He said Shasta's seven glaciers are the only ones scientists have identified as getting larger, with the exception of a small glacier in the shaded crater of Washington state's Mount St. Helens. It formed after the 1980 eruption blasted away slightly more than half the mountain's ice, and scientists believe it will not grow in area once it stretches outside the shade of the crater.

Glaciologists say most glaciers in Alaska and Canada are retreating, too, but there are too many to study them all.

Although Mount Shasta's glaciers are growing, researchers say the 4.7 billion cubic feet of ice on its flanks could be gone by 2100. For the glaciers to remain their current size, Shasta would have to receive 20 percent more snowfall for every 1.8-degree Fahrenheit increase in temperature, Tulaczyk said.

The Shasta glaciers have been advancing since the end of a drought in the early 20th century. The mountain's smallest glaciers — named Konwakiton, Watkins and Mud Creek — have more than doubled in length since 1950.

Hikers seeking to cross Shasta's glaciers — marked with crevasses as deep as 100 feet — say they are much larger than the boundaries drawn on geological maps.

"I noticed I was traveling down farther than the maps were showing it," said Eric White, a U.S. Forest Service ranger who has climbed Shasta for 23 years.

Four glaciers at Washington's Mount Rainier are staying about the same size. Those glaciers — shielded from the sun on the mountain's north and east sides — have received just enough snow to keep them from shrinking.

The added ice on Mount Shasta might be good for the state's water supplies. Hydrologists believe the glaciers feed springs and aquifers, though they say it's unclear precisely how the water travels underground.

Until recently, the same phenomenon that is benefiting Shasta's glaciers was feeding glacier growth in southern Norway and Sweden, the New Zealand Alps and northern Pakistan, according to the U.N. Intergovernmental Panel on Climate Change.

In each area, scientists say, more snowfall temporarily offset warming temperatures in the 1990s and early 2000s. But rising temperatures since then have begun to shrink the ice.

Climate change is causing roughly 90 percent of the world's mountain glaciers to shrink, said Thompson, the Ohio State glacier expert.

"Best that we keep our eye on the big picture," Thompson said in an e-mail about Shasta's unique position. "The picture points unfortunately (to) massive loss of ice on land, which has huge implications for future sea level rise."

Global forecasts show temperatures warming from 2 degrees to 11.5 degrees Fahrenheit by the end of the century if no major efforts are undertaken to reduce greenhouse gas emissions. At that rate, California's snowpack and its remaining glaciers are among the most vulnerable of its natural resources.

Even without global warming, another threat to Shasta's glaciers could come far more quickly. A volcanic eruption could melt them, creating mud flows that could bury the surrounding small communities.

Over the last 4,000 years, Shasta has erupted about every 250 to 300 years, and did so most recently about 200 years ago, said William Hirt, a geology instructor at the College of the Siskiyous.

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On the Net:
Shasta-Trinity National Recreation Area: www.fs.fed.us/r5/shastatrinity/recreation/nra/index.shtml