Walnut Creek, Calif. ? The number and spread of major wildfires has exploded in western states since the late 1980s in part due to a warming climate, says a study in today’s edition of the journal Science.

Researchers found that large wildfires in national forests burned four times as often and charred more than six times the number of acres since 1987 than they did during the period between 1970 and 1986. And the conventional wisdom that the increasing fury of those forest fires is caused by decades of firefighting, logging and other activities is only part of the answer, the study says.

Rather, the study found a temperature increase of 1.5 degrees Fahrenheit caused earlier snowmelt, which in turn caused forests to dry out earlier in the year. The result: a longer fire season with more opportunities for fires to start and more time for fuel to dry out.

“I see this as one of the first big indicators of climate change impact in the continental United States,” said one of the paper’s authors, Thomas Swetnam, director of the University of Arizona’s Laboratory of Tree-Ring Research.

The study, led by scientists at the Scripps Institution of Oceanography in La Jolla, Calif., allows that it is still uncertain whether the recent warming is due to the buildup of greenhouse gases in the atmosphere or an “unusual natural fluctuation.”

But it adds that there is scientific consensus that global warming will further increase spring and summer temperatures in the region in the coming decades – and that ensures that wildfires will continue to worsen.

There is broad consensus among scientists that the climate is warming and that it is caused at least in part by human activities, especially the burning of fossil fuels.