Yosemite's Lyell Glacier may be receding YOSEMITE

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Yosemite's famed Lyell Glacier has stopped moving downhill and may actually be shrinking - another probable sign that the world's climate is warming, scientists report.

"It appears to have stagnated, and we strongly suspect that it has thinned to less than half the size that would keep it moving," said Greg Stock, Yosemite National Park's geologist who has been measuring the Lyell and nearby Maclure Glacier for the past four years with Robert Anderson of the University of Colorado.

The Lyell is small by world standards - only about a quarter of a mile wide and less than that long - but it stands atop the headwaters of the Tuolumne River, which feeds San Francisco's Hetch Hetchy reservoir.

The glacier is the largest of 14 in the High Sierra that have shrunk by more than half during the past century, according to a recent survey by geologists at Portland State University in Oregon.

"The most logical reason for the shrinking is because of more loss from melting snow as the climate warms," Stock said.

At Yosemite, Stock has monitored the 12,000-foot-elevation Lyell glacier with stakes driven into the ice at its margin and made the most recent measurements during four climbing trips there last summer. It has shrunk by about 60 percent since 1900 and has thinned by about 120 vertical feet, which is probably the reason it has stopped moving, he said.

The two scientists also measured the Maclure Glacier, at 11,400 feet, and found that although it has lost a similar amount of ice as the Lyell, it is still moving downhill at about an inch a day. Although it has thinned considerably, the scientists said, it is still thick enough to move and flow because of increased amounts of meltwater.

The Lyell Glacier's continued shrinking mirrors the fate of hundreds of glaciers worldwide that are also losing ice. Bruce Molina, a glaciologist with the U.S. Geological Survey, recently reported he has photographed 2,000 of the world's glaciers during the past 35 years and has found that at least 1,975 are clearly receding.

Although the Lyell glacier is at the top of the 150-mile Tuolumne River, any loss of ice would have no effect on the supply of water to Hetch Hetchy, Stock said. Far more water flows into the river from all the streams that make up the river's huge watershed, which covers almost 2,000 square miles, he noted.