Western wildfires linked to Atlantic Ocean temps

BOULDER, COLO.
December 27, 2006 10:58am

Wildfires in California and other parts of the West may be linked to the surface temperature of the Atlantic Ocean, 3,000 or more miles to the east, according to a new tree-ring study led by the University of Comahue in Argentina and involving the University of Colorado at Boulder.

The conclusions indicate the wildfires may be getting worse.

The new study links episodic fire outbreaks in the past five centuries with periods of warming sea surface temperatures in the North Atlantic.

States like California, Washington, Oregon, Colorado, New Mexico, Arizona and South Dakota all had an increased prevalence of wildfires in recent centuries when a phenomenon known as the Atlantic Multi-Decadal Oscillation -- similar but longer in duration than the better known El Nino-Southern Oscillation -- periodically shifted from a cool to a warm mode that lasted roughly 60 years each time, say the study authors.

Warmer waters in the North Atlantic correspond with episodes of drought and subsequent fires in the West as shown by fire scars in annual tree rings studied by the researchers, says Thomas Kitzberger of the University of Comahue, who led the study with researchers from CU-Boulder, the University of Arizona, the U.S. Forest Service and Rocky Mountain Tree-Ring Research Inc., a private lab in Fort Collins, Colo.

The North Atlantic warming trend, coupled with warming temperatures and the earlier onset of spring in the West, poses "an increased hazard for wildfires that may continue for decades," Mr. Kitzberger says.

The research paper was published this week in the Proceedings of the National Academy of Sciences.

While previous tree-ring studies have linked fires in different regions of western North America to drought associated with the warm El Nino phase or cool La Nina phase of the Southern El-Nino Southern Oscillation phenomenon in the Pacific, the new study is the first to correlate the Atlantic Multi-Decadal Oscillation with increased North American fires on such a large scale, say the authors.

The team analyzed nearly 34,000 individual fire scar dates from tree rings, primarily ponderosa pine and Douglas fir, at 241 sites -- the largest record of tree rings linked to past wildfires ever assembled.

"This trend of warmer sea-surface temperatures in the North Atlantic appears to be correlated with dry spells we have seen in the West since the late 1990s," says study co-author and CU-Boulder geography Professor Thomas Veblen. "If the trend continues for the next 60 years or so as it has in the past, the degree of fire occurrence in the West could be unprecedented compared to anything in recent memory."