COEUR D'ALENE, Idaho (AP) -- A study carried out in a central Idaho pine forest suggests global warming - not overly thick forests - is the leading cause of huge wildfires that have been burning in the West every season for nearly 20 years.
The findings are published in the latest issue of the journal Nature.
The study analyzed 8,000 years of sediment accumulated in the valley of the South Fork Payette River near Lowman, where records of massive, prehistoric wildfires are buried in the sediment.
After the fires, rain and snowmelt flowed over a floor sealed by the heat and ash, creating a cement-like slurry of charcoal, boulders and bits of trees. The most severe fires caused the thickest bands of sediment.
Most fire history is based on studying tree rings and typically extends back no further than 500 years, the age of the oldest ponderosa pines, said Grant A. Meyer, co-author of the study and associate professor of earth and planetary sciences at the University of New Mexico.
The period from about 1350 to the beginning of the 20th century was known as the Little Ice Age. Rocky Mountain glaciers grew larger during this period. And the wet, cool conditions allowed grasses to flourish on forest floors, which provided tinder for frequent but cool ground fires.
Foresters see this time as a period when cool fires and old trees seemed to be in harmony, Meyer said. However, the time before the Little Ice Age probably provides a more accurate glimpse of the interplay between forests and fire.
Before the Little Ice Age was a 400-year period of hot, dry weather with multiple, decades-long droughts, Meyer said. Several massive wildfires burned the forest to the ground above the Payette River.
``The fires must have been quite severe,'' Meyer said.
It appears that temperatures today are even higher than they were during the medieval period, the scientists wrote.
Most scientists believe the current warming trend is the result of a buildup of greenhouse gases, such as carbon dioxide, caused by the burning of coal, oil and other fossil fuels.
Fire had always been allowed to rid the forest floor of excess fuel, yet this did not protect the forest from large-scale destruction by fire during the warming trends, said Jennifer Pierce, a University of New Mexico graduate student and lead author of the study.
This may mean that forests cannot be protected by thinning projects, because historical evidence shows that fires will only be hotter and larger on a planet that continues to heat up, Pierce said.
``Management policy needs to incorporate climate change, instead of trying to get back to these Little Ice Age conditions where fires were small,'' Pierce said. ``That's just not realistic. Maintenance of those conditions will be difficult in the face of global warming.''
Retired U.S. Forest Service fire ecologist Stephen Arno said climate should not be ignored, but forest thinning remains the best defense against catastrophic wildfire.
Recent years have provided ``innumerable examples'' where thinned forests have slowed the rapid spread of forest crown fires, said Arno, author of the 1999 book ``Flames in Our Forest.''
``Fuels management can work to mitigate the severity of fires. Even with climate change, you're still going to need to control fuel buildup,'' he said.