When smoke from blazing forests in the West tinted skies ochre across much of the U.S. last year, we asked, “How much worse can fire seasons get?” Fire paleoecologist Philip Higuera has spent his career trying to determine the answer by looking at history. “If we’re all wondering what happens when our forests warm up,” he says, “let’s see what happened in the past when they warmed up.”

The central Rocky Mountains’ subalpine forests grow in cool, wet conditions and burn less readily than their lowland counterparts. To find how frequently these tough woodlands still caught fire through the ages, Higuera and his colleagues combined records from modern satellite-observed fires, fire scars in tree rings from the 1600s onward and flecks of charcoal that settled in lakes over thousands of years. The study found that from 2000 to 2020, the forests burned 22 percent faster than they did during an unusual warming period that started in A.D. 770 and saw the area’s highest temperatures before the 21st century.* Most of this burn rate increase was caused by the 2020 fire season alone. And 72 percent of the total area burned between 1986 and 2020 resulted from fires in just the latter year.

Graph shows fire rotation periods for 100-year intervals between 50 B.C. and 2000, plus FRPs based on data from 1984 to 2020.
Credit: Amanda Montañez; Sources: “Rocky Mountain Subalpine Forests Now Burning More Than Any Time in Recent Millennia ,” by Philip E. Higuera, Bryan N. Shuman and Kyra D. Wolf, in Proceedings of the National Academy of Sciences USA, Vol. 118, No. 25, Article No. e2103135118; June 14, 2021; Dryad https://doi.org/10.5061/dryad.rfj6q579n (data and code for study)

Overall, these forests have not burned frequently—until the past two decades. The gap between extreme fire years in the U.S. is narrowing as the climate warms, and Higuera does not think this pattern will reverse any time soon. The new research is detailed in Proceedings of the National Academy of Sciences, USA.

Line graph shows total area burned annually in the subalpine forests of the focal study area from 1984 to 2020.
Credit: Amanda Montañez; Sources: “Rocky Mountain Subalpine Forests Now Burning More Than Any Time in Recent Millennia ,” by Philip E. Higuera, Bryan N. Shuman and Kyra D. Wolf, in Proceedings of the National Academy of Sciences USA, Vol. 118, No. 25, Article No. e2103135118; June 14, 2021; Dryad https://doi.org/10.5061/dryad.rfj6q579n (data and code for study)

Higuera finds the recent unprecedented fire seasons unsurprising but still agonizing. “I’ve spent 20 years writing about this,” he says. “But I have not spent 20 years thinking about how this would feel.”

This research shows that the past may no longer guide us when it comes to understanding and handling wildfires, says environmental geographer Rosemary Sherriff, who was not involved in the study. “We have to accept that we’re going to see an increase in fire activity,” she says. “We have to adapt to the new norm.”

Many communities hit by recent wildfires have fire protection plans, Higuera adds, but “they’re based on our expectations that these forests burn once every few centuries. That’s not where we are. That’s not where we're going.”

*Editor’s Note (7/28/21): This sentence was edited after posting to correct the earlier 21st-century date.