Recent Declines in Northern Rocky Mountain Snow and Runoff
Pederson, G.T., Gray, S.T., Woodhouse, C.A., Betancourt, J.L., Fagre, D.B., Littell, J.S., Watson, E., Luckman, B.H. and Graumlich, L.J. 2011. The unusual nature of recent snowpack declines in the North American Cordillera. Science 333: 332-335.
To address this important question, Pederson et al. "developed annually resolved, multi-century to millennial length snowpack reconstructions for the headwaters of the Columbia, Missouri and Colorado Rivers," which collectively "serve as the primary water source for more than 70 million people," supplying "60 to 80% of their water needs." This they did "by using existing tree-ring collections from areas where precipitation is dominated by snowfall and by sampling trees known to be sensitive to snowpack," and, finally, by comparing their results, in the case of the Upper Colorado River, against existing long-term records of streamflow variability (Woodhouse et al., 2006; Meko et al., 2007), since "periods of high snowpack generally coincide with high flows and vice versa," and since the ultimate parameter of direct importance to plants, animals and people is, after all, riverflow.
The nine researchers report that "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." Almost unprecedented, however, is not the same as truly unprecedented. And the same holds for river flow. They note, for example, that "warmer temperatures and severe decadal-scale snowpack reductions combined to produce an extreme low-flow interval during the period of the Medieval Climate Anomaly (~1143 to 1155 CE," which according to graphs contained in their Supporting Online Material was embedded within a low-flow period of ~50 years duration, which far exceeded the durations of the next three longest low-flow intervals, which all fell within the ~25-30-year category.
The results of this study do not suggest that snowpack losses of the late 20th century are unprecedented over the past millennium or more, nor do they imply that they are caused by "unparalleled springtime warming that is due to positive reinforcement of the anthropogenic warming by decadal variability," as suggested by the authors of this work, since the truly unprecedented period of low streamflow during the Medieval Warm Period could not have had a significant anthropogenic or CO2-induced component associated with it, indicating that nature -- on its own -- is fully capable of having produced the late 20th-century snowpack and riverflow declines.
Meko, D.M., Woodhouse, C.A., Baisan, C.A., Knight, T., Lukas, J.J., Hughes, M.K. and Salzer, M.W. 2007. Medieval Drought in the Upper Colorado River Basin. Geophysical Research Letters 34: 10.1029/2007GL029988.
Woodhouse, C.A., Gray, S.T. and Meko, D.M. 2006. Updated streamflow reconstructions for the Upper Colorado River basin. Water Resources Research 42: 10.1029/2005WR004455.