Himalayan glaciers are melting and retreating at their edges because of global warming. But they also conceal a more ominous effect of climate change: they are deflating. They are losing internal ice mass to melting, which can substantially hasten their disappearance. Scientists have recently captured real-time video showing a glacier purging its own meltwater, and at rates far faster than the experts had imagined.

Ulyana Horodyskyj, a geologist at the University of Colorado at Boulder, climbed to 5,000 meters on the Ngozumpa Glacier in Nepal.

Scientists had surmised that such lakes slowly fill over time with water from surface melt, but Horodyskyj's videos show that the lakes can empty in just two days and then refill in less than a week, and repeat the cycle again and again, flushing out large volumes of melted ice through interior channels that eventually lead down into underlying rock or out to the glacier's edges. The regular purging indicates that far more ice is melting than previous thought. In one instance Horodyskyj scrambled down to a lake after it emptied to inspect the white ice walls that had been exposed below the high-water line. The lake would soon fill in.

The rise and fall of water in glacial lakes can also aggravate calving—the sudden collapse of ice walls alongside the water.

Deflation may cause glaciers to disappear faster than the melting that occurs at their edges. "South American and Himalayan glaciers are losing ice the most rapidly, but most of it is from vertical deflation," says Horodyskyj, who reported some of her findings at the recent American Geophysical Union meeting in San Francisco. Certain glaciers on the north side of Mount Everest, for example, have lost 100 meters in height over the past century, she says.