Snow is melting weeks earlier in the American West

Scientists can measure the timing of snowmelt by tracking daily water flow levels in rivers and streams fed mainly by snow and not rain. Indeed, a larger fraction of water is flowing earlier in the year than used to be the case for these waterways.[[Dettinger, M.D., and D.R. Cayan, “Large-Scale Atmospheric Forcing of Recent Trends toward Early Snowmelt Runoff in California.” (Abstract) J. Climate, 8 (March 1995), 606–623.]] [[Stewart, Iris, Daniel Cayan, and Michael Dettinger. “Changes toward Earlier Streamflow Timing across Western North America.” (PDF) Journal of Climate 18 (April 15, 2005): 1136-1155.]] Yet, the same is not true of rivers and streams fed mainly by rain. This pattern implies that changes in snowmelt are responsible for the difference. Between 1948 and 2000, the change in snowmelt timing has been from 1 to 3 weeks on most snow-fed western rivers.[[Stewart, I. T., D. R. Cayan, and M. D. Dettinger. “Changes in snowmelt runoff timing in western North America under a ‘business as usual' climate change scenario.” (PDF) Climatic Change 62, no. 1 (2004): 217-232.]] 

These observed changes in the timing of river flows have been shown to be too rapid to be due entirely to natural climate cycles; hence human-caused warming, acting on the American West, appears at least partly implicated.

Snowmelt timing can also be measured by looking at the amount of snow remaining on the ground on April 1, the traditional date for measuring accumulated winter snowpack. There is now less snow on the ground on that date, on average, than there was a half-century or so ago in most regions of the west. These reductions are more clearly associated with temperature increases than with reductions in precipitation.[[Mote, P.W., Climate-Driven Variability and Trends in Mountain Snowpack in Western North America.(PDF) J. Climate, 19 (2006), 6209–6220.]] [[Hamlet, Alan F., Mote, Philip W., Clark, Martyk P., and Lettenmaier, Dennis P. “Effects of Temperature and Precipitation Variability on Snowpack Trends in the Western United States” (Abstract) Journal of Climate 18, no. 2 (November 2005): 4545-4561.]] Logically, the cause must be more precipitation falling as rain, or earlier snowmelt — and in fact, both factors are involved.

Two important consequences of earlier snowmelt are drier forests and more wildfires. Trout habitat is also threatened. In addition, many water systems depend on melting snow to keep reservoirs full in late spring and summer; so earlier snowmelt leads to increased water scarcity.