Study: Forest fires flourishing from human-caused climate change

Human-caused climate change has dramatically increased the scope of wildfires over the past three decades by increasing the degree to which Western forests dry during summer months.

According to a study published in the Proceedings of the National Academy of Sciences on Monday, human-caused climate change has almost doubled the amount of forest land burned in the West since 1979, accounting for an additional 16,000 square miles or more than 10.2 million acres.

"We're no longer waiting for human-caused climate change to leave its fingerprint on wildfire across the Western U.S. It's already here," said lead author John Abatzoglou, an associate professor of geography at the University of Idaho. "Over the last several decades, we've seen longer fire seasons, larger fires and more area burned - and those observations led us to ask 'Why?' What we found was that human-caused climate change played a resounding role in observed increases in forest fire activity."

Abatzoglou and co-author Park Williams, a bio-climatologist at Columbia University, found that spring and summer temperatures have increased by 2 to 2.5 degrees since 1950 and relative humidity has dropped slightly. That warming and reduced humidity have worked together to dry timber, brush, grasses and dead fuel like downed trees, making them more ripe for fire. Firefighters closely track fuels and the degree to which they are either dry or wet during the summer months in an attempt to gauge fire danger.

"Wildfire is a function of several processes, some natural and some human. But what we know is on a year-to-year basis, warm, dry summers enable large fire seasons," Abatzoglou said. "There is a remarkable relationship between the extent of forested area burned and fuel dryness that allows us to implicate climate as the preeminent driver of annual variability in forest fire over the past three decades."

According to their work, man-made climate change accounted for 55 percent of the increase in forest aridity observed between 1979 and 2015, while normal climate variability, such as natural drought, accounted for 45 percent. The bulk of the increase attributed to human-caused climate change has come since the turn of the century. According to the authors, since 2000, human-caused climate change produced a 75 percent increase in the extent of forested land with elevated dryness and has added about nine days to the average fire season.

Other factors, such as a build up of fuels attributed to past fire suppression efforts has probably increased the effect climate change is having on forests and fires.

"Opposite of how climate change would have zero effect on forest fire activity if there was zero fuel to burn, climate change probably has had an amplified effect on forest fire activity due to an artificially high number of trees in many forested areas due to past fire suppression," Williams said.

Abatzoglou said firefighters and land managers can attempt to mitigate the effects of the warming climate on fire activity by working to reduce fuels when conditions allow. For example, he said in years where fire conditions are lower, they could allow more fires to burn naturally and reduce the severity of future fires.

"Going forward we should expect to see summers like we have seen over the past 10 years occurring more frequently. That will translate into more fire," he said. "Hopefully, what we will learn to do is to co-exist with fire and take advantage of years where conditions are relatively moist where if fires start we can let fires burn and consume fuels."

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