A Millennium of Snowpack Variability and Change in the North American Cordillera

In the mountains of western North America, snowpack controls the amount and timing of runoff, while also influencing myriad ecosystem processes.  Within the Columbia, Missouri, and Colorado River drainages, 60-80% of streamflow originates as snowpack, and the timing and amount of snowmelt driven runoff is critical for western fisheries.  In much of this region, observational records show diminished snowpack since the 1950s, with further declines projected for the 21^st century.  However, questions remain as to whether observed declines might result from natural variability, as well as to how the magnitude and spatial extent of recent events fits into the context of the Medieval Climate Anomaly, Little Ice Age and other key climatic periods of the past 1,000+ years.  In the first study of its kind, we use tree rings to produce millennial-length reconstructions of snowpack at multiple spatial scales for key runoff generating areas in the North American Cordillera.  Results confirm that snowpack has declined significantly across the Northern Rockies during the 20^th century, and over the entire cordillera since the 1980s.  Such coherent and persistent snowpack declines are rare in the reconstructions; before the 1950s the region exhibits substantial inter-basin variability with northern areas tending toward wetness when the south was dry, and vice-versa (i.e. the north-south moisture dipole).  Cordillera-wide periods of low snowpack shown for the 1350s, 1400s, and post-1980s era correspond with times of anomalous warmth at regional and hemispheric scales.  This implies Pacific Basin forcing of winter precipitation, and the resulting north-south dipole, have been defining features of snowpack variability for at least the last millennium, but temperature also has the potential to synchronize snowpack anomalies across the entire cordillera.  When combined with the high likelihood of future warming, these results may herald a fundamental shift in regional snowpack and seasonal runoff dynamics, with potentially large impacts on fisheries and aquatic ecosystems.