At the same time, ice slabs at Mt. Shasta have grown. A warming trend is responsible for both developments, researchers say.
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.
When temperatures rise, the ice naturally melts away. This has resulted in the shrinkage of glaciers in the Sierra Nevada.
But higher winter temperatures can also increase snow and fuel glacial growth in some areas, such as Mt. Shasta, by allowing the air to hold more moisture.
"The climate of these two places is different," Tulaczyk said. Mt. Shasta, which he calls "a lonely mountain," sticks out and captures weather that is passing by. The Sierra Nevada, in contrast "makes its own weather."
As warm, moist air rises up Mt. Shasta, it is released as snow in something Tulaczyk calls "the snow-gun effect."
Such a phenomenon has been recorded on some of Norway's glaciers, which are growing as well, he said.
The warming trend is expected to continue, Tulaczyk said, to the point where it will overcome the increase in precipitation. When that happens, the Shasta glaciers could start to retreat.
The new data on California's glaciers come after many decades in which the region was ignored by glaciologists, who tend to focus on immense glaciers in the Himalayas, the Arctic and the antarctic, where ice sheets are large enough to break off chunks as large as small states.
When the topic of California glaciers comes up, "glaciologists tend to raise their eyebrows and say, 'Oh, those small things down there,' " Basagic said.
But as climate research has become a high priority, even puny glaciers have become important.
Glaciers big or small are sentinels of climate change. Monitoring them closely can reveal how fast the climate is changing and what role humans may be playing in that change.
The smaller the glacier is, the more valuable it may be in studies of recent climate change, Tulaczyk said. California's glaciers are small and thin enough that they can respond to climate changes within a decade.
Some of Alaska's hefty glaciers could take 1,000 years to show the effects of modern warming.
Basagic's glacier survey also aims to refine estimates of how many glaciers exist in California. Those estimates range from several dozen to nearly 500. The survey also could provide a modern baseline against which the erasing of the ice could be measured in the future.
While Basagic and Stephenson expected to see the glaciers of the Sierra in retreat, they were not prepared for how quickly the ice had disappeared. The rapid shrinking is obvious, even when today's glaciers are compared with photos taken in 1976, the last time Stephenson approached and photographed Goddard glacier.
"I thought, 'Oh, my gosh. In my lifetime I'm seeing this change,' " Stephenson said.
Other Sierra Nevada glaciers that the survey found were retreating include Junction Peak, Mendel, Maclure and Dana. As it has elsewhere across the globe, ice in California has waxed and waned in response to natural climatic shifts.
During the last ice age, 18,000 to 22,000 years ago, miles of ice as much as 1,500 feet thick filled and scoured the now ice-free Yosemite Valley.
That ice retreated after a few thousand years, but more flowed downward from the peaks again more than 13,000 years ago, 3,000 years ago and about 1,100 years ago, said Alan Gillespie, a geologist at the University of Washington who has studied the history of California's glaciers for nearly three decades.
Today's glaciers are the last remnants of the Little Ice Age period, when "glaciers were much more advanced and a few hundred feet thicker," Gillespie said.
In cooler times, even sunny Southern California had its share of glaciers. Permanent ice ran down the slopes of 11,502-foot Mt. San Gorgonio, Southern California's highest peak, as recently as 5,000 years ago, said Lewis Owen, a geologist at UC Riverside.
Geologists have long known the mountain once carried glaciers because of the rocky debris, or glacial moraine, left behind by the ice. But many thought glaciers had last occurred there about 70,000 years ago, during a cool period that caused a major expansion of global ice.
A study led by Owen attempted to date more precisely when rocks had last been moved by glaciers by measuring how long their exposed surfaces had been bombarded by cosmic rays.
It suggests that Mt. San Gorgonio's glaciers advanced and retreated several times during the past 20,000 years and existed as recently as 5,000 years ago. The study was published in August in the journal Geology.
The report of such recent glaciation so far south is confusing to experts like Gillespie, who believes Owen's results but said there is no evidence that the much colder Sierra Nevada saw increased glaciation during that same period.
"If the Sierras are warm and dry, how is it that just a couple of hundred miles south in L.A. of all places you have glaciers?" he asked.
Understanding the climatic variations between neighboring regions could explain why glaciers are behaving so differently now in the Sierra Nevada and nearby Mt. Shasta.
Geologists like Owen see the state's changing ice as "a natural part of the ebb and flow" of the world's glaciers. Southern California's glaciers could return one day, he said, as the Earth cycles back into another ice age in the next 10,000 years.
"It's inevitable we'll go back to a glacial time, unless we offset it with our own activities," he said. "At the moment, we are changing climate at a greater rate than at any time in human history."