Although many ecosystems require fire, excessive wildfire can permanently alter ecosystem integrity. Climate change has led to an increase in the area burned by wildfire in the western United States. Climate change has driven the wildfire increase particularly by drying forests and making them more susceptible to burning. With continued greenhouse gas emissions, models project more wildfire across the Southwest region. Under higher emissions (SRES A2), fire frequency could increase in parts of the southwest by 25% by the end of the century, and the frequency of very large fires (greater than 5,000 hectares) could triple.
Projections of annual fire probability, calculated solely with climate data and physical principles, are highly variable across the region. Dry ecosystems that are already fuel-limited may actually see a decline in wildfire risk, if warmer temperatures and moisture limitations prevent them from producing enough plant matter to carry fire. Wetter, cooler, high-elevation ecosystems in the Southwest will likely have increasing wildfire probability, however. The incidence of atmospheric conditions that contribute to large and erratic fire behavior, measured by the Haines Index, is also projected to occur more frequently (2 to 11 percent increase) by the end of the century, with the largest increases the Arizona and New Mexico. The limitation for these sorts of projections is that they do not account for changes in land use, fire suppression rates, or vegetation changes.