WASHINGTON — Wildfires almost quadrupled in American West over 16 years as increasingly early snowmelt left mountain forests drier in hot weather, according to a study showing climate played a bigger role than land-use patterns.

The northern Rocky Mountains area, which includes Idaho and Montana, accounted for 60% of the blazes from 1987 to 2003, according to data in Friday’s issue of the journal Science. The wildfires burned an area more than six times larger than from 1970 to 1986, scientists said.

Climate explains about two-thirds of the variation in big forest fires in the mountains, which cost federal agencies more than $1 billion a year, the researchers said. Less snow and rising temperatures dry out vegetation by the peak of summer heat in August, creating more tinder and a longer fire season, the lead investigator, Anthony Westerling, said in a telephone interview.

“I was surprised by just how important temperature was for wildfire frequency,” Mr. Westerling, a school of engineering assistant professor at the University of California at Merced, said. “The biggest increase is in the northern Rockies and the pattern that we see is a little bit drier winter, significantly warmer spring, and an earlier snowmelt.”

Investigators studied 1,166 large forest wildfires from 1970 to 2003. Each fire involved more than 988 acres, a little more than the 843 acres of New York City’s Central Park. More than half of the fires occurred in early snowmelt years, compared with 11% when snow melted later, they said.

Most wildfires occur at about 7,000 feet, and these blazes have the strongest association with changes in snowmelt timing, scientists said. Wildfire seasons expanded by about 11 weeks from 1987 to 2003, compared with 1970 to 1986. It now takes about a month longer to bring a fire under control, researchers said.

“It provides pretty compelling evidence that climate has a distinct influence on both the number of acres and the severity of fires in Western forests,” a wildland fire analyst for the National Interagency Fire Center, Tom Wordell, said in a telephone interview yesterday.

Wildfire increases in other Western forests include 18% in the southern Cascades, which cross Oregon; the coastal ranges of southern Oregon and northern California, and the Sierra Nevada. The study was financed by the National Oceanic and Atmospheric Administration’s Office of Global Programs; the National Fire Plan, through the southern research station of the U.S. Department of Agriculture’s Forest Service, and the California Energy Commission.

The Northern Rockies Coordination Center reported four new large fires as of July 5, in northern California, the Northwest, the northern Rockies, and Alaska, and 11 fires that were contained, according to the agency’s Web site.

“The article kind of implies the cards are stacked against us, especially given the climate model forecast for the future,” Mr. Wordell said. “But it also implies that we should be taking every opportunity we can to manage” fires caused by lightning and other natural events.