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 the 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 Niño, a warming of water in the South Pacific.
When El Niño is strong, the Northwest typically has drought and severe fire seasons, and the Southwest has rain.
When the cycle reverses, known as La Niña, 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 Niño-La Niña 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.
In 2006, 89,000 fires burned across 9.5 million acres. The U.S. Forest Service spent $1.5 billion fighting those fires — about $100 million over budget.
Another factor in the larger fires, said Swetnam, is that after a century of fighting wildfires, fuel is building up in the nation's forests.
The study gathered data from 241 logging sites around the West, compiling the dates of 33,795 fire scars on 4,700 stumps to develop a history of fires in the West dating to 1550.
The fire history was compared to a reconstruction of the Atlantic Decadal Oscillation, the Pacific Decadal Oscillation, and El Niño.
The most severe fire seasons fell between 1660 and 1710, when the Atlantic Multidecadal Oscillation was at its warmest, the study found.
The least severe fire seasons happened from 1787 to 1849, when North Atlantic temperatures were at their coolest.
The study comes after another published this year in the journal Science found that a sudden and dramatic increase in western wildfires in the late 1980s was related to a pattern of earlier springs and warmer summers. Swetnam and Cayan both took part in that study.
Greg McCabe, a climatologist for the U.S. Geological Survey in Denver, said his research has been showing a connection between North Atlantic Ocean temperatures and the drought that is gripping much of the West, which creates conditions for major fires.
"I think what Tom has written is really good," McCabe said. "More and more people are starting to see there is something there. We do know the tropical Pacific (home to the ocean warming condition known as El Niño) is a key player in global climate. But on longer time scales it looks like the Atlantic also has some influence."