Medieval warming initiated exceptionally large wildfire outbreaks in the Rocky Mountains

Abstract

Many of the largest wildfires in US history burned in recent decades, and climate change explains much of the increase in area burned. The frequency of extreme wildfire weather will increase with continued warming, but many uncertainties still exist about future fire regimes, including how the risk of large fires will persist as vegetation changes. Past fire-climate relationships provide an opportunity to constrain the related uncertainties, and reveal widespread burning across large regions of western North America during past warm intervals. Whether such episodes also burned large portions of individual landscapes has been difficult to determine, however, because uncertainties with the ages of past fires and limited spatial resolution often prohibit specific estimates of past area burned. Accounting for these challenges in a subalpine landscape in Colorado, we estimated century-scale fire synchroneity across 12 lakesediment charcoal records spanning the past 2,000 y. The percentage of sites burned only deviated from the historic range of variability during the Medieval Climate Anomaly (MCA) between 1,200 and 850 y B.P., when temperatures were similar to recent decades. Between 1,130 and 1,030 y B.P., 83% (median estimate) of our sites burned when temperatures increased ∼0.5°C relative to the preceding centuries. Lake-based fire rotation during the MCA decreased to an estimated 120 y, representing a 260% higher rate of burning than during the period of dendroecological sampling (360 to −60 y B.P.). Increased burning, however, did not persist throughout the MCA. Burning declined abruptly before temperatures cooled, indicating possible fuel limitations to continued burning.wildfire | climate change | Medieval Climate Anomaly

Fire-regime variability impacts forest carbon dynamics for centuries to millennia

“…While the Chickaree Lake record clearly identified the most recent high-severity fire in the watershed (Dunnette et al, 2014), we cannot quantify accuracy over the past four millennia. However, the range of variability in individual fire return intervals reconstructed at Chickaree Lake (20-330 years) is consistent with the range of intervals recon-structed from other lake-sediment records in Colorado subalpine forests (Calder et al, 2015;75-885, 45-750, 30-645, 30-103575-885, 45-750, 30-645, 30- years, Higuera et al, 2014, suggesting that the C dynamics highlighted here are not unique to this single fire history reconstruction.…”

“…Ultimately, the implications of fire-regime variability for biogeochemical states will depend strongly on the synchrony of fire activity across spatial scales larger than a single watershed. If fire activity is synchronized on landscape to regional scales, as in the past (Calder et al, 2015;Marlon et al, 2012;Morgan et al, 2008) and as anticipated for the future (Westerling et al, 2011) in Rocky Mountain forests, we would expect to see similar centennial-to millennial-scale dynamics in biogeochemical states revealed here, which would have important implications for carbon cycling, including potential feedbacks to CO 2 -induced warming.…”

Human-caused climate change is now a key driver of forest fire activity in the western United States

“…1 A and B), eclipsing the previous high mark set just one decade prior (2). Scientists have known for some time that climate is a key driver of forest fires; records from the past and present (3)(4)(5) provide strong evidence that warmer temperatures are associated with spikes in fire activity. Therefore, recent increases in wildfire activity as the planet warms are not a surprise.…”

Taking time to consider the causes and consequences of large wildfires

“…Because wildfires have immediate and long-term impacts on social and ecological systems (3), these events motivate critical questions about the precedence, causes, and consequences of large wildfires, and ultimately what the future may hold under varying global-change scenarios. In PNAS, Calder et al (4) offer a unique perspective informing these questions in subalpine forests in northern Colorado, a region that has experienced extensive fires in the past two decades, as part of the trend of increased fire activity across the western United States (5). Calder et al's (4) insights come from 2,000 y of fire history, developed from sediment-charcoal records from 12 lakes spanning a 100,000 ha study area.…”

“…By extending the fire history record in their study region back 2000 y, Calder et al (4) effectively use the past as a natural experiment to study the response of subalpine forest fire regimes to climatic warming. They focus on the Medieval Climate Anomaly (MCA), a period that occurred about 1,000 y ago (750-1100 C.E.)…”