In the Southwest, normal changes in seasonal climate make the landscape ripe for fires. Every winter, precipitation spurs plant growth, while the dry months of April, May, and June turn the vegetation into tinder. At the time in which the landscape is most primed for fire, convective monsoon storms generate lightning, providing the match.
Figure 1. The Rodeo-Chediski fire on June 21, 2002 before the fires converged.
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Credit: Jesse Allen, NASA
Under normal conditions, the Southwest experiences recurring fires. Add in fire suppression by federal agencies, population growth that increases numbers of campfires and careless people, and human-caused climate change and the relationship between fire and climate becomes complicated.
In recent years, wildfires have charred increasing areas of western U.S. forests, burning homes and wildlands and regularly siphoning more than $1 billion per year from federal land-management agencies. With the general belief that future climate in the Southwest will become hotter and drier, recent research has focused on understanding:
- Observed links between climate and fire
- Climate change and wildfire in forests and deserts
- The predicted character of future fires
Observed links between climate and fire
In recent years, the size and frequency of fires have increased. In some cases, either human-caused forest change or climate variability is primarily responsible for this increase. In other cases, both factors play important roles.
Figure 2. Relationship between Western U.S. forest wildfires and spring-summer temperature. The top graph shows the positive relationship between annual frequency of large (>400 hectare) wildfires (bars) and average spring and summer temperatures (line) in Western U.S. forests. Using the same x-axis, the bottom graph shows the first principal component of the center timing of streamflow in snowmelt dominated streams (pink = early, white = average, blue = late).
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Credit: The American Association for the Advancement of Science
The frequency and size of fires in the Southwest is influenced by many variables. Climate variability, both short- and long-term, and natural- and human-caused, is one variable. For example, shifts in temperature can influence the timing of snowmelt, leading to drier conditions later in the year. Oscillations between wet and dry periods, such as those related to phase changes of the El Niño/Southern Oscillation can also influence the amount and size of fires in a given year. Fluctuations in the vigor of monsoons can influence the number of lightening strikes. Other influences on fires are the incursion of invasive species, particularly in desert environments where exotic grasses often provide fine fuels that feed fires; land use changes, such as grazing and logging; and changes in fire management policy.
Although human actions and the climate both influence the timing and size of fires, analysis of historical fire records have helped identify relationships between climate and fire. Some of the more conclusive relationships are:
- In western mountains during the last two to three centuries, years that had a high number of fires coincided with drought years.
- The frequency of fires increases more during hotter springs and summers than during cooler springs and summers (Figure 2).
- Wildfire activity at elevations between 5500 and 8500 feet has occurred predominantly during warm years and is associated with earlier spring snowmelt, which is driven by warmer temperatures.
- In general, the size of fires in the Southwest is greatest during intense La Niña years in which winter and spring precipitation is low, while fewer acres burn during El Niño years characterized by exceptionally wet springs.
Climate change and wildfire in forests and deserts
Historically, wildfires have been a recurring disturbance in conifer forests, pinyon-juniper woodlands, chaparral shrublands, and grassland ecosystems of the Southwest. Lightning strikes frequently initiated surface fires in ponderosa pine forest, clearing out much of the surface brush and vegetation while killing only a few of the thick-barked mature ponderosa trees. These low intensity fires created a more open forest, preventing the build-up of vegetation that could fuel more severe wildfires. Meanwhile, fires were rare in the higher elevation spruce-fir forests, but were likely severe when they did occur.
Since about the mid-1970s, however, the total acreage of area burned and the apparent severity of wildfires in pine and mixed-conifer forest have increased, both from climate change and human activities. Management practices have made many forests more susceptible to severe wildfires; fire suppression and clear-cutting promote a high density of small trees, while grazing has reduced the grassy understory that formerly sustained surface fires.
Warming temperatures also have a clear link to the observed frequency of large western wildfires in recent times. An analysis of U.S. Forest Service and National Park Service data from 1970–2003 revealed that high spring and summer temperatures correlated with an unusually high number of large western wildfires. The researchers identified earlier snowmelt as a contributing factor to the increase.
Wildfires have also been occurring in desert ecosystems that are ill-adapted to flame. A major contributor to desert fires in the Southwest is invasive grass. Red brome, Mediterranean grass, and buffelgrass are now growing in southwestern deserts and are providing fine fuels for wildfires. Desert fires now expand more easily than in the past, especially after wet winters, which boost the growth of the grasses. Wildfires in desert ecosystems have also increased the fire risk in urban areas and housing developments where urban areas and desert meet.
Predicted character of future fires
Increases in future temperatures are nearly certain. Regardless of precipitation, temperature alone will likely amplify the frequency and total area burned by wildfires. If drought conditions become more common or if precipitation becomes more variable, these will likely combine with temperature change to exacerbate future fire risk. In addition, continued population growth will likely cause greater human-started fires since nearly half of the fires in the Southwest are started by humans. In 2002, for example, the Rodeo-Chediski fires in northern Arizona (Figure 1, above) were both started by humans and combined to burn nearly half a million acres, the largest fire on record in Arizona.
The frequency and size of fires will likely increase in the future because:
- warmer annual temperatures will cause the snowpack to melt earlier in the spring, therefore increasing the length of the dry season between winter and monsoon rains—a longer dry season creates drier fuels as well as a longer period in which fires can occur
- increases in summer temperatures will cause trees to be more moisture-stressed, making forests more susceptible to fire
- fire suppression has created an over abundance of snags and small trees that have increased fuel supplies
- growth in the human population will increase the number of wildfire starts
- Whitlock, C. 2004. Forests, fires and climate. Nature, 432:28-29.
- Consortium for Integrated Climate Research in Western Mountains (CIRMOUNT). 2006. Mapping New Terrain: Climate Change and America’s West. U.S. Department of Agriculture.
- Westerling, A.L., H.G. Hidalgo, D.R. Cayan, and T.W. Swetnam. 2006. Warming and earlier spring increase western U.S. forest wildfire activity. Science, 313:940-943.
- Swetnam, T.W. and J.L. Betancourt. 1998. Mesoscale disturbance and ecological response to decadal climate variability in the American Southwest. Journal of Climate, 11:3128-3147.
- Grissino-Mayer, H.D. and T.W. Swetnam. 2000. Century-scale climate forcing of fire regimes in the American Southwest. The Holocene, 10(2):213-220.