The western U.S. has seen more and bigger wildfires over thelast 30 years, a trend expected to continue as temperatures riseand droughts become more severe, according to a new study.

The study, accepted for publication in Geophysical ResearchLetters, which is published by the American Geophysical Union,examined the region from Nebraska to California from 1984 to 2011.Over that period, the number of wildfires over 1,000 acres in sizeincreased by a rate of seven per year, the study says. Total acresburned increased by 90,000 over that time, and the largestwildfires grew by 350 acres per year.

The total area these fires burned increased at a rate of nearly90,000 acres a year – an area the size of Las Vegas, according tothe study. Individually, the largest wildfires grew at a rate of350 acres a year, the new research says.

The study used satellite data to measure areas burned by largefires since 1984, and also looked at climate variables such asseasonal temperature and rainfall, during the same time. Most areasthat saw increases in fire activity also experienced increases indrought severity during the same time period, the study says.

Max Moritz, a co-author of the study and a fire specialist atthe University of California-Berkeley Cooperative Extension, saysin a statement, “Twenty eight years is a pretty short period ofrecord, and yet we are seeing statistically significant trends indifferent wildfire variables—it is striking.”

A statement on the study says the trends “suggest thatlarge-scale climate changes, rather than local factors” could bedriving increases in fire activity, but the study “stops short oflinking the rise in number and size of fires directly tohuman-caused climate change. Observed changes in fire activity,though, are in line with long-term, global fire patterns thatclimate models have projected, the statement says.

Philip Dennison, an associate professor of geography at theUniversity of Utah in Salt Lake City and lead author of the paper,says, “We looked at the probability that increases of thismagnitude could be random, and in each case it was less than1%.Most of these trends show strong correlations withdrought-related conditions which, to a large degree, agree withwhat we expect from climate-change projections.”

A research ecologist not connected to the study, Jeremy Littella research ecologist with the U.S. Geological Survey (USGS) at theAlaska Climate Science Center in Anchorage, AK, who was notconnected with the study, says in the statement fire activityreported in the paper resembles what would be expected from risingtemperatures caused by climate change, but he says other factorssuch as a response to decades of fire suppression could also be atplay.

“It could be that our past fire suppression has caught up withus, and an increased area burned is a response of more continuousfuel sources,” Littell says. “It could also be a response tochanges in climate, or both.”