Fire season in the western US is intensifying

In November 2018 a faulty electric transmission line provided the spark that started the Camp Fire in Northern California. The wildfire became one of the worst in US history, killing at least 85 people and causing damage totaling over $16 billion. Some 95% of the buildings in Paradise, California, were destroyed.

Observations and models predict that wildfire activity over the 21st century will increase and soon surpass the historical range of variability. To assess the magnitude of such changes and place contemporary fire activity in a millennial-scale context, Philip Higuera and Kyra Wolf of the University of Montana and their colleague Bryan Shuman at the University of Wyoming analyzed the paleofire record of the central Rocky Mountains. The network of observations they constructed shows that the subalpine forests in the region, driven to warmer and drier conditions because of climate change, are burning at a higher rate than in any other period over the past 2000 years.

The researchers derived the data set from 20 previously published reconstructions of fire activity based on lake sediments and tree rings. Higuera and his colleagues compared paleofires with contemporary ones by using fire rotation periods (FRPs)—the time required to burn the same area of forest as their study region. In the historical record of the central Rocky Mountains, the median FRP was 230 years, as shown in the figure below. The 21st century FRP of 117 years reflects nearly half the historical average—and the FRP of the last decade, just 68 years.

The previous peak in wildfire activity, with an FRP of 150 years, occurred about 1000 years ago, during a warm period known as the Medieval Climate Anomaly (MCA). The fires at that time fundamentally altered the subalpine forests they burned: Closed-canopy forests transitioned to lower-density areas of vegetation that are still present today. During the MCA, persistent forest burning created a negative feedback that decreased forest connectivity. Despite the elevated temperatures of the MCA, Higuera and his colleagues argue that the less dense, patchier forests in the latter half of the MCA were insufficient to fuel large, widespread conflagrations.

But that feedback mechanism took centuries, and now residents of towns such as Paradise, California, are being threatened again. The North Complex Fire in 2020, for example, incinerated some 320 000 acres near the town before firefighters were able to get the blaze under control. (P. E. Higuera, B. N. Shuman, K. D. Wolf, Proc. Natl. Acad. Sci. USA 118, e2103135118, 2021 .)