A new survey of glaciers in the Sierra Nevada shows the thick slabs of ice that have frosted many of the state's high peaks for the last thousand years are dramatically shrinking and, in some cases, disappearing altogether.
Darwin Glacier near Bishop is an estimated 50 to 100 feet thinner today than it was in historical photos from the early 1900s. The Lyell Glacier off the popular John Muir Trail in Yosemite National Park is retreating to the peaks above Tuolumne Meadows from which it springs.
Seven Sierra Nevada glaciers that were surveyed and rephotographed over the summer are all smaller than they were a century ago, said Hassan Basagic, a graduate student at Portland State University who initiated the survey. The mountain range is home to most of the state's glaciers.
"There's been lots of melt," said Nathan L. Stephenson, a research ecologist with the U.S. Geological Survey based at Sequoia National Park. Stephenson led a glacier survey of the Evolution Range in Kings Canyon National Park in August.
At the same time, a few glaciers to the north, atop Mt. Shasta, are growing -- an unexpected development, given that the majority of the world's glaciers are in retreat.
All seven of Mt. Shasta's glaciers, including three-mile long Whitney, the state's largest, have grown in recent decades. Three of the mountain's glaciers have doubled in size since 1950, said Slawek Tulaczyk, a glaciologist at UC Santa Cruz who began the Mt. Shasta Glacial Survey in 2002.
"We totally expected them to have shrunk, and they've grown dramatically," he said.
The changes in California's ice -- both its growth and retreat -- are a product of vast climatic cycles that have caused ice to ebb and flow across the Earth's surface for millions of years.
Most ice worldwide has retreated in the past 100 years as the planet has recovered, through natural processes, from a bout of cooling called the "Little Ice Age," which ended around 1850.
But many experts also suspect that the transformation of the glaciers may be accelerating as the planet warms in response to the human production of greenhouse gases, such as carbon dioxide.
"I would never point the finger and say this is all human-induced warming," Stephenson said. "But maybe we are speeding it up now."
Glaciers are a product of both temperature and precipitation. To grow, they require snowfall and temperatures low enough that winter snow does not melt during the summer but accumulates each year and eventually compresses itself into a slowly moving slab of ice.
