"I think there was a tendency to think that the overwhelming factor (driving forest fires) was short-term weather. There’s this idea that drought matters, and it does. But it’s taking time and a lot of research to show that climate plays a big role as well." -- Anthony Westerling
Six years ago, climate scientist Anthony Westerling began obsessively poring over the meticulously detailed invoices that U.S. Forest Service and National Park Service land managers use to itemize firefighting expenses.
"These things will have 170-plus fields," says Westerling — including information on when a fire was first reported, when firefighters finally controlled it, and how many acres were burned. Westerling, who works at the Scripps Institution of Oceanography (which also studies climate and earth sciences) in La Jolla, Calif., didn’t aspire to be an accountant, nor was he searching for fraud in government spending. He was hoping to answer a question that had not been seriously asked before: How do rising global temperatures affect wildfire behavior?
Along with fellow researchers in La Jolla and at the University of Arizona in Tucson, Westerling wove the information in the invoices together with data from streamflow gauges, soil moisture measures, and temperature and precipitation records to form a comprehensive picture of the driving forces behind the West’s fires.
The group will present its findings in the journal Science next month; a preliminary article appeared in the July 6 issue of Science Express. The basic conclusion may not startle: Large forest fires increased beginning in the mid-1980s — particularly in the Northern Rockies, the Sierra Nevada and the southern Cascades — and the changes closely correlated with an increase in spring and summer temperatures during the same time period.
But some of the nuances are surprising. Westerling and his colleagues found that a delicate "tipping point" exists, particularly in forests in the Northern Rockies and Northern California. When snowpack melts earlier in the spring — even just a few days sooner — the severity of the fire season intensifies greatly. "It didn’t take a very big temperature increase (to) switch from very few fires to a lot of fires," says Westerling. As spring and summer temperatures gradually increased, "You were getting closer and closer to this tipping point, so that the (climate) variability from year to year just pushes you over it easily."
Westerling grew up in Los Angeles but vacationed at a family cabin on the east side of the Sierra, and later lived abroad in places like Saudi Arabia, China and Brazil. He returned to California to get his Ph.D., and in 2000, he went to work at Scripps. That year proved to be a banner one for fire, and that’s when Westerling began looking more closely at the relationship between climate and fire.
Because of the wide variability in fire regimes throughout the West — from ponderosa forests in the Southwest to chaparral in Southern California to lodgepole pine and spruce-fir forests in the Northern Rockies — long-term trends can be hard to pick out from the year-to-year "noise."
That was precisely the value of Westerling’s research: Zooming out to look at the entire region helped bring the phenomenon into relief.
Usually, blame for the bigger and more frequent fires of the past 20 years is ascribed to the federal government’s aggressive firefighting policies, which have left a lot of dead and small-diameter trees in Western forests ready to burn. Westerling’s research suggests that rising temperatures, not land-management practices, may play a greater role in driving forest fires.
But, he writes, if warming is the main driver of increased fire activity, "ecological restoration and fuels management alone will not be sufficient to reverse current wildfire trends." Scientists and planners must also plug global climate change into their equations.
The author is HCN’s West Coast correspondent.