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Warmer temperatures tied to wildfires, scientists say

  • Story Highlights
  • Study: Number of large wildfires began to increase in the mid-1980s
  • Greatest increase in large fires in the Northern Rockies, scientists found
  • Wet winters in Southern California promote growth of fuel plants
  • Region suffering extreme drought, record heat this year
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By David E. Williams
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(CNN) -- Huge wildfires, such as the ones that have charred more than 460,000 acres this week in Southern California, are becoming more common in the Western United States, and scientists say warming trends and other climate factors may be responsible.

Scientists say that warming trends appear tied to increased wildfire activity in parts of the U.S. West.

A study published last year in the journal Science found that the number of large wildfires (greater than 1,000 acres) in Western forests increased dramatically in the mid-1980s.

The length of the fire season also increased, the study found. It noted that the average length of the fire season between 1987-2003 was 78 days longer than it was between 1970 and 1986.

That's a 64 percent increase, the study found.

Thomas Swetnam, one of the study's authors, said that there's a pretty strong connection between the early arrival of spring and increased wildfire activity.

"Fuel has a longer season to dry out, and the soils have a longer season to dry out so the fire season starts earlier and lasts longer," Swetnam said. Video Watch what's fueling the flames »

The greatest increase in wildfire frequency was in the Northern Rockies, which made up 60 percent of the rise in large fires, according to the study.

"I think the evidence is lining up that indeed there is some connection between the warming trends and especially the early arrival of spring," he said. "The strongest signal is actually in the northern states of the West -- Oregon, California, Idaho, Montana -- and also Alaska and Canada are showing a pretty clear connection between warming temperatures and wildfire activity."

Swetnam, director of the University of Arizona's Laboratory of Tree-Ring Research, said different climate factors are fueling the Southern California fires. See where the fires are burning »

First there's the hot, dry Santa Ana winds, which sweep into the region and can fan the tiniest embers into a roaring blaze.

"I don't think there are any really good studies that show that the increased severity or duration of Santa Ana winds could be related to global warming. That link has not been established yet," Swetnam said.

Wet winters in 2005 and 2006 promoted the growth of shrubs, grasses and other fine fuel plants, he said.

This year's extreme drought and record temperatures have made those plants tinder dry.

Fire weather expert Rob Balfour said surface temperatures are 10 to 20 degrees hotter than the air in the hills around San Diego.

"So we're looking at 120 degrees for the grasses and things like that," Balfour said. "So if an ember lands in there, it's just going to ignite immediately."

Plants such as mountain mahogany, chamise and sage are filled with oils that burn fast and hot.

"In those shrubby systems, you don't have surface fires," said Monique Rocca, a wildfire fire science professor at Colorado State University. "The fire just goes through all the shrubs and kills them, but because that's the normal fire regime for down there the shrubs are highly adapted to re-sprout or reseed after a fire."

Those plants grow back quickly after a fire, and several species of wildflowers only grow in areas that have burned, Rocca said.

Swetnam said smaller, surface fires are good for forests because they destroy shrubs and other brush without hurting trees.

If that brush builds up, it can fuel more intense crown fires, which can reach the tops of trees and kill them.

"In many cases the extent of these crown fires are so large that the canopy holes that are created won't recover back to forest for a long, long time," Swetnam said. See how fires grow »


Grasses and brush often establish themselves in those clearings because they grow faster than trees, he said. They're also well-adapted for fire, so they may burn again and kill young trees growing in the area.

"If you lose the forest canopy and it reverts to shrubs or grasslands, it may not revert to forests," Swetnam said. E-mail to a friend E-mail to a friend

CNN's Rob Marciano contributed to this report.

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