If John Muir were here today," says Greg Stock, Yosemite National Park's head geologist, "he'd be walking on a surface of ice hundreds of feet over our heads." Stock is standing along the frozen shoulder of Mount Lyell, Yosemite's highest peak and home to its largest – but vanishing – glacier. From here, about 13,000 feet up, the dimensions of the ice's retreat are clear. Two survey points are visible thanks to bright orange letters, k and l, painted on opposite ends of the rugged basin. These places, first marked in the 1930s, once identified the high point of the surface of the ice. Today, almost 120 vertical feet of bare rock stand between k and l and the glacier.
Stock, 40, initially spent much of his seven-year tenure at Yosemite evaluating the risks posed by falling rocks to the park's buildings and visitors. Over the past four years, however, he and his colleague, glaciologist Bob Anderson, have begun using GPS and laser range finders to assess the size, shape, and movement of the Lyell and adjacent Maclure glaciers. What they've found is disheartening: Both have shrunk by more than 60 percent over the past century. Worse yet, the Lyell, having shed too much ice and snow to be drawn downhill, has stopped moving entirely. What this means is the largest glacier in Yosemite National Park is now a large, dead chunk of ice – and it's melting fast.
When the rest of Yosemite's glaciers disappear completely – and Stock thinks they will within a matter of decades – so will the glacier-fed waterways that support countless plants and animals and feed reservoirs like the Hetch Hetchy, the source of water for 2.4 million people in northern California. For now, Stock is recording the loss of the ancient ice structures throughout the park. "Glaciers around the world are retreating so quickly," he says. "At least with the Lyell and Maclure, we're getting a chance to study them before they're gone."
On a 40-degree September day in Yosemite National Park, there's a biting wind traveling up the steep, granite spine leading to the glacier where Stock works with six volunteers. Using a six-foot ice auger, he drills into the cadaverous, blue-gray ice near the peak of Lyell with an undeniable sense of urgency. Into the holes, he secures PVC pipes, which he then zeroes in on with one of his laser range finders. By surveying these pipes over successive months and years, he can determine, within a few centimeters, the rate of the glacier's movement as it advances or, in the case of Lyell, comes to a halt.
This decline is a global problem: Glaciers, which store about 75 percent of the world's freshwater, in the Sierras, Rockies, Alps, and the Himalayas all are experiencing similar trends. The Andes have suffered some of the most extreme melt in the world – much of the mountain range could be glacier-free by mid-century – and the impact is already evident. The 1.8 million people of Quito, Ecuador, draw a significant portion of their water from glaciers. La Paz, Bolivia (population: 1.6 million), gets 30 percent of its water from melt.
Northern California still has water, for now, but it's losing an important part of its landscape and a central piece of its national park. "You can now rappel into crevasses and climb into ice caves in Yosemite," says Stock. "That's an experience my eight-year-old daughter may not be able to have in her lifetime, because the glaciers may be gone."