So far this calendar year, wildfires in the US have burned 3.1 million acres, according to the NIFC.
Persistent drought along southern tier
Conditions leading to this year's fires along the southern tier include the persistence of long-term droughts, which in the Southwest in particular have lasted for a decade, although with some minor breaks, notes Tom Swetnam, a fire ecologist at the University of Arizona's Laboratory of Tree Ring Research.
In addition, he says, La Nina conditions – cooler than normal surface water – in the tropical Pacific, in place since October, have tended to drive storm tracks along the northern US. This has tended to further dry an already parched landscape.
As La Nina weakens, and the rainy season begins in the Southeast, forecasters at the National Oceanic and Atmospheric Administration's Climate Prediction Center say they see some easing of drought conditions in many regions along the southern tier – but in many cases not enough to erase the drought itself.
But the longer-term trend in large, longer-lasting fires looks to be driven at least in part by global warming, which most atmospheric scientists attribute to a build-up of carbon-dioxide in the atmosphere – released through burning fossil fuels and through land-use changes.
Researchers documented the change in a study published in 2006 – in the middle of a four-year period when wildfires consistently consumed more than 8 million acres a year in the US. The increase in larger fires began somewhat abruptly in the mid-1980s, according to the study, which analyzed fire, climate and hydrological data between 1970 and 2004.