Climate Change Blamed for Half of Increased Forest Fire Danger

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The Loma Fire rages on the Santa Cruz Mountains summit beyond the Giant Dipper Roller Coaster in Santa Cruz. Credit Shmuel Thaler/The Santa Cruz Sentinel, via Associated Press

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Forest fires are burning longer and stronger across the western United States, lighting up the landscape with alarming frequency. Residents are forced to flee, homes are incinerated, wildlife habitats are destroyed, lives are lost. Last year, the Forest Service spent more than half its annual budget fighting fires.

Scientists have long theorized that climate change has contributed to the longer fire seasons, the growing number and destructiveness of fires and the increasing area of land consumed, though some experts suggest that the current fire phenomenon is not just a result of a changing climate, but also fire-suppressing policies practiced by the government for the last century or more.

In a new study published Monday in Proceedings of the National Academy of Sciences, scientists from the University of Idaho and Columbia University have calculated how much of the increased scope and intensity of Western wildfires can be attributed to human-caused climate change and its effects. They state that, since 1979, climate change is responsible for more than half of the dryness of Western forests and the increased length of the fire season. Since 1984, those factors have enlarged the cumulative forest fire area by 16,000 square miles, about the size of Massachusetts and Connecticut combined, they found.

The study uses “fuel aridity,” or dryness of the climate and the forests, as a way to measure the influence of climate change on forest fires. The combination of a long period of drought in the West and hot temperatures have caused trees and undergrowth to become particularly tinderlike. Warmer air can draw more moisture, in general, from trees and plants, turning them into kindling.

The scientists used the dryness of the climate to determine the dryness of the forests themselves, using eight metrics that corresponded with fuel dryness and fire danger, said A. Park Williams, one of the study’s authors and an assistant research professor at Columbia’s Lamont-Doherty Earth Observatory.

The authors found that fuel aridity in a given year has a direct relationship with the forest fire area, and that climate change accounts for 55 percent of the increased aridity from 1979 to 2015. Cyclical climate variations would have dried out the landscape some, but human-caused climate change on top of those patterns caused this drying process to double.

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