YOSEMITE -- As melting water gushed off the ice in a tinseled maze of rivulets and tumbled through a gaping chasm, the hikers watched, wondered and worried.
Unlike most backcountry travelers who pitch their tents along the John Muir Trail in the upper reaches of the Lyell Fork of the Tuolumne River, these visitors had not pushed on to scale the summit of Mount Lyell -- Yosemite's highest peak.
Instead, they scrambled up a ridge of rose-tinted granite and over a mound of dark, unstable boulders to tromp across this less well-known corner of the national park, a silvery-white sheet of ice fast becoming one of the first California landmarks to succumb to climate change.
Later in the September day, Pete Devine, a veteran glacier observer who manages educational programs for the nonprofit Yosemite Association, sat on a log and opened a notebook. "Gaunt remnant of what I saw 10, 20 years ago," he wrote in his journal. "Lots of large boulders dot the surface. Lots of melt- water flow."
As signals of climate change begin to come into focus in the Sierra Nevada, its melting glaciers spell trouble. Not only are they in-your-face barometers of global warming, they also reflect what scientists are beginning to uncover: that the Sierra snowpack -- the source of 65 percent of California's water -- is dwindling, too.
More of the Sierra's precipitation is falling as rain instead of snow, studies show, and the snow that blankets the range in winter is running off earlier in the spring. And snow in the Sierra touches everything. Take it away and droughts deepen, ski areas go bust and fire seasons rage longer.
Some glaciers already have melted away, including the first Sierra glacier discovered in Yosemite by John Muir in 1871. Today, the remaining 100 or so are withering, including Lyell, the second-largest, which could be gone inside a century.
"All across the Sierra, glaciers are transitioning into ice patches. Ice patches are transitioning to snow fields. And snow fields are transitioning into bedrock," said Greg Stock, a geologist with Yosemite National Park who joined Devine this fall on an annual survey of the Lyell glacier.
While this is not the first time glaciers have receded across the Sierra Nevada -- they last did so about 20,000 years ago -- this meltdown is more ominous, Stock said, because scientists increasingly believe it is sparked not by natural forces but by rising carbon dioxide levels from the burning of fossil fuels.
"We have entered new terrain with what's going on in the atmosphere," he said. "We haven't seen anything like this in tens of millions of years."
Witnesses to warming planet
Stock and Devine were accompanied by four tourists who came to see the signature of climate change firsthand, to pay their respects to a Sierra sentinel before it slips away. The trip, organized through the Yosemite Association, cost $360 -- and the melting glacier put on quite a show.
Amid the whoosh of rushing water and the wind's freight train roar, they looked up at boulders riding on the surface of the ice that not long ago were entombed in the glacier.
"You don't have to be Al Gore to realize something is changing and we need to do something," said Jerel Steckling, a safety officer at Hilmar Cheese Co., who was on the trip. "This is a bigger deal than most of us want to believe."
The four tourists were not the only ones to make the pilgrimage. A few days later, 81-year-old Hal Klieforth inched himself up a pale pink ridge below the glacier on a return visit to an ice sheet he last set foot on 58 years ago.
On Aug. 20, 1950, Klieforth crossed the glacier on a youthful scramble to the summit of Mount Lyell with four friends. Now, at The Bee's urging, he had come back to see how global warming had altered it.
As he crested a towering wall of rocks at the foot of the glacier and gazed across it, a boyish smile spread across his face. "Oh, my goodness, this takes me back," he said.
His joy was tempered by reality, for the ice sheet was a shrunken remnant of the one he'd known nearly six decades ago. Gone was the massive, furrowed brow of ice and snow that curved around a ridge of rock, linking the glacier into a mile-wide seamless wall of white. Gone were the scalloped waves of thigh-deep snow that covered much of the glacier in 1950 and made hiking difficult.
Critics of global warming say glaciers have been retreating for more than a century and climate change is caused by natural forces, such as increased sun spot activity. But Stock cites scientific studies that point to a new culprit: rising carbon dioxide levels.
Much of that knowledge comes from drilling into Antarctic and Greenland glaciers and extracting ice formed hundreds of thousands of years ago. Bubbles trapped inside can be analyzed for carbon dioxide.
"As far back as we can see in time, we don't see anything like what's happening today," Stock said.
As he tromped around the west lobe of the Lyell glacier, Stock saw signs of warming everywhere. One of the most vivid examples, he pointed out, was the daunting perch at the west end of a "transect line" -- a fixed route anchored by two stationary points on bedrock on each side of the glacier -- that geologists walk each year to chart changes.
"In the 1930s, you could step right off the bedrock and onto the glacier," Stock said. "Now we have to step off a rock shelf, climb down a series of cliffs for 100 feet. It's not often you see that much change in nature."
In his daypack, Devine carried historical photos of Lyell glacier, including one taken just four years ago. Even in that short span, the ice sheet has changed. No longer is its icy surface so marshmallow white. Today, parts of it are speckled with rocks. Boulders the size of cars, kayaks and kitchen tables litter the surface of the west lobe -- and most are melting out of the shrinking ice sheet, not tumbling down from the cliffs above.
The melting is so swift that even U.S. Geological Survey maps no longer reflect reality.
"All this is gone," Devine said as he kneeled down to look at a 1985 topographical map that one member of the tour group -- Chicago artist Bonnie Peterson -- had pulled from her pack.
The highly detailed Mount Lyell Quadrangle map showed the glacier in the right place -- tucked into a gigantic spoon-shaped cirque on the north-facing side of the mountain. But the ice sheet was too large. "Just eyeballing this, a third of it" has vanished, Devine said.
"Maybe 40 percent," Peterson said.
The most dramatic spectacle was not on the map or in the historic photos, though. It was the sight and sound of the water gushing off the glacier. "I don't think I've ever seen that much meltwater before," Stock said.
All glaciers melt naturally. But to remain stable or grow, they must replace meltwater with snowfall. And that's not happening at Lyell or elsewhere. Today, scientists are finding that not only is more precipitation across the region arriving as rain, but less snow overall is falling. The current drought -- now heading into a possible third year -- could be catastrophic for California, state officials say.
"With a healthy glacier, we expect to see the upper half covered in snow and the lower half bare ice," Stock said in base camp one morning. "Instead, virtually all of the glacier is bare ice. It's losing a lot more than it's gaining. It's definitely deteriorating."
That lack of snowfall worries many observers the most. "The most valuable resource in the Sierra Nevada is not the timber or gold or recreation or second homes -- it's the snowpack," Devine said.
"The Central Valley of California wouldn't be the richest valley in the world without the snowpack that's above it."
Carrying experience back
Few people are better acquainted with the Sierra snowpack than Klieforth, the meteorologist from Bishop, who has spent much of his life studying the range's climate, vegetation and weather patterns.
Willow-thin and wiry, Klieforth had begun this journey at the Tuolumne Meadows trailhead 13 miles down the canyon with an old World War II Army surplus sleeping bag stuffed inside a plastic grocery bag. His pack included plastic dinnerware he has carried on camping trips for half a century.
On this journey to the Lyell glacier, Klieforth logged another 30 miles and spent a part of each day looking at grainy, black-and-white photographs he had taken of the glacier back in 1950 -- pictures of a much bigger ice sheet than the one he was seeing today.
No global warming skeptic, Klieforth is nonetheless a cautious man, careful about jumping to conclusions. He was moved by his trip to the glacier, calling it a watershed event.
"Climate change is sneaking up on us," he said. "And the rate of the sneaking is going from a slow walk to a sprint."
He wanted to share the visit with some special friends: his old climbing buddies -- now in their 80s -- who had joined him on the original 1950 trip.
"I'm going to tell them that they can believe in global warming," Klieforth said. They can "take it to the bank."