West’s Wildfires Linked to Global Warming

The raging Western wildfires of recent years have often been blamed on management practices that promoted dense, overpacked forests. But a new study indicates global warming may be the main culprit.

Challenging the conventional wisdom that today’s severe wildfires are unnatural and unprecedented, researchers have found that parts of the West experienced destructive blazes during a warm, drought-plagued period in the Middle Ages.

The linkage suggests that as the climate warms, damaging wildfires will continue to strike the West. “If we are just at the beginning of dramatic warming ... we can simply expect larger, more severe fires,” said Grant A. Meyer, a co-author of the study, published in today’s journal Nature.

Meyer and two other researchers sifted through soil deposits as old as 8,000 years in ponderosa pine forests in central Idaho, finding a record of severe fire activity during the 400-year-long Medieval Warm Period from about 950 to 1350.

The sediments contained charcoal as well as landslide and mudflow debris washed into mountain basins following severe burns. To the east, in Yellowstone National Park, the researchers also found records of greater fire activity during the same period.

“Occasionally you do have these big fires and you get a lot of erosion with them and that’s part of the system,” said Meyer, a University of New Mexico associate professor of Earth and planetary sciences.

The study greatly expands the record of fire in western ponderosa pine forests, suggesting it is more varied and extreme than often thought. Much of the earlier research, based on 500-year-old tree ring data, points to a pattern of frequent, low-intensity burns that cleared out small growth and maintained more open forest conditions than prevail today.

That cycle of frequent, less severe fires was encountered by the first European settlers and is often held up as a model by advocates of increased logging and forest thinning in the West. They argue the big wildland fires that have charred millions of acres in recent years are unnatural, stoked by dense growth that is the result of logging declines on public land and a century of government efforts to quickly douse forest fires.

But the authors note the pre-European pattern of frequent, low burns coincided with a cooler, wetter period known as the Little Ice Age -- and may therefore be difficult to replicate in these warmer, drier times

“Trying to make things look like they did at European contact really misses the bigger picture of climatic change,” argued Cathy Whitlock, a Montana State Earth sciences professor who wrote an accompanying commentary to the Nature article. “These fire regimes are tightly linked to climate and climate has been changing continuously.”

The study’s authors acknowledge that dense forest growth has contributed to the severity of today’s wildfires. “Stand density has certainly had an effect as well,” said Jennifer Pierce, a University of New Mexico graduate student and the article’s lead author.

But “a one-size-fits-all management strategy -- everything-must-be-thinned approach -- just doesn’t make sense,” Meyer said.

Jon E. Keeley, a U.S. Geological Survey scientist at the Western Ecological Research Center, said the Idaho study was “a nice addition” to fire research. “It does illustrate the growing consensus that large catastrophic fires were not unknown in the past,” he said.

That is not to say nothing can be done to reduce the severity of future fires, he added. “But we can’t justify thinning forests to prevent large [severe] fires because it’s the natural course.”