A new study says that human-induced climate change has doubled the area affected by forest fires in the U.S. West over the last 30 years. According to the study, since 1984 heightened temperatures and resulting aridity have caused fires to spread across an additional 16,000 square miles than they otherwise would have—an area larger than the states of Massachusetts and Connecticut combined. The authors warn that further warming will increase fire exponentially in coming decades.

A new study says that human-induced climate change has doubled the area affected by forest fires in the U.S. West over the last 30 years. According to the study, since 1984 heightened temperatures and resulting aridity have caused fires to spread across an additional 16,000 square miles than they otherwise would have—an area larger than the states of Massachusetts and Connecticut combined. The authors warn that further warming will increase fire exponentially in coming decades. The study appears today in the journal Proceedings of the National Academy of Sciences.

“No matter how hard we try, the fires are going to keep getting bigger, and the reason is really clear,” said study coauthor Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory. “Climate is really running the show in terms of what burns. We should be getting ready for bigger fire years than those familiar to previous generations.”

Fires in western forests began increasing abruptly in the 1980s, as measured by area burned, the number of large fires, and length of the fire season. The increases have continued, and recently scientists and public officials have in part blamed human-influenced climate change. The new study is perhaps the first to quantify that assertion. “A lot of people are throwing around the words climate change and fire—specifically, last year fire chiefs and the governor of California started calling this the ‘new normal,’ ” said lead author John Abatzoglou, a professor of geography at the University of Idaho. “We wanted to put some numbers on it.”

Warmth drives fire by drying out the land. Warmer air can hold more moisture, and the air ends up sucking it out of plants, trees, dead vegetation on the ground, and soil. Average temperatures in forested parts of the U.S. West have gone up about 2.5 degrees F since 1970, and are expected to keep rising. The resulting drying effect is evident in the rise of more fires. Williams published a study last year showing how climate-driven removal of moisture from land worsened the recent California drought, which was accompanied by widespread fires.

The overall increase in fire since the 1980s is about twice what the researchers attribute to climate change; the rest is due to other factors, they say. One has been a long-term natural climate oscillation over the Pacific Ocean that has steered storms away from the western United States. Another: firefighting itself. By constantly putting out fires, authorities have allowed areas they “saved” to build up more dry fuel, which later ignites, causing ever more catastrophic blazes, the researchers say. The costs of fire fighting have risen sharply in step; last year the federal government alone spent more than $2.1 billion. “We’re seeing the consequence of very successful fire suppression, except now it’s not that successful anymore,” said Abatzoglou.

Continue reading at Columbia University Earth Institute.

Image credit: Mike Daniels via Columbia University.