Taking time to consider the causes and consequences of large wildfires.

Over the past several decades we have witnessed record-setting wildfires across nearly every continent (1), with recent months as no exception. Widespread burning across western North America includes some of the largest wildfires on record in Washington State, with 2015 total area burned on track to exceed observations of the past two decades (2). 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 combining these records over space and time, the authors develop a composite fire history record that reveals the timing and regional synchrony of past wildfire activity. Their results help elucidate the precedence of regionally extensive wildfires, the long-term dynamics that govern fire activity during climatic warming, and the potential implications of warmer conditions for fire regimes in the 21st century. Such paleoecological perspectives highlight that our understanding of wildfires—and the way we interact with and plan for them— is strongly shaped by the timescales we consider (6, 7) (Fig. 1). The nature of wildfire in most subalpine forests is that a single large event or a year with regionally extensive burning is inherently rare in any one location (8). Wildfires in these forests typically occur every one to several centuries at a given point on a landscape, making them literally a once-in-a-lifetime experience when witnessed by humans. The extreme fire behavior and extensive tree mortality associated with these events makes them genuinely dramatic. When combined with their immediate and often negative impacts on human health and livelihood, it’s easy to view any single wildfire as a disaster or harbinger of change. This perspective helps drive fire management practices that focus on fire suppression, at increasingly high costs [e.g., regularly exceeding $1.5 billion annually in the United States (2)]. However, as research following the 1988 fires in Yellowstone National Park has shown, when viewed over decades, centuries, and millennia, events that appear devastating in the moment are “business as usual” for many subalpine forest ecosystems (6, 9, 10). Calder et al. (4) provide a similar perspective for subalpine forests in the Colorado Rocky Mountains, where our understanding of fire history is largely based on tree-ring records of stand-replacing fires that extend back four centuries (e.g., ref. 11). The composite record presented by Calder et al. (4) is consistent with tree-ring records in their study region, both suggesting that over the past four centuries an individual forest stand burned approximately every 300 y on average. This correspondence helps validate the use of charcoal from lake sediments for developing long-term records of fire history, and it provides an important benchmark for assessing fire activity in the past and into the future. 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.) when mean annual temperatures were probably similar to today, but ∼0.5 °C warmer than previous or subsequent centuries (12). Before and after the Fig. 1. Conceptual model describing the controls of fire across spatial and temporal scales, adapted from Moritz et al. (8). Much of fire science focuses on understanding fire behavior, which is sensitive to weather, fuels, and topography. Fire regimes describe the characteristic patterns of wildfires over large spatial and temporal scales, and they are sensitive to changes in climate, vegetation, and ignitions. Calder et al. (4) use paleoecological records to study fire regimes in a large subalpine landscape over the past 2,000 y. By combining space and time, their work reveals how climate warming can promote regionally extensive wildfire activity, and it suggests important feedbacks among climate, vegetation, and fire that are relevant for anticipating fire-regime responses to 21st century climate change. Author contributions: P.E.H. wrote the paper.