LOS ANGELES — Climate change is taking a visible toll on Yosemite National Park, where the largest ice mass in the park is in a death spiral, geologists say.

During an annual trek to the glacier deep in Yosemite’s backcountry last month, Greg Stock, the park’s first full-time geologist, found that Lyell Glacier had shrunk visibly since his visit last year, continuing a trend that began more than a century ago.

Lyell has dropped 62 percent of its mass and lost 120 vertical feet of ice over the last 100 years. “We give it 20 years or so of existence — then it’ll vanish, leaving behind rocky debris,” Stock said.

The Sierra Nevada Mountains have roughly 100 remaining glaciers, two of them in Yosemite. The shrinkage of glaciers is also occurring around the world. Great ice sheets are dwindling, prompting concerns about what happens next to surrounding ecological systems after perennial rivulets of melted ice disappear.

“We’ve looked at glaciers in California, Colorado, Wyoming, Washington and elsewhere, and they’re all thinning because of warming temperatures and less precipitation,” said Andrew Fountain, professor of geology and geography at Portland State University. “This is the beginning of the end of these things.”