Forests and wildfires form an unlikely ecological yin and yang: In a healthy climate, the apparently destructive blazes actually keep an entire ecosystem in balance. Fire accelerates the decomposition of organic material, returning nutrients to the soil; it also helps evergreens' cones to open and lodgepole and jack pine seeds to germinate. But if flames come too often, or burn too intensely, they can sterilize the landscape, harming wildlife and opening the door to invasive species. Unfortunately, numerous studies have documented that global warming creates conditions that invite larger and more frequent conflagrations–which lead in turn to higher carbon emissions. Here's a look at the vicious cycle.
 Mercury rises
Average spring and summer temperatures throughout the West have increased 1.6°F in just the last two decades. As a result, mountain snowpacks, which keep the soil moist and serve as the main water source for Western forests and rivers, are melting one to four weeks earlier.
 Forests dry out
Within a month of snowmelt, trees weaken and vegetation dries, making forests more vulnerable to parasites and fires. Dry seasons now last 78 days longer, on average, than they did 20 years ago.
 Beetles thrive
The combination of warmer year-round temperatures and longer summers has spurred the proliferation of forest-slaying mountain pine beetles by speeding up the bugs' maturation and metabolism. "What's normally a two-year life cycle has shortened to one," says Don McKenzie, research ecologist with the Pacific Wildland Fire Sciences Lab in Seattle and lead author of a 2004 study examining the link between climate change and wildfires. The beetles lay their eggs in pines, eventually killing the trees (see page 56) and adding more fuel to the ecological tinderbox. Spruce beetles have killed more than 90 percent of spruce trees on Alaska's Kenai Peninsula.
 Fire becomes more devastating
Parched vegetation, beetle-killed trees, and longer, warmer summers are provoking more fires, larger fires, and longer-lasting fires. A 2006 Scripps Institution of Oceanography study reports that four times as many fires burned 6 1/2 times more acreage between 1987 and 2003 than in the previous 16 years. The reason? Higher spring and summer temperatures.
 Nonnative plants invade
This increase in fire damage clears the way for invasive flora such as cheatgrass to enter areas like the Front Range and Great Basin. "Cheatgrass intensifies fires, because there used to be patches of nonvegetated areas on the plains that didn't burn," McKenzie says. "Now that the grass is there, the fire can spread more." Impact: the largest fire in Colorado history, the 2002 Hayman Fire, which burned 137,760 acres and cost $39 million in damages and manpower.
 Gases increase
Forests act as a carbon sink, extracting CO2 from the atmosphere during photosynthesis. Western forests alone account for 20 to 40 percent of carbon sequestration in the United States. But the combustion of trees and vegetation by fire releases enormous amounts of carbon dioxide–more than 3.8 billion tons last year (or nearly four times the 1980 level, according to one estimate). "Any time you release CO2, you're accelerating the cycle of global warming and forest fires," says McKenzie.
Since the 1970s, the average duration of wildfires has increased from 7.5 days to 37.1 days. Average summer temperatures in the western United States are expected to increase up to 9°F by 2069. The last two decades have been the warmest years in the West since recordkeeping began in 1895.