A new study led by scientists at Scripps Institution of Oceanography at the University of California, San Diego, implicates rising seasonal temperatures and the earlier arrival of spring conditions in connection with a dramatic increase of large wildfires in the western United States.
In the most systematic analysis to date of recent changes in forest fire activity, Anthony Westerling, Hugo Hidalgo and Dan Cayan of Scripps Oceanography, along with Tom Swetnam of the University of Arizona, compiled a database of recent large western wildfires since 1970 and compared it with climate and land-surface data from the region.
Western United States forest wildfire activity is widely thought to have increased in recent decades, but surprisingly, the extent of recent changes has never been systematically documented. Nor has it been established to what degree climate may be driving regional changes in wildfire. Much of the public and scientific discussion of changes in western United States wildfire has focused rather on the effects of 19th and 20th century land-use history.
...The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks, and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.
The new findings, published in the July 6 issue of Science Express point to climate change, not fire suppression policies and forest accumulation, as the primary driver of recent increases in large forest fires.
The increase in large wildfires appears to be another part of a chain of reactions to climate warming. The recent ramp-up is likely, in part, caused by natural fluctuations, but evidence is mounting that anthropogenic effects have been contributing to warmer winters and springs in recent decades.—Dan Cayan, co-author and director of Scripps’ Climate Research Division
The scientists compiled a comprehensive time series of 1,166 forest wildfires of at least 1,000 acres that had occurred between 1970 and 2003 from wildfire data covering western US Forest Service and National Park Service lands. To investigate what role climate might play, the researchers compared the time series, the timing of snowmelt and spring and summer temperatures for the same 34 years.
For the timing of peak snowmelt in the mountains for each year, they used the streamflow gauge records from 240 stations throughout western North America. The team also used other climatic data such as moisture deficit, an indicator of dryness.
The results point to a marked increase in large wildfires in western US forests beginning around 1987, when the region shifted from predominantly infrequent large wildfires of short duration (average of one week) to more frequent and longer-burning wildfires (five weeks).
The authors found a jump of four times the average number of wildfires beginning in the mid-1980s compared with the 1970s and early 1980s. The comparison showed that the total area burned was six and a half times greater. Also in the mid-1980s, the length of the yearly wildfire season (March through August) extended by 78 days, a 64% rise when comparing 1970-1986 with 1987-2003.
The researchers determined that year-to-year changes in wildfire frequency appear “to be strongly linked to annual spring and summer” temperatures with “many more wildfires burning in hotter years than in cooler years.”
They established a strong association between early arrivals of the spring snowmelt in the mountainous regions and the incidence of large forest fires. An earlier snowmelt, they said, can lead to an earlier and longer dry season, which provides greater opportunities for large fires. Overall, 56% of the wildfires and 72% of the total area burned occurred in early snowmelt years. By contrast, years when snowmelt happened much later than average had only 11% of the wildfires and 4% of the total area burned.
The greatest wildfire increases occurred in the Northern Rockies, where forest ecosystems in middle elevations were found to be highly susceptible to temperature increases. Other significant wildfire increases were found in the Sierra Nevada, the southern Cascades and the Coast Ranges of northern California and southern Oregon.
I see this as one of the first big indicators of climate change impacts in the continental United States. We’re showing warming and earlier springs tying in with large forest fire frequencies. Lots of people think climate change and the ecological responses are 50 to 100 years away. But it’s not 50 to 100 years away—it’s happening now in forest ecosystems through fire.—Thomas Swetnam, director of the Laboratory of Tree-Ring Research at The University of Arizona in Tucson
Fire-fighting expenditures for wildfires now regularly exceed one billion dollars per year.
The authors state that climate model projections, driven by potential increases in atmospheric greenhouse gas concentrations, indicate that warmer springs and summers will likely continue and intensify in the coming decades, accentuating conditions favorable to large wildfires.
The authors conclude that the increased frequency of large and devastating wildfires may significantly change forest composition and reduce tree densities, transforming the western US forests’ role as a storage sink for sequestering some 20 to 40% of all US carbon to a source for increasing carbon dioxide in the atmosphere.
In a companion Perspective in the same issue of Science Express, Steven Running of the Numerical Terradynamic Simulation Group, University of Montana draws the same conclusion:
Wildfires add an estimated 3.5 × 1015 g to atmospheric carbon emissions each year, or roughly 40% of fossil fuel carbon emissions. If climate change is increasing wildfire, as Westerling et al. suggest, these new sources of carbon emissions will accelerate the buildup of greenhouse gases and could provide a feed-forward acceleration of global warming.
The research was supported by the National Oceanic and Atmospheric Administration’s Office of Global Programs, the National Fire Plan via the United States Forest Service’s Southern Research Station and the California Energy Commission.
“Warming and Earlier Spring Increases Western U.S. Forest Wildfire Activity”; Anthony Leroy Westerling, Hugo G. Hidalgo, Daniel R. Cayan, Thomas W. Swetnam; Science Express 6 July 2006; DOI: 10.1126/science.1128834
“Is Global Warming Causing More, Larger Wildfires?”; Steven W. Running; Science Express, 6 July 2006; DOI: 10.1126/science.1130370