Data and code for: Rocky Mountain subalpine forests now burning more than any time in recent millennia

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.