Colorado recorded its most destructive fire in 2013 with the Black Forest Fire that tallied more than $420 million in insured losses. Washington saw the largest fire in the state’s history when the 2014 Carlton Complex Fire burned more than 250,000 acres (100,000 hectares). California’s Rim Fire in 2013 was the third largest in the state’s history, and the 2012 Rush Fire, the second largest. And last year’s Butte and Valley fires were some of the most destructive in state history.
These grim statistics are part of an alarming trend in western states: The number of large fires is growing, and so is the area burned and the length of the annual fire season. There is not one single cause for this, but a big contributor is temperature. Most of the region’s large fires are happening in warmer years that result in earlier spring runoff.
“The conclusion of our work is very direct, warming temperatures are drying out western forests and warmer and earlier springs are lengthening the fire season and the consequence of these factors is that there are more opportunities for large severe fires,” said Anthony LeRoy Westerling, associate professor of environmental engineering at the University of California, Merced. “As a result, fire activity is increasing.”
Even though the amount of precipitation varies across western states from year to year, warming temperatures mean that regardless of a wet or dry year, there will be more evaporation and less moisture retained in the soil and by vegetation.
Temperatures are rising throughout the country with the western U.S. particularly hard hit in many places. Climate Central found that since 1970, when warming trends began accelerating, western states have seen an average increase of 2.1F (1.2C), with New Mexico (2.67F), Arizona (2.34F) and Colorado (2.05F) experiencing the fastest rates of warming.
If a few degrees of temperature change doesn’t seem like a lot, consider that during the last ice age, the temperature was only 5–9F (2.8C–5C) colder and parts of the U.S. were buried under 3,000ft (900m) of ice.
“No single wildfire can be attributed to climate change,” the World Resources Institute reported. “However, research shows that climate change is increasing the duration and severity of wildfires in certain regions, and is expected to continue doing so in a warmer world.”
Research led by Westerling proves this point. In looking at forests managed by federal agencies in the Rockies, the Pacific Northwest, the Sierra Nevada and the Southwest, they found that wildfire activity is increasing with each decade, starting from around 1980.
Their most current research, published in March, uses data up to 2012 and found that since the window between 1973 and 1982, the number of large fires in the west has increased more than 500 percent across the region and the area burned has increased more than 1,200 percent. But certain areas have seen a much larger uptick – the area burned in the northern Rockies is up 3,000 percent and in the Northwest, 5,000 percent.
Their research also found that fire season now runs 220 days, an increase of 84 days in recent decades.
“There is great diversity around region’s forests, but all show a substantial trend in warming and drying and a longer fire season,” said Westerling.
Although the number of large fires is increasing, the total number of ignitions (or reported fires of all sizes) is actually not increasing on the federal lands they studied, said Westerling. And these areas tend to be more remote and sparsely settled, with the majority of fires coming from lightning strikes and not people.
Not all areas of the western mountains are equally sensitive to these climate changes, either. Forests that are cooler and have more moisture have been the most impacted. This is part of the reason why the northern Rockies and the Northwest have shown such huge increases in the amount of land burned by wildfires in recent decades.
But Westerling said they are also starting to see another trend that wasn’t visible in their research a decade ago. The areas burned by large fires are now increasing not just in mountain forests but also in grass and shrublands at lower elevations.
“Our review of historical data demonstrates how closely linked drier years and earlier springs are to the frequency of wildfires,” Westerling wrote. “Given projections for further drying in the West due to human-induced warming, this study points to a future with more wildfire activity.”