An additional 16,000 square miles of Western forests — an area more than 30 times the size of the city of Los Angeles — burned from 1984 to 2015 because of higher temperatures and drier conditions caused by the rise of greenhouse gases from the burning of fossil fuels.
“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 John Abatzoglou, the study’s lead author and an associate professor of geography in U of I’s College of Science.
Fire seasons are more than 70 days longer than they were 30 years ago, and measured summer temperatures are higher. Firefighters are able to put out 98 percent of the fires that start, but the 2 percent that escape grow larger, such as the 188,400-acre Pioneer Fire still burning in the Boise National Forest.
“What we found was that human-caused climate change played a resounding role in observed increases in forest fire activity,” Abatzoglou said.
The study was published Monday in the online early edition of the Proceedings of the National Academy of Sciences. It’s the first to quantify the degree to which human-caused climate change has increased wildfire activity in western U.S. forests. The study area included the area west of the Rocky Mountains.
“Knowing that human-caused warming is responsible for approximately half of the western U.S. forest fire area in the past few decades, and understanding that this effect is becoming increasingly dominant, helps us better anticipate continued changes in forest fire activity in the coming decades,” said co-author Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory.
“This knowledge will allow us to make more educated fire and land management decisions.”
The United States has seen dramatic fire seasons in recent years. In 2015, 10.1 million acres burned across the country — the most since the National Interagency Fire Center began documenting wildland fire areas in 1983. Federal firefighting costs for the 2015 fire season hit a record-high $2.1 billion.
Abatzoglou likened the many factors that lead to forest fires to instruments in an orchestra: When any instrument plays louder, the volume of the whole orchestra increases. His and Williams’ research finds that both human-caused climate change and natural variability have been playing with increasing volume in recent decades, leading to record-breaking fire seasons.
Abatzoglou did not dismiss the influence of decades of fire suppression. Fewer fires have led to overgrowth in many forests, so there is more fuel available to burn when conditions get dry.
“The legacy of fire suppression has probably amplified the effect of climate change on forest fire,” Williams said.
Increased temperature and reduced relative humidity increase fire activity by drying out timber. Such conditions allow fires to start and spread, and make them more difficult to suppress.
“There is a remarkable relationship between the extent of forested area burned and fuel dryness that allows us to implicate climate as the pre-eminent driver of annual variability in forest fire over the past three decades,” Abatzoglou said.
Abatzoglou and Williams compiled data from eight measures of aridity, or dryness, in western U.S. forests. They then used climate model experiments to identify how human-caused climate change influenced these aridity measures.
They found that spring and summer temperatures warmed by 2 to 2.5 degrees Fahrenheit since 1950, which is in agreement with the warming that climate models attribute to human-caused climate change. This warming led to significant drying within western U.S. forests — about 55 percent of the documented increases in forest aridity from 1979 to 2015. The remaining 45 percent was due to natural climate variations that reduced humidity and precipitation in some regions.
This influence has been particularly notable since the turn of the 21st century: Abatzoglou and Williams estimate that since 2000, human-caused climate change led to a 75 percent increase in forested lands with elevated aridity and contributed about nine additional days per year with exceptionally dry fuels and high fire potential.
The continued warming, scientists predict, will intensify over the next several decades, as will the effects on fire and forests.
“So long as there are trees in our forests, climate change will continue to set the table for large fire seasons,” Abtzoglou said.