Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 118, Issue 25, Pages -Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.2103135118
Keywords
climate change; fire ecology; wildfires; extreme events; paleoecology
Categories
Funding
- NSF [DEB-1655121, OISE-0966472, DEB-1655189, BCS-0845129]
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The 2020 fire season in the western United States marked a decades-long trend of increased fire activity, particularly in subalpine forests where fire frequency historically has been low. Observing fire activity over the past 2000 years reveals that the current fire rotation period and burning rates are significantly higher than historical averages. This highlights how extreme events in recent years are shaping new fire regimes as temperatures continue to rise, impacting subalpine forests in the Rocky Mountains.
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 similar to 0.3 degrees 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.
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