Size of US Wildfires, 1983-2008. Data on wildland fires in the United States show that the number of acres burned per fire has increased since the 1980s. National Interagency Fire Center 2008 via globalchange.gov

Data on wildland fires in the United States show that the number of acres burned per fire has increased since the 1980s. In the western United States, both the frequency of large wildfires and the length of the fire season have increased substantially in recent decades, due primarily to earlier spring snowmelt and higher spring and summer temperatures. These changes in climate have reduced the availability of moisture, drying out the vegetation that provides the fuel for fires. Alaska also has experienced large increases in fire, with the area burned more than doubling in recent decades. As in the western United States, higher air temperature is a key factor. In Alaska, for example, June air temperatures alone explained approximately 38 percent of the increase in the area burned annually from 1950 to 2003.

Global Climate Change Impacts in the United States [pdf]