The Earth's climate warmed steadily during the 20th century, and mean annual air temperatures are estimated to have increased by 0.6 C (Intergovernmental Panel on Climate Change, 2007). Although many cycles of warming and cooling have occurred in the past, the most recent warming period is unique in its rate and magnitude of change (Siegenthaler and others, 2005) and in its association with anthropogenic emissions of greenhouse gases (Intergovernmental Panel on Climate Change, 2007). The climate in the western United States warmed in concert with the global trend but at an accelerated rate (+0.8 C during the 20th century; Saunders and others, 2008). The region could also prove especially sensitive to future changes because the relatively small human population is growing rapidly, as are demands on limited water supplies."