From the Christian Science Monitor (Pete Spotts):
A team of researchers has found preliminary evidence that when an increasing proportion of winter precipitation falls as rain, rather than snow, the amount of water flowing through drainage basins and into rivers in the US undergoes a long-term decline. The study, published Sunday in the journal Nature Climate Change, estimates that for a increase of 2.4 degrees Celsius (4.3 degrees F.), increases in the proportion of rain falling at the expense of snow could lead to a decline of 12 percent or more in stream flows…
If the observations hold up to additional scrutiny “and if they reflect some fundamental response to climate change, then the problems for water managers are going to be even more complicated that we already fear they are,” says Peter Gleick, who heads the Pacific Institute, a think tank based in Oakland, Calif., that focuses on sustainable use of water and other environmental issues.
For nearly 30 years, research has shown that as the climate warms, a rising percentage of precipitation will fall as rain, rather than snow, even if global warming had no effect on the overall amount of rain or snow falling in a region. In addition, spring comes earlier, reducing the time available for snow to accumulate.
During the past 50 years, these trends have taken hold throughout the Southwest, according to the Third National Climate Assessment, released in early May. Between 2001 and 2010 flows in major rivers such as the Colorado, Rio Grande, and the Sacramento-San Joaquin system have experienced reductions in flow ranging from 5 to 37 percent compared with the 20th century average. The US geological Survey has measured a 20 percent decline in snow cover in the Rocky Mountains since 1980, due a mix of global warming and natural variability.