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Warming climate fuels fires in Rockies

July 6, 2006 By Richard Van Noorden This article courtesy of Nature News.

Western United States feels the heat of climate change.

Are fiercer wildfires driven by climate change or poor forest management? Scientists and the media have been debating this burning question for decades. Now a large study of recent western US forest fires shows that, for the Rockies at least, climate is to blame.

Every year, forest fires burn hundreds of homes, severely damage natural resources, and attract more than US$1 billion in fire-fighting costs across the United States. As the fires rage more strongly, so does the debate about their causes.

Some explanations cite changing land-use patterns: livestock grazing and extensive logging in the early twentieth century was followed by forest regrowth and accumulation of burnable matter, which has increased the ferocity and spread of wildfires. But growing amounts of scientific research have indicated that local climate changes may also be to blame.

Now a research team led by Anthony Westerling of the Scripps Institution of Oceanography in La Jolla, California, has published a large-scale study of western US forest fires that shows what many had suspected: forest-fire activity has dramatically increased in recent years, particularly in the northern Rockies, in step with local climate change.

"Most people studying this are ecologists, not climatologists," says Westerling. "We looked at things on a much larger scale; only then do you start seeing patterns." The researchers looked at 1,166 large forest wildfires in the western United States over 34 years. They found a dramatic jump in fire behaviour after 1987. In the 17-year period following that date, wildfire frequency was nearly four times higher, and area burned six times higher, than in the previous 17 years.

Early meltdown

The team compared their year-on-year forest fire data with the corresponding spring and summer temperatures and seasonal timing of snowmelt. Forest fires in the northern Rockies, which accounted for most of the wildfire increase, were strongly associated with increased temperatures, when snows melted earlier and dry seasons lasted longer.

Lots of people think climate change and the ecological responses are 50 to 100 years away. But it's happening now.
Thomas Swetnam,
University of Arizona.
"I see this as one of the first big indicators of climate-change impacts in the continental United States," said Thomas Swetnam, a member of the research team and director of the Laboratory of Tree-Ring Research at the University of Arizona in Tucson. "Lots of people think climate change and the ecological responses are 50 to 100 years away. But it's happening now in forest ecosystems through fire."

The study has implications for current US fire suppression policies, which focus on removing shrubs and selectively logging trees to create open forests. Because local climate is the driving factor behind fires in the Rockies, this vegetation management may not work, says Westerling. "There is always a tendency to have a 'one size fits all' policy,'selectively log everywhere', for example, but that would be inappropriate in places such as the Rockies."

Wood to burn

In the Rockies, lodgepole pine and spruce-fir forests are densely packed. But trees in the ponderosa pine forests of the southwest of the study region are more sparsely scattered. Fires in these open forests are generally carried in kindling near the forest floor, says Westerling, which might profitably be removed by a forest management scheme. Even so, the team found that climate oscillations between wet and dry periods (creating lush plants that can later burn) were a factor in these wildfires too.

For the areas most affected by climate change, it looks like things can only get worse. All climate model projections suggest that hotter springs and summers will be coming to the region in the next few decades if global warming trends continue, the authors write.

The researchers suggest that more large wildfires could burn enough biomass to make western US forests produce carbon dioxide than they soak up. "It is alarming that forests in the West and elsewhere may become important carbon sources in the greenhouse-gas story, something that is not included in current climate projections," comments Cathy Whitlock of Montana State University in Bozeman, who has studied the fire and climate history of the western United States.

One test of the Westerling findings, she says, will be to examine fire-climate linkages on longer time scales, looking back to tree-ring and lake-sediment records when climate and greenhouse gases were quite different.

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  1. Westerling A., et al. Science. doi: 10.1126/science.1128824 (2006).


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