Climate change may be responsible for increasing the number of forest fires in the western United States, a new study shows.

Researchers analyzed data on 1,166 forest fires on federal land since 1970 and compared it to climate data such as snowmelt measurements.

A forest burns across the river from Stony Rapids.
(Courtesy of Julie Duff)

"We show that large wildfire activity increased suddenly and dramatically in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations and longer wildfire seasons," the team wrote in Thursday's online issue of Science Express.

The fire season is now 64 per cent longer than it was 30 years ago, with the mid-elevation Northern Rockies showing the greatest increase in wildfires, the researchers found. 

Land-use changes in the 19th and 20th centuries were traditionally the focus of discussions on forest fire increases, but had relatively little effect in the region, the team concluded. 

The findings add to the debate about whether the warmer conditions are a sign of climate change or a 20-year heat wave.

"They have really demonstrated this link between fire activity and climate," said John Clague, who holds a Canada Research Chair in natural hazards research at Simon Fraser University in Burnaby, B.C. "Now where you get a little more on thin ice is, what is the driver for that?"

Initially, study author Thomas Swetnam, director of the Laboratory of Tree-Ring Research at the University of Arizona in Tucson, considered fuel such as dried timber to be the primary driver. But the findings convinced him that climate change from human sources and natural fluctuations is the main contributor to the increase in forest fires.

"I see this as one of the first big indicators of climate change impacts in the continental United States," said Swetnam. "Lots of people think climate change and the ecological responses are 50 to 100 years away," he added in a statement. "But it's not 50 to 100 years away — it's happening now in forest ecosystems through fire." 

Forest fires burned communities in the U.S. last year and have forced the evacuation of hundreds in Western Canada this week.

Effect of longer dry season

Since 1987, the pattern changed from infrequent fires burning for about one week on average to more frequent fires blazing for five weeks or more, the team reported. Scientists found a similar increase in forest fire activity in Canada from 1920 to 1999.

There was a strong link between early arrival of spring snowmelt in mountainous regions and incidence of large forest forests.

An earlier snowmelt at higher elevations can lead to an earlier and longer dry season that creates more opportunities for large fires, said study author Anthony Westerling of the Scripps Institution of Oceanography in San Diego, Calif.

The increasing frequency of large, devastating wildfires may significantly change the makeup of forests. That could change the role of forests from storing atmospheric carbon dioxide to a source of the gas, exacerbating the fire problem.