The Long Burn: Warming drove recent upswing in wildfires

Major forest fires in the western United States have become more frequent and destructive over the past 2 decades. The trend has occurred in step with rising average temperatures in the region.

"Climate change in the West is a reality," says Thomas Swetnam of the University of Arizona in Tucson. "Now, we're starting to see the effects."

Earlier spring snowmelts, which kick off longer fire seasons, account for the trend, he says. The melt's timing influences how parched—and therefore how vulnerable to fire—the landscape gets later in the year.

Western snow packs now typically melt a week to a month earlier than they did half a century ago, recent studies have shown.

The northern Rockies have borne the brunt of the shift in fire patterns. In 1