Warmer Atlantic, Climate Change Presage More, and Worse, Western Wildfires

Historical record suggests the American West is “primed” for climate change-inspired conflagrations.

Between 1650 and 1749, fires raged across western North America from what is now British Columbia down into northern Mexico. In contrast, the following century saw scattered, sporadic wildfires. By combining tree-ring records stretching back 450 years as well as fire-scar data from more than 4,700 burned trees, scientists have now created an extended log of the climate in the western U.S. and its attendant wildfires. And this record has revealed that temperature shifts in the waters of the northern Atlantic help determine the scale and intensity of western wildfires.

Forest ecologist Thomas Swetnam of the University of Arizona and a team of international colleagues collected tree-ring and fire-scar data from 241 logging sites across western North America. The scars revealed the dates of at least 33,975 individual fires stretching back to 1550. “We got most of our fire-scar samples from dead trees, stumps and logs. They’re cookies,” or cross sections, Swetnam explains. “A single tree might have 10 or even 20 different scars on it.”

The researchers compared this long fire record with weather patterns: the well-known El Nino and La Nina cycles that occur every two to seven years, as well as longer cycles called the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation (AMO). “Fire activity correlated with–as we expect–drought, but also with these interesting oscillations in the Pacific and Atlantic,” Swetnam notes. “The North Atlantic, and warming temperatures there, have apparently had some importance in drought occurrence and fire activity.”

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