ON THE LYELL GLACIER – 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 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 in bold font. 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 last month 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 a 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.

They trudged past sofa-sized slabs of granite perched atop thawing pedestals of ice and peered awestruck into the icy abyss and yawning death-trap of a moulin – a water-carved hole in the ice – and scrambled across a widening moat of super-sized stones along the glacier's receding lower edge.

"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 the Hilmar Cheese Co. near Turlock 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.

At one spot, he slipped and his walking stick – an old metal broom handle – went flying. A bit later, he lowered himself onto his hands and knees and willed himself up a ladder-steep wall of bedrock.

Call The Bee's Tom Knudson, (530) 582-5336. Terray Sylvester provided logistical support to The Bee for Hal Klieforth's hike.