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September 2007

Phenomenon: The Life Cycle of Wildfires

Global warming creates conditions that invite larger and more frequent forest fires. Here's how.

[4] 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.

[5] 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.

[6] 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.

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