The Unusual Nature of Recent Snowpack Declines in the North American

Citation: Pederson, G. T., S. T. Gray, C. A. Woodhouse, J. L. Betancourt, D. B. Fagre, J. S. Littell, E. Watson, B. H. Luckman, and L. J. Graumlich, 2011. The Unusual Nature of Recent Snowpack Declines in the North American Cordillera. Science, 9 June 2011: 1201570Published online 9 June 2011 [DOI:10.1126/science.1201570]
l Abstract l Reprint l

 Study map. Snow shapes the ecosystems and economies of the semi-arid landscapes of western North America by generating the streamflow that allows agriculture and cities to flourish. The patterns of snowfall across space and through time impact aquatic and terrestrial ecosystems by governing processes such as: 1) the onset of spring, 2) the amount, timing, and temperature of streamflow runoff, 3) probability forest fires, 4) size and distribution of glaciers, and 5) the elevation and position of alpine treeline and meadows. We're understandably concerned when scientists report that snowpack in the West has been steadily declining since the 1950s and obvious questions that come to mind include: 1) Is this normal and part of some long-term pattern of waxing and waning conditions?, or 2) Does this reflect recent anomalous trends towards declining snow the West? To answer these questions, we need records that encompass multiple centuries thereby allowing us to assess trends and cycles in snowpack.

Our project seeks to address this gap using tree-rings to reconstruct snowpack over several centuries for three key high-mountain watersheds: the Upper Colorado River basin, the Upper Missouri River basin, and the headwaters of the Columbia River. Snowpack reconstructions were produced at multiple watershed scales (Click here for map), and the resulting high-resolution maps of past snowpack provide insights into natural controls on snowpack variability linked to changes in the Pacific Ocean basin (e.g., El Niño, Pacific Decadal Oscillation). Additionally, both data and mapping tools have been developed as planning and research resources for water managers and research scientists. This work is particularly relevant because the three target regions form the headwaters for the West's three most important watersheds.

In order to calibrate tree-ring data to snowpack data, we have assembled large spatial databases using recently collected and existing tree-ring chronologies and records of April 1st snow water equivalent (SWE) in the headwaters of our three target watersheds. The final reconstructions have been completed, and results indicate that the tree-ring based reconstructions skillfully capture decade-scale and longer-term variability in snowpack at multiple watershed scales.


In western North America snowpack has declined in recent decades, and further losses are projected through the 21st century. Here we evaluate the uniqueness of recent declines using snowpack reconstructions from 66 tree-ring chronologies in key runoff generating areas of the Colorado, Columbia and Missouri River drainages. Over the past millennium, late-20th century snowpack reductions are almost unprecedented in magnitude across the northern Rocky mountains, and in their north-south synchrony across the cordillera. Both the snowpack declines and their synchrony result from unparalleled springtime warming due to positive reinforcement of the anthropogenic warming by decadal variability. The increasing role of warming on large-scale snowpack variability and trends foreshadows fundamental impacts on streamflow and water supplies across the western USA.

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Project co-authors and key data contributors:
Greg Pederson, U.S. Geological Survey | NOROCK and University of Arizona
Stephen Gray, University of Wyoming
Connie Woodhouse, Geography Department and Laboratory of Tree Ring Research, University of Arizona
Lisa Graumlich, School of Natural Resources, University of Arizona
Daniel Fagre, U.S. Geological Survey, Glacier Field Station
Julio Betancourt, U.S. Geological Survey, National Research Program
Jeremy Littell, Climate Impacts Group, University of Washington
Brian Luckman, University of Western Ontario
Emma Watson, Environment Canada
Dave Meko, University of Arizona, Laboratory of Tree-Ring Research
Troy Knight, St. John’s University, Minnesota

Funding Support from:
National Science Foundation (NSF) | Geography and Regional Science – Grant# 0620793
National Science Foundation (NSF) | Ecology – Grant# 0734277
U.S. Geological Survey (USGS) | Western Mountain Initiative