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 decades. The size of the range of suppression estimates reflects only a portion of this uncertainty; climate change could cause actual suppression expenditures to increase by less than the lower bound or more than the upper bound. In addition, the model used in this assessment does not incorporate several key factors—including four in particular that could have substantial bearing on the pattern and implications of fire over the course of the 21st century.

First, while average maximum temperature is a reasonable proxy for many of the effects of climate change on fire, it may not capture the full impact of climate change. In particular, it may represent increased incidence of temperature extremes fairly well, but may only partly capture increases in prolonged high-temperature periods and drought expected to occur with climate change. For example, fire season length, while related to temperature, may also increase due to other climate change phenomena, and this could affect expenditures in ways not captured by this analysis. Increased variability in precipitation and changes in fuel moisture and water availability for suppression are also not represented.

Second, both fire and climate are expected to substantially change vegetation composition over the coming 85 years, including the prevalence of vegetative fuels that enable and sustain fires. Detailed vegetation modeling would be required to determine the extent to which these changes would occur, and the extent to which they would alter area burned or suppression expenditures.

Third, growth in the WUI could influence wildfire ignitions and area burned and hence heighten efforts and suppression expenditures to protect life and property. Population and income variables were not used in the model as they were generally found either not to be significant or not to appreciably increase the predictive power of the area-burned model—perhaps due to relatively small sample size given a limited historical data set. The estimates provided above for the compounding effect of WUI development, based on Gebert et al. (2007), are intended to illustrate the potential sensitivity of fire suppression expenditures to WUI development.

Finally, the model holds constant the general approach to wildland fire management. Changes in wildfire suppression strategies and technologies, wildland fuels management practices, and other risk management strategies could affect the area expected to burn or the expenditures incurred per unit area. Some of these changes may occur as an adaptive response to growing wildland fire risks and/or as part of efforts to enhance carbon storage in the land sector. Previous shifts in management regime can be detected in historical data. For example, an upward shift in expenditure pattern occurred in FY 2000, corresponding to changes associated with the National Fire Plan. A second structural shift is detectable in FY 2011. However, any future shifts would be policy-dependent and uncertain in their timing and magnitude, so they are not projected.

The USDA Forest Service white paper detailing the modeling conducted for this assessment provides additional discussion of major uncertainties and limitations.