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Rocky Mountain subalpine forests now burning more than any time in recent millennia

Significance Climate change is increasing wildfire activity across the western United States, with unprecedented rates of burning expected in many western forests by mid-century. Here, we use a unique network of fire history records to show that after the extreme 2020 fire season, Rocky Mountain subalpine forests are now burning more than at any point in the past 2,000 y, exceeding variability experienced in response to past climate extremes. Increasingly warm, dry conditions in the 21st century are enabling the exceptional rates of burning, including 2020, consistent with long-standing links between climate and fire in subalpine forests. Continued warming will reinforce newly emerging fire regimes, with significant implications for ecosystems and society. The 2020 fire season punctuated a decades-long trend of increased fire activity across the western United States, nearly doubling the total area burned in the central Rocky Mountains since 1984. Understanding the causes and implications of such extreme fire seasons, particularly in subalpine forests that have historically burned infrequently, requires a long-term perspective not afforded by observational records. We place 21st century fire activity in subalpine forests in the context of climate and fire history spanning the past 2,000 y using a unique network of 20 paleofire records. Largely because of extensive burning in 2020, the 21st century fire rotation period is now 117 y, reflecting nearly double the average rate of burning over the past 2,000 y. More strikingly, contemporary rates of burning are now 22% higher than the maximum rate reconstructed over the past two millennia, during the early Medieval Climate Anomaly (MCA) (770 to 870 Common Era), when Northern Hemisphere temperatures were ∼0.3 °C above the 20th century average. The 2020 fire season thus exemplifies how extreme events are demarcating newly emerging fire regimes as climate warms. With 21st century temperatures now surpassing those during the MCA, fire activity in Rocky Mountain subalpine forests is exceeding the range of variability that shaped these ecosystems for millennia.