the relationship is complex. For just one illustration, it may seem as though rainy seasons would dampen fuels and reduce the risk of fire but they can actually have the opposite effect in some ecosystems because moisture encourages vegetation growth, which can become more potent fuel in subsequent dry conditions.1
Some models estimate that the western US will see 54 to 100 percent more land burned annually by 2050.
Finding links between climate, wildfires, and other conditions
Scientists examining wildfire generally structure their research around several key “drivers.” Their studies look at factors like vegetation, climate variables (such as temperature, precipitation, and the timing of these), and sources of ignition to understand how various conditions influence fire. Some of these studies are empirical analyses—they look at past data, hunting for correlations between the frequency and severity of fires and the presence of these drivers. Of course, none of these elements are perfect predictors of wildfire, but the studies identify general correlations crucial to interpreting the link between climate and wildfire. When researchers are able to identify links, they can deploy models that forecast changes to drivers—including a changing climate—in order to make projections about future wildfire characteristics.
Climate models reveal how rising concentrations of greenhouse gases in the atmosphere will alter seasonal temperature and precipitation. Relative humidity, wind, cloud cover, and lightning strikes are also expected to change as the planet warms, though projections for these elements are somewhat less certain. As researchers keep refining their models of future regional climate conditions, they can make estimates for increases in fire severity, area burned, and changes to the fire season.
Some correlations are relatively clear. Take summer precipitation: dry summers are strongly correlated with an increase in how many acres are burned by wildfire.2 Nationwide, fire records predating European settlement in the US show a clear relationship between drought and fire activity.3 Similarly, droughts preceding fire season lead to higher tree mortality after wildfire.4
Drier conditions are on the rise. From 1979 to 2016, summer precipitation trended downward in nearly all western US forests, with a third seeing significant declines.5 Data from 1984 to 2011 show a statistically significant increase in the number of large fires in the mountain ranges of the Sierra, Cascade, and Rocky Mountains (though they do not show a significant increase in the overall number of acres burned).6 The increase tracks with analyses of underlying fire conditions in those regions, where the northern Sierras and northern Rockies are some of the most drought-stressed forests in the West. Overall, fire seasons in the western US have already gotten 64 percent longer since the mid-1980s with the frequency of large wildfires increasing by four times.7
In the western United States, human-caused climate change has led to an additional 4.2 million hectares of burned area over the period from 1984 to 2015.
Other correlations are less clear, such as how snowpack affects wildfires. The question of snowmelt matters because the Rocky Mountain and Sierra Nevada ecosystems rely on late snowmelt to get through arid summer and there is evidence that wildfires in these regions are highly sensitive to spring arriving early.8 The region spanning from southern Washington into Oregon is also vulnerable to stress from moisture deficit arising from earlier spring snowmelt.
Trends show that early snowmelt correlates to increased wildfire frequency, in part because early snowmelt allows for an earlier start to fire season, which typically means there are more opportunities for a fire to spread.9 Researchers who track the timing of snowmelt lump years into thirds—early, on-time, and late—and they find that the one-third of years with early snowmelt account for 56 percent of the number of wildfires and fully 72 percent of acres burned.10 Yet when statistical analysis controls for summer rains, it turns out that snowpack is only weakly associated with increases in fires (though the correlation is still statistically significant).11 In other words, the timing of snowmelt matters, but its influence is moderated by summertime rain.
Not surprisingly, temperature plays a role in wildfire too. Heat stresses fire-prone trees, reducing their ability to withstand and recover from fires. It also creates arid conditions that can lead to more severe fires, although risks associated with soaring temperatures and arid conditions can, like the timing of snowmelt, be moderated by summer rains.12
The basics of wildfire science
Among the most foundational concepts of wildfire is this: fires need fuel. While other elements such as ignition sources and topography are also important to understanding wildfire, researchers typically pay the most attention to fuel characteristics—the density, type, and condition of a forest or other wildland—and these are affected by many factors, climate, and land use chief among them.
Climate conditions and fuel are relatively easy to study because detailed data are readily available. And it’s obvious that long term climate conditions impact fuels directly. Hot dry summers in eastern Oregon produce different forests than, say, the temperate, damp climate of Washington’s Olympic Peninsula, even as shorter-term climate conditions like drought or El Niño affect fuels too.
Both human- and lightning-caused fires are expected to increase because researchers believe that climate change will increase the number of close-to-ground lightning strikes.
Vegetation is the primary source of fuel for wildfires, so it matters a lot to researchers. Some areas are covered with vegetation that is inherently more flammable than the vegetation that grows in other places. In fact, researchers believe that the influence of vegetation is so strong that it can amplify or override the effect of climate changes on wildfire frequency.13 In Alaska, there’s been a thousand-plus year shift from deciduous to coniferous forests, which coincides with an increase in wildfires, even though the regional climate was becoming cooler and wetter.14 Yet even this relationship is complex because vegetation—it’s type, condition, and extent—is influenced by both long-term and short-term climate changes, as well as by a range of others factors, including topography, forest management practices, and even wildfire itself.
Does fighting fire cause more wildfires?
The way we use and manage land can also dramatically affect wildfire fuels. Logging, housing developments and roads, and even fire-fighting can all shape the nature of fuels, just as they may increase potential sources of ignition. Yet land use and land management variations often happen at a small geographic scale, so the research can be more complicated.
Some people blame what they see as an ill-conceived strategy of fighting forest fires for a rise in blazes over the past few decades. Though there is likely some truth to this notion, it is almost certainly applied too broadly.
It is true that forests in the Northwest, such as the ponderosa pine forests east of the Cascade Mountains, are naturally adapted to frequent fires that burn at low severity, maintaining comparatively wide open stands of trees.15 These forests are adapted to frequent low-intensity fires that burn away brush and other vegetation, thereby preventing a buildup of fuels that can trigger more destructive fires capable of burning into the forest canopy.16 In these forests, it appears that many decades of fire suppression is, ironically, a driver of wildfires today because successful fire-fighting tactics allowed so much fuel to build up. That said, the research on the relationship between fire suppression and wildfire is complicated by imprecise data on both land use and the history of disturbance in these forests.17
It is also important to refrain from applying this narrative—that fire suppression in the past is to blame for wildfires today—too broadly to other ecosystems. In fact, scientists find that fire suppression makes little to no difference in other places, such as the lodgepole pine forests of the northern Rockies, the temperate forests of the Puget Sound Basin, and the boreal forests of North America, where infrequent but major conflagrations that kill off entire areas of forest naturally occur.18 These forests are not terribly affected by a buildup of fuels from fire suppression because they naturally produce shade-tolerant species that can move wildfire into the crown of the forest.19
Models based on scenarios from the IPCC, the world’s most scientifically robust climate science body, predict that 78 percent more Northwest forestland will burn annually.
Even where fire suppression is a factor in wildfire, the evidence shows that it is not the sole driver. That’s true in California, where many forests are adapted to frequent fires and where an increase in wildfires can likely be attributed both to historical fire suppression and to a changing climate.20 It’s important to keep in mind, too, that the increase in fires correlates to more than just fire-fighting.
In order to understand the relative importance of forest management techniques like fire-fighting and climate, researchers look to trends occurring across the globe, zooming out beyond the specifics of regional and national practices. These studies reveal a close correlation between wildfires and climate, strongly suggesting that while forest management practices are implicated in wildfire trends, climate plays a crucial role too.21
The future of wildfires in the Northwest
Even the most respected research bodies agree that more wildfire is in our future—because of climate change. The US National Academy of Sciences estimates that in the western United States, human-caused climate change has caused over half of the increase in “fuel aridity” (basically, the likelihood of vegetation burning) from 1979 to 2015; added nine days to the fire season; and led to an additional 4.2 million hectares of burned area over the period from 1984 to 2015.22 Looking forward, scientists expect this trend to continue.
Eastern Oregon summers have warmed at over twice the global rate over the past four decades
Scientists are also applying climate models to develop projections for the extent of area burned by wildfire. One study estimates the area burned in Canada will increase by a staggering 74 to 118 percent and by fully 80 percent in the US.23 Both human- and lightning-caused fires are expected to increase because researchers believe that climate change will increase the number of close-to-ground lightning strikes. Other models estimate that the western US will see 54 to 100 percent more land burned annually by 2050. Models based on scenarios from the IPCC, the world’s most scientifically robust climate science body, predict that 78 percent more Northwest forestland will burn annually.24
Though the Pacific Northwest will likely not experience changes as dramatic as in other regions, scientists expect temperatures to rise and summer rains to decrease in the western US, both of which up the odds of more wildfire in the future.25 In fact, Cascadia’s summer highs are expected to increase by three to four degrees Celsius (roughly 5 to 7 degree Fahrenheit) when comparing the thirty-year period from 1971 to 2000 to projections for 2041 to 2070 and much of the region stands to see less summer precipitation. Overall, common measures of fire risk are already high in much of the West and will be getting even higher. The largest increase in fire potential is expected in the Rocky Mountains, with moderate increases projected along the Pacific Coast.26
These larger regional studies appear to be borne out in specific Northwest locations too. Eastern Oregon summers have warmed at over twice the global rate over the past four decades, and the number of fires is trending upward, averaging 2.3 additional fires per decade since 1970.27 And a recent case study of the 2015 fire season in eastern Washington (the awful year that when three firefighters died in Twisp) found that the acreage burned that season was more than three times larger than the next largest year in the previous 32 years, perhaps related to the fact that it was 1.4 degrees Celsius warmer than the previous 32 years. The study attributes the record 2015 fire season to a combination of conditions—climate, a buildup of fuels, and lightning storms—and notes “the combination of conditions, especially climate and fuel loads, are likely to recur in the future.”28
How the future develops in Northwest forests is unknowable, not least because the severity of climate impacts in the region are difficult to predict. Still, there is little doubt that late-summer wildfire risk will remain high in Northwest forests, and good scientific evidence that global warming is likely to worsen the danger. At the same time, it’s important to remember that many of the causes of worsening wildfire are far more local in nature, some are possible for us to control, and some are a product of natural forces that have forever shaped Cascadia.
Eric de Place is Sightline’s Director of Thin Green Line. He is a leading expert on coal, oil, and gas export plans in the Pacific Northwest, particularly on fossil fuel transport issues, including carbon emissions, local pollution, transportation system impacts, rail policy, and economics. For questions or media inquiries about Eric’s work, contact Sightline Communications Manager Anne Christnovich.
Paelina DeStephano is a contract researcher and environmental policy nerd with a deep appreciation for government recordkeeping, from permits to public disclosures. In the midst of pursuing a Masters of Public Policy at Duke University, she also studies clean energy transitions and carbon offsets.
Footnotes:
1. David M. J. S. Bowman et al., “Fire in the Earth System,” Science 324, no. 5926 (April 24, 2009): 481–84, https://doi.org/10.1126/science.1163886.↩
2. Zachary A. Holden et al., “Decreasing Fire Season Precipitation Increased Recent Western US Forest Wildfire Activity,” Proceedings of the National Academy of Sciences 115, no. 36 (September 4, 2018): E8349–57, https://doi.org/10.1073/pnas.1802316115.↩
3. Jeremy S. Littell et al., “A Review of the Relationships between Drought and Forest Fire in the United States,” Global Change Biology 22, no. 7 (July 2016): 2353–69, https://doi.org/10.1111/gcb.13275.↩
4. Phillip J. van Mantgem et al., “Climatic Stress Increases Forest Fire Severity across the Western United States,” Ecology Letters 16, no. 9 (2013): 1151–56, https://doi.org/10.1111/ele.12151.↩
5. Hold Zachary A. Holden et al., “Decreasing Fire Season Precipitation Increased Recent Western US Forest Wildfire Activity,” Proceedings of the National Academy of Sciences 115, no. 36 (September 4, 2018): E8349–57, https://doi.org/10.1073/pnas.1802316115.↩
6. Philip E. Dennison et al., “Large Wildfire Trends in the Western United States, 1984–2011,” Geophysical Research Letters 41, no. 8 (2014): 2928–33, https://doi.org/10.1002/2014GL059576.↩
7. Westerling et al.; Johnson, Miyanishi, and Bridge, “Wildfire Regime in the Boreal Forest and the Idea of Suppression and Fuel Buildup.”↩
8. Anthony LeRoy Westerling, “Increasing Western US Forest Wildfire Activity: Sensitivity to Changes in the Timing of Spring,” Philosophical Transactions of the Royal Society B: Biological Sciences 371, no. 1696 (June 5, 2016), https://doi.org/10.1098/rstb.2015.0178.↩
9. Westerling et al.; Johnson, Miyanishi, and Bridge, “Wildfire Regime in the Boreal Forest and the Idea of Suppression and Fuel Buildup.”↩
10. Westerling et al.; Johnson, Miyanishi, and Bridge, “Wildfire Regime in the Boreal Forest and the Idea of Suppression and Fuel Buildup.”↩
11. Zachary A. Holden et al., “Decreasing Fire Season Precipitation Increased Recent Western US Forest Wildfire Activity,” Proceedings of the National Academy of Sciences 115, no. 36 (September 4, 2018): E8349–57, https://doi.org/10.1073/pnas.1802316115.↩
12. Zachary A. Holden et al., “Decreasing Fire Season Precipitation Increased Recent Western US Forest Wildfire Activity,” Proceedings of the National Academy of Sciences 115, no. 36 (September 4, 2018): E8349–57, https://doi.org/10.1073/pnas.1802316115.↩
13. Aurélie Terrier et al., “Potential Changes in Forest Composition Could Reduce Impacts of Climate Change on Boreal Wildfires,” Ecological Applications 23, no. 1 (2013): 21–35, https://doi.org/10.1890/12-0425.1.↩
14. Philip E. Higuera et al., “Vegetation Mediated the Impacts of Postglacial Climate Change on Fire Regimes in the South-Central Brooks Range, Alaska,” Ecological Monographs 79, no. 2 (2009): 201–19, https://doi.org/10.1890/07-2019.1.↩
15. Matthew D. Hurteau and Matthew L. Brooks, “Short- and Long-Term Effects of Fire on Carbon in US Dry Temperate Forest Systems,” BioScience 61, no. 2 (February 1, 2011): 139–46, https://doi.org/10.1525/bio.2011.61.2.9.↩
16. Sean A. Parks et al., “Wildland Fire Limits Subsequent Fire Occurrence,” International Journal of Wildland Fire 25, no. 2 (2016): 182, https://doi.org/10.1071/WF15107; Johnson, Miyanishi, and Bridge, “Wildfire Regime in the Boreal Forest and the Idea of Suppression and Fuel Buildup.”↩
17. Philip E. Higuera et al., “Vegetation Mediated the Impacts of Postglacial Climate Change on Fire Regimes in the South-Central Brooks Range, Alaska,” Ecological Monographs 79, no. 2 (2009): 201–19, https://doi.org/10.1890/07-2019.1.↩
18. Westerling et al., “Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity”; Johnson, Miyanishi, and Bridge, “Wildfire Regime in the Boreal Forest and the Idea of Suppression and Fuel Buildup.”↩
19. Tania Schoennagel, Thomas T. Veblen, and William H. Romme, “The Interaction of Fire, Fuels, and Climate across Rocky Mountain Forests,” BioScience 54, no. 7 (July 1, 2004): 661–76, https://doi.org/10.1641/0006-3568(2004)054[0661:TIOFFA]2.0.CO;2.↩
20. Westerling et al., “Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity.”↩
21. Bowman et al., “Fire in the Earth System.”↩
22. John T. Abatzoglou and A. Park Williams, “Impact of Anthropogenic Climate Change on Wildfire across Western US Forests,” Proceedings of the National Academy of Sciences 113, no. 42 (October 18, 2016): 11770–75, https://doi.org/10.1073/pnas.1607171113.↩
23. M. D. Flannigan et al., “Forest Fires and Climate Change in the 21ST Century,” Mitigation and Adaptation Strategies for Global Change 11, no. 4 (July 2006): 847–59, https://doi.org/10.1007/s11027-005-9020-7.↩
24. D. V. Spracklen et al., “Impacts of Climate Change from 2000 to 2050 on Wildfire Activity and Carbonaceous Aerosol Concentrations in the Western United States,” Journal of Geophysical Research: Atmospheres 114, no. D20 (2009), https://doi.org/10.1029/2008JD010966.↩
25. Westerling et al.; Johnson, Miyanishi, and Bridge, “Wildfire Regime in the Boreal Forest and the Idea of Suppression and Fuel Buildup.”↩
26. Yongqiang Liu, Scott L. Goodrick, and John A. Stanturf, “Future U.S. Wildfire Potential Trends Projected Using a Dynamically Downscaled Climate Change Scenario,” Forest Ecology and Management, The Mega-fire reality, 294 (April 15, 2013): 120–35, https://doi.org/10.1016/j.foreco.2012.06.049.↩
27. Lawrence C. Hamilton et al., “Wildfire, Climate, and Perceptions in Northeast Oregon,” Regional Environmental Change 16, no. 6 (August 1, 2016): 1819–32, https://doi.org/10.1007/s10113-015-0914-y.↩
28. Ruth A. Engel, Miriam E. Marlier, and Dennis P. Lettenmaier, “On the Causes of the Summer 2015 Eastern Washington Wildfires,” Environmental Research Communications 1, no. 1 (February 2019): 011009, https://doi.org/10.1088/2515-7620/ab082e.↩
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