For the past 20 years, wildfires in western states have raged more often and more intensely, an alarming trend that has been blamed mostly on decades of aggressive fire suppression and century-old grazing and logging practices.

Recently, however, evidence has emerged that appears to show global warming also could be playing a significant role.

And computer models developed last year show that if the climate continues to warm, the risk of large wildfires in California could increase by more than 50 percent. In Northern California, they could nearly double in number.

The Bay Area, where neighborhoods are often on the edges of wooded areas, also could be threatened by more fires.

"The warmer it gets, the more forest fires you get, especially in Northern California," said Anthony Westerling, an environmental engineer and geographer at UC Merced who developed the models with a colleague.

"Some of the biggest increases were in the foothills of the Sierra and the Sierra themselves," he added.

Warmer temperatures mean earlier snowmelt, longer fire seasons and drier vegetation and soils.

In the summer, Westerling published a study in the journal Science that suggests a possible link between the increase in western wildfires and global warming..

The study found that large wildfires in western states increased "suddenly and dramatically" beginning in the late 1980s, and that the increases were largely driven by warming temperatures.


Advertisement

Westerling cautioned that the study could not definitively link global climate change and increased fires because it looked at only 34 years' worth of data. The warmer temperatures that drove the increase could have been part of natural variability, he said.

But the study found that the earlier snowmelt was a big reason for the increase in fire danger, and other studies have linked that trend to global climate change.

"It's very clearly driven by temperature and the timing of snowmelt," Westerling said.

One of the co-authors of the paper, Thomas Swetnam, director of the University of Arizona's Laboratory of Tree-Ring Research, said when the paper was published, "I see this as one of the first big indicators of climate change impact in the continental United States."

Overall, the study found that from 1987 to 2003, large wildfires quadrupled in number and burned more than six times the acreage compared with the period from 1970 to 1986.

The largest increases occurred in parts of the Rocky Mountains where forest management practices were not expected to have a large effect on fires.

Large increases were also recorded in the Sierra Nevada, but in those forests the impact of fire suppression, grazing and logging was more likely to contribute to the greater risk of fires.

The reason for the increase? The paper's authors wrote that "while land use history is an important factor" in some types of forests, including those in the Sierra, "the broad-scale increase in wildfire frequency across the western United States has been driven primarily by sensitivity of fire regimes to recent changes in climate over a relatively large area."

In some types of forests, especially the ponderosa pine forests of the Southwest, aggressive firefighting policies have had a larger effect. Those forests historically burned at low intensities every decade or so, but decades of fire suppression have severely disrupted the natural fire regime and led to dense forests and fuel buildup.

By contrast, other types of forests, such as lodgepole pine forests that are more common in the Rockies, historically burned far less frequently. As a result, aggressive fire suppression policies that began in the middle of the 20th century were not around long enough to significantly disrupt natural fire patterns.

Yet those forests, too, have seen a dramatic increase in fire.

The Science study said that if the snowpack is smaller and melts earlier in the year, vegetation and soils dry out earlier. That, in turn, means more opportunities for fires to start and a longer fire season. The extended fire season will also lead to a longer drying period for vegetation and soils.

According to a separate modeling study done by Westerling, the increase in wildfires as a result of global warming is expected to be especially high in the Sierra Nevada and its foothills -- and poses an especially high risk to nearby residents in rapidly growing places such as Placer County. But Westerling said if the climate continues to warm, the Bay Area also will face a greater risk of wildfires and property losses.

That modeling study used two different climate models -- one that assumes that climate is very sensitive to greenhouse gases and one that assumes it is less sensitive. It also used two emissions scenarios -- one that assumes emissions continue on present trends and one that assumes emissions are curtailed.

In the worst case, with a more sensitive climate and emissions continuing unabated, the study predicted the number of large fires would increase by 53 percent by as early as 2070 in California, and by 90 percent in Northern California.

If the climate turns out to be less sensitive to emissions, and those emissions are curtailed, the number of large fires would still increase. Statewide, the number of fires would increase by 12 percent and in Northern California, they would increase by 15 percent.

In Southern California, a warmer climate could mean fewer fires because there could be less vegetation to burn.

Southern California also faces less-severe risks of property losses because the highly urbanized part of the state also has relatively fewer houses in fire-prone areas, Westerling said.

Mike Taugher covers natural resources. Reach him at 925-943-8257 or mtaugher@cctimes.com.