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Wildland Fire and Climate Change
Preparers: David L. Peterson and Don McKenzie, Fire and Environmental Research Applications Team, Pacific Northwest Research Station, Seattle, WA.
Issues
Vegetation dynamics, disturbance, climate, and their interactions are key ingredients in predicting the future condition of ecosystems and landscapes and the vulnerability of species and populations to climate change (Schmoldt et al. 1999). The relative influence of climate and fuels on fire behavior and effects varies regionally and subregionally across the western United States (McKenzie et al. 2004). In wet forests and subalpine forests with high fuel accumulations, climatic conditions usually act to limit the frequency and severity of fire, whereas fuels are rarely limiting (Bessie and Johnson 1995). In these forests, prolonged drought of one or more years and extreme fire weather are required to carry fire. In drier forests, ignition and fire behavior at small scales were historically limited by fuels, but large fires required extreme fire weather.
Large severe fires (>300 acres) account for most of the area (>95 percent) burned in the Western United States in a given year. Regional-scale relationships between climate and fire differ, depending on seasonal and annual variability in climatic drivers, fire frequency and severity, and the legacy of previous-year climate in live and dead fuels (Veblen et al. 2000 Hessl et al. 2003). Current-year drought is typically associated with more area burned, but the effects of antecedent conditions differ owing to interactions among climatic effects (Littell et al. in press). In the American Southwest, large-fire years are associated with current-year drought but wetter than average conditions in the 5 previous years, whereas small fire years are strongly associated with drought in the previous year (Swetnam and Betancourt 1990). In the Pacific Northwest, direct associations exist only between fire extent and current-year drought (Hessl et al. 2003, Wright and Agee 2003). In boreal forest and wetter areas of the Pacific Northwest, where fine fuel production is not limited by climatic variability, short-term synoptic fluctuations in atmospheric conditions play an important role in forcing extreme wildfire years (Gedalof et al. 2004, Johnson and Wowchuk 1993).
Likely Changes
A warmer climate will lead to more frequent fires, possibly more severe fires, and a longer fire season in the Western United States (e.g., Westerling et al. 2006). McKenzie et al. (2004) built statistical models of the associations between seasonal and annual precipitation and temperature and fire extent for 1916-2002 for the 11 contiguous Western States. They found that relatively modest changes in mean climate will lead to substantial increases in area burned, particularly in crown-fire ecosystems in which distinct thresholds of fuel moisture and fire weather exist. For a mean temperature increase of 4 °F (expected by the mid-21st century), annual area burned by wildfire is expected to increase by a factor of 1.5 to 5. At one extreme, fire in Montana, Wyoming, and New Mexico is acutely sensitive to temperature changes, and may respond dramatically to global warming. At the other extreme, fire in California and Nevada is relatively insensitive to changes in climate, and area burned may not respond strongly to altered climate.
Summer temperature forces the change in area burned, likely as a result of overall drought patterns and fuel dryness. In most states, there is a greater range of area burned under hot, dry conditions than under cool, wet conditions. Whereas cool, wet conditions nearly always lead to reductions in area burned, favorable to fire conditions do not necessarily lead to increases. This difference in response is linked to the sequence of events required to cause large fire increases: although drought is important, fires will not occur unless drought is accompanied by an ignition source and strong winds. As long as weather conditions do not become unfavorable for wildfire, forests will remain flammable, and ignition and rapid spread can occur at any point during the fire season.
Options for Management
Aggressive thinning and surface fuel treatment (including prescribed burning) to increase landscape diversity are one set of approaches to reducing fire hazard in western dry forests (Peterson et al. 2005). Fuel modification can reduce severity in wildfires, although maintenance treatments are required every 20 to 40 years. Wildland fire use can be used more frequently as an appropriate management response, especially in areas in which fire suppression is difficult, expensive, or counterproductive to resource objectives. Realistically, fuel treatments and wildland fire use will be challenging to implement at spatial scales large enough to have much impact, especially if wildfire increases greatly in the future. However, these management approaches can be used to create resilience in specific landscapes with high resource, economic, and political values (e.g., the wildland-urban interface).
Additional adaptation options (Joyce et al. in press; Millar et al. 2007) include:
- Maintain biological diversity–Identify species, populations, and communities that are sensitive to increased fire and develop conservation plans for them.
- Plan for postdisturbance management–Treat fire and other ecological disturbances as normal processes and incorporate fire management options directly in planning.
- Implement early detection / rapid response–Monitor postfire conditions, and eliminate/control exotic species.
- Manage for realistic outcomes–Determine thresholds for species and functions, and identify those species and vegetation structures tolerant of increased fire. Abandon hopeless causes.
- Incorporate climate change into restoration planning–Reduce emphasis on historical references, and develop performance standards (e.g., lower stand densities) appropriate for accomplishing realistic restoration trajectories in an era of increased fire.
Anticipate big surprises–Expect that mega droughts, larger fires, system collapses, and species extirpations will occur, and incorporate these events in planning.
Metric Equivalents
1 acre = 0.405 hectars
Degree Fahrenheit (F) to Celsius (F-32) x 0.56 = C
Recommended Reading
Keeley, J.E.; Aplet, G.; Christensen, N.L.; Conard, S.G.; Johnson, E.A.; Omi, P.N.; Peterson, D.L.; Swetnam, T.W. [In Press]. Ecological foundations for fire management in North America.
McKenzie, D.; Gedalof, Z.; Peterson, D.L.; Mote, P. 2004. Climatic change, wildfire, and conservation. Conservation Biology. 18: 890-902.
References Cited
Bessie, W.C.; Johnson, E.A. 1995. The relative importance of fuels and weather on fire behavior in subalpine forests. Ecology. 76: 747-762.
Gedalof, Z.; Peterson, D.L.; Mantua, N.J. 2004. Atmospheric, climatic and ecological controls on extreme wildfire years in the Northwestern United States. Ecological Applications. 15: 154-174.
Hessl, A.E.; McKenzie, D.; Schellhaas, R. 2003. Drought and Pacific Decadal Oscillation affect fire occurrence in the inland Pacific Northwest. Ecological Applications. 14: 425-442.
Johnson, E.A.; Wowchuk, D.R. 1993. Wildfires in the southern Canadian Rocky Mountains and their relationships to mid-tropospheric anomalies. Canadian Journal of Forest Research. 23: 1213-1222.
Littell, J.S.; McKenzie, D.; Peterson, D.L.; Westerling, A.L. [In press]. Climate and wildfire area burned in Western U.S. ecoprovinces, 1916-2003. Ecological Applications.
McKenzie, D.; Peterson, D.L; Agee, J.K. 2000. Fire frequency in the Columbia River Basin: building regional models from fire history data. Ecological Applications. 10: 1497-1516.
McKenzie, D.; Gedalof, Z.; Peterson, D.L.; Mote, P. 2004. Climatic change, wildfire, and conservation. Conservation Biology. 18: 890-902.
Millar, C.I.; Stephenson, N.L.; Stephens, S.L. 2007. Climate change and forests of the future: managing in the face of uncertainty. Ecological Applications. 17: 2145-2151.
Swetnam, T.W.; Betancourt, J.L. 1990. Fire-Southern Oscillation relations in the southwestern United States. Science. 249: 1017-1020.
Veblen, T.T.; Kitzberger, T.; Donnegan, J. 2000. Climatic and human influences on fire regimes in ponderosa pine forests in the Colorado Front Range. Ecological Applications. 10: 1178-1195.
Wright, C.; Agee, J.K. 2003. Fire and vegetation history in the east Cascade Mountains, Washington. Ecological Applications. 14: 443-459.
Figure 1—Thinning and surface fuel removal can improve resilience of forests to increased fire expected as a result of a warming climate. Photo courtesy of Lassen National Forest.
Figure 2—A warmer climate will increase area burned by wildfire, resulting in significant changes in fire regimes and vegetation. From McKenzie et al. (2004).
Figure 3—A warmer climate in combination with increased fire and insect outbreaks can result in long-term changes in dominant vegetation.
Recommended Citation
Peterson, David L.; McKenzie, Don. 2008. Wildland Fire and Climate Change. (May 20, 2008). U.S. Department of Agriculture, Forest Service, Climate Change Resource Center. http://www.fs.fed.us/ccrc/topics/wildland-fire.shtml
US Forest Service - Climate Change Resource Center
Last Modified: Jan 15, 2009 04:59:56 PM