The largest ice mass in Yosemite, a glacier that is the California park's key source of water, is melting fast and could be gone in 20 years, scientists say.

Lyell Glacier has shrunk by more than half in the past century and hasn't moved in years, researchers in the park said.

"We give it 20 years or so of existence -- then it'll vanish, leaving behind rocky debris," Greg Stock, the park's first full-time geologist, told the Los Angeles Times.

Climate change has caused the glacier to drop 62 percent of its mass and lose 120 vertical feet of ice in the last 100 years, he said.

Yosemite's other glacier, Maclure, is shrinking but is still moving at a rate of about an inch a day.

Lyell, the second-largest glacier in the Sierra Nevada and the headwater of the Tuolumne River watershed, no longer fits the definition of a glacier because it has ceased moving, Stock said.

"Lyell Glacier is stagnant -- a clear sign it's dying," he said. "Our research indicates it stopped moving about a decade ago.

"Glaciers tend to flow like honey down a plate, or slide over meltwater beneath them," Stock said. "We suspect Lyell just isn't thick enough anymore to drive a downhill motion."

The rate of its retreat has accelerated in the last decade, he said.

"Eventually, there'll be nothing left."