Sierra Snowpack Could Shrink
Faster Than Previously Thought

MIKE TAUGHER / Contra Costa Times 16jul2008

The same phenomenon that is causing Arctic sea ice to melt at an accelerating rate also poses a bigger threat to California's snowpack than previously thought, a new study suggests.

Spring snowmelt could begin up to two months earlier in parts of the western United States by the end of the century, the study predicts.

That could further dry the West and increase the number of forest fires.

It also poses a serious threat to California's water supply because the Sierra snowpack is, effectively, the state's largest surface water reservoir, releasing water over a period of months rather than all at once.

Researchers say that as the snowpack retreats in a warmer environment, sunlight that would otherwise reflect off the white, snowy surface instead will be absorbed into the ground, which in turn causes more snow to melt. The same effect is contributing to sea ice melting in the Arctic, leading to more energy being absorbed by the ocean.

In other words, the warming caused by heat-trapping greenhouse gases is simply a trigger for a more problematic feedback loop in the Arctic and in the West.

"It's a very similar mechanism," said Noah Diffenbaugh, the paper's senior author and a professor of Earth and atmospheric sciences at Purdue University. "We're seeing a response in terms of the snowmelt season that is about two times as big as we expected up to now."

The paper is to be published in Geophysical Research Letters.

Scientists have known for some time that the loss of reflective snow and ice would warm the ground and increase snowmelt, and other models have predicted spring runoff a month earlier than historical averages. But this latest research points to a doubling of that effect.

"It sends a message that what we see from the (global climate models) may be underestimating this kind of impact," said climatologist Gregg Garfin.

Diffenbaugh and others arrived at their conclusions by using detailed topographical information to help the computers more accurately predict when and where snow would melt. Some of the previous global models, for example, incorrectly had Santa Cruz and Lake Tahoe at the same elevation, Diffenbaugh said.

Diffenbaugh said the model's results matched up accurately with recent history in the West, especially in the Sierra and some of the West's other mountain ranges. Garfin, a University of Arizona climatologist, said that because the study only predicts what will happen by the end of the century, the model could lead to an exaggerated result. Still, it confirms previous studies that predict earlier snowmelt.

"It looks like the better the models get, it turns out the more conservative we were. That's bad," said Peter Gleick, executive director of the Pacific Institute, an Oakland research group, and an expert on climate change and water.

source: 17jul2008