In a warming West, expect more fire

Overall wildfire size likely to double by 2100, new study concludes

  • Wildfires in the West, like this one in California's Sequoia National Forest, already are dramatic. They will just get worse as global warming brings hotter summers, says a new report


The average area burned by wildfires in the West will roughly double by the end of the century, according to a study published in the August issue of the journal Conservation Biology. And that sobering conclusion is based on the "best-case" — or least dramatic — climate-change scenario.

The researchers — Donald McKenzie and David Peterson of the Forest Service Pacific Wildland Fire Sciences Lab in Seattle, Ze’ev Gedalof of the University of Victoria in British Columbia, and Philip Mote of the Climate Impacts Group at the University of Washington — analyzed historical wildfire records from the 11 Western states. They compared 86 years of fire data with climate records, finding that for most states, the hotter and drier the summer weather, the larger the area burned by wildfire.

Then, the researchers looked at the projected climate for the years 2070 to 2100, using a model from the U.S. National Center for Atmospheric Research that predicts relatively small increases in average summer temperature and precipitation in the West.

When the historical relationship between fire and climate is extended into this warmer and generally wetter future, a stark picture emerges. In most Western states, if the average summer temperature rises just 1.6 degrees Celsius by the end of the century, the average area burned by wildfire will increase significantly. That’s assuming that average summer precipitation increases by the 11 percent projected by the model.

Though many previous studies have looked at the historical relationship between fire and climate, says University of Arizona tree-ring scientist Thomas Swetnam, this study uses a longer historical record than most, looks at a larger area, and most significantly, extends the existing relationship into an altered future. "It confirms what some of our fears have been," he says.

The results of the study, published as part of a special section on wildfire in the West, raise several general environmental concerns for the region. One is the fate of threatened and endangered species, many of which are isolated in small islands of old-growth forest habitat. "Some of these species have managed to escape fire for a long time," says McKenzie. "But if the habitat is gone, if the house is gone, there’s not going to be any place to live."

Bigger fires could also accelerate weed invasions, already considered by Forest Service Chief Dale Bosworth to be one of four "top threats" to national forests (the other three are wildfire, loss of open space, and unmanaged outdoor recreation). Cheatgrass, an invasive exotic that thrives in burned areas — and speeds up the fire cycle by providing more fuel for future fires — would almost certainly gain ground in a more fiery West.

The most dramatic changes in fire area — increases of between four- and fivefold — are projected for New Mexico and Utah. Even more significant shifts would be expected under the conditions projected by other climate-change models.

McKenzie, the study’s lead author, says state-by-state distinctions shouldn’t be overemphasized, since landscape types vary widely within each state. "We need to think of it in terms of a trend rather than as precise predictions," he says. In future studies, he and his colleagues plan to break this trend down to smaller areas within states — separating the chaparral ecosystem of Southern California from the conifer forests of the Sierra Nevada, for instance.

Nonetheless, these results suggest that contingency planning is in order — for land managers and the general public alike. "One lesson is that you have to anticipate a wide range of possibilities," says McKenzie. "The further ahead you can think, the less chance you have of getting stuck."

The author is HCN’s contributing editor.

This story is funded in part by a grant from the Engel Fund of the San Diego Foundation.

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