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Climate change is erasing 'flammability barrier' that protects high-elevation forests

Global warming and ongoing drought conditions in the western U.S. are drying out such forests, making them particularly susceptible to blazes.
Image: Cameron Peak Fire burns outside Estes Park
The Cameron Peak Fire burns outside Estes Park, Colo., on Oct. 16, 2020. Jim Urquhart / Reuters file

Wildfires in the western United States are increasingly happening at high elevations, in mountainous areas that were previously too wet to burn, according to a new study.

Scientists say climate change and ongoing drought conditions in the West are drying out high-elevation forests, making them particularly susceptible to blazes. With several Western states plunging deeper into a megadrought, and experts predicting a hot and dry summer, the findings add to a distressing outlook for this year's wildfire season.

For their study, which was published Monday in the journal Proceedings of the National Academy of Sciences, the researchers examined records from 1984 to 2017 of all fires in the western U.S. that were larger than 1,000 acres. They found that the amount of scorched land increased across all elevations during that period, but observed that the biggest increase was at elevations above 8,200 feet.

Mojtaba Sadegh, an assistant professor of civil engineering at Boise State University and one of the authors of the study, said the shift to higher-elevation fires was noticeable beginning in the year 2000.

From 2001 to 2017, the area of land burned above 8,200 feet more than tripled compared to in the previous 16-year period, according to the study. For context, Aspen, Colorado, sits at an elevation of 8,000 feet above sea level.

Typically, at such high elevations, snow lingers through the summer, leaving little time for vegetation in these mountainous forests to dry out before rain and snow fall anew in the autumn and winter.

Now, global warming is melting mountain snowpack earlier in the year and disrupting that natural cycle.

"There used to be a flammability barrier, where the forest was too wet to burn," Sadegh said. "With earlier snowmelt, vegetation has time to dry and so that flammability barrier is not there anymore for much of the western U.S."

As such, more than 31,000 square miles of high-elevation forests — or roughly 11 percent of forest cover in the western U.S. — could be at risk this year and in future wildfire seasons, according to the study.

The new research focused on a 34-year period ending in 2017, the most recent year with reliable data available from the Monitoring Trends in Burn Severity program, which is overseen by the U.S. Geological Survey, the U.S. Forest Service and several other government agencies.

If more recent years were taken into account, Sadegh said the trend toward higher-elevation fires would likely be even more pronounced. During last year's record-breaking fire season, for instance, several blazes in Colorado and California burned at high elevations.

"If you look at two 2020 fires in Colorado, the Cameron Peak Fire and East Troublesome Fire, those were two of the biggest fires in the modern history of that state," he said. "Both of them basically burned all the way to the upper tree line."

Scientists are concerned that wildfires in high-elevation forests could have worrisome consequences for the ecosystem.

Climate change combined with longer and more intense wildfire seasons could alter the rate of snowmelt in these mountainous regions, which subsequently affects water that normally flows into reservoirs and rivers, according to the study.

Tree canopies that are lost in fires can also expose land and streams to more sunlight, which increases temperatures across elevations and could have negative implications for fish and other animals that live in these regions, the researchers said.

Sadegh said he hopes the findings will act as a wake-up call about how climate change is compounding the risks of wildfires.

"Fires are going to continue at lower elevations, but now we see how higher elevations that have not been exposed to a lot of fires are now susceptible," he said. "There are just a lot more risks as the climate warms."