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MOUNT SHASTA, Calif.–Global warming is shrinking glaciers all over the world, but the seven tongues of ice creeping down Mount Shasta's flanks are a rare exception: they are the only known glaciers in the continental U.S. that are growing.
Reaching more than 4,200 metres above sea level, Mount Shasta is one of the state's tallest peaks, dominating the landscape of high plains and conifer forests in far northern California.
Nearby Indian tribes referred to its glaciers as the footsteps made by the creator when he descended to Earth. Hikers flock to Shasta's peak every summer to scale them.
With glaciers retreating in the Sierra Nevada, the Rocky Mountains, and elsewhere in the Cascades, Mount Shasta – a volcano in the southern end of the Cascade range – is actually benefiting from changing weather patterns over the Pacific Ocean.
"When people look at glaciers around the world, the majority of them are shrinking," said Slawek Tulaczyk, an assistant professor of earth sciences at the University of California, Santa Cruz, who led a team studying Shasta's glaciers.
"These glaciers seem to be benefiting from the warming ocean."
Climate change has cut the number of glaciers at Montana's Glacier National Park from 150 to 26 since 1850, and some scientists project there will be none left within a generation. Lonnie Thompson, a glacier expert at Ohio State University, has projected the storied snows at Africa's Mount Kilimanjaro might disappear by 2015.
But for Shasta, about 435 kilometres north of San Francisco, scientists say a warming Pacific Ocean means more moist air. On the mountain, precipitation falls as snow, adding to the glaciers enough to overcome a temperature rise of 1C in the last century, scientists say.
"It's a bit of an anomaly that they are growing, but it's not to be unexpected," said Ed Josberger, a glaciologist at the U.S. Geological Survey in Tacoma, Wash.
By comparison, the glaciers in the Sierra Nevada, more than 800 kilometres south of Mount Shasta, are exposed to warmer summer temperatures and are retreating.
The Sierra's 498 ice formations – glaciers and ice fields – have shrunk by about half their size over the past 100 years, said Andrew Fountain, a geology professor at Portland State University. He said Shasta's seven glaciers are the only ones scientists have identified as getting larger.
Glaciologists say most glaciers in Alaska and Canada are retreating, too, but there are too many to study.
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.
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 UN 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.
Even without global warming, another threat to Shasta's glaciers could come more quickly: a volcanic eruption. 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.
