Western wildfires linked to Atlantic warming cycle

GRANTS PASS, Ore. — Using fire scars on nearly 5,000 tree stumps dating back 450 years, scientists have found that extended periods of major wildfires in the West occurred when the North Atlantic Ocean was going through periodic warming.

With the North Atlantic at the start of a recurring warming period that typically lasts 20 to 60 years, the West could be in for an extended period of multiple fires on the scale of those seen in 2002 and 2006, said Thomas W. Swetnam. He's director of the Laboratory of Tree Ring Research at the University of Arizona and a co-author of the study published in the Dec. 26 issue of the Proceedings of the National Academy of Sciences.

"This study and others have demonstrated that there is an underlying climatic influence on fuels and then on the weather conditions that promote fires," said Dan Cayan, climate researcher at the Scripps Institution of Oceanography, who did not take part in the study.

Ron Neilson, a U.S. Forest Service scientist who has developed models that predict wildfire danger based on climate models, agreed with the study's conclusions and noted all the oceans are affected by global warming. And that in turn could exacerbate the wildfire cycle.

Scientists have long seen a relationship between weather in the United States and El Nino, a warming of water in the South Pacific.

When El Nino is strong, the Northwest typically has drought and severe fire seasons, and the Southwest has rain. When the cycle reverses, known as La Nina, the South Pacific cools, the Northwest has more rain, and the Southwest has drought and fires.

Less well understood are two other climate drivers, the Pacific Decadal Oscillation, known as the PDO, centered in the North Pacific, which typically changes every 10 to 20 years, and the Atlantic Multidecadal Oscillation, or AMO, which is marked by warming and cooling periods of 20 to 60 years in the North Atlantic.

El Nino-La Nina is thought to be the most influential cycle, but the Atlantic and Pacific oscillations can magnify or diminish those effects when strong phases of the three cycles come together, Swetnam said.

"Over the last 400-plus years in our fire history study, when the AMO was positive (producing warm temperatures in the North Atlantic), then you would get big fires breaking out synchronously across the West," Swetnam said. "That's what we saw in 2002 and 2006."

The year 2002 saw three huge fires that stretched firefighting resources to the breaking point: Biscuit burned 500,000 acres in southwestern Oregon, Rodeo-Chedeski burned 462,000 acres in Arizona, and the Hayman fire burned 136,000 acres in Colorado.