This story is part of a special National Geographic News series on global water issues.
Mountain glaciers long have been known to be in retreat as the planet warms. But the process is occurring even more rapidly than previously believed, scientists said earlier this month in San Francisco at a meeting of the American Geophysical Union.
For example, said Garry Clarke, professor emeritus of glaciology at the University of British Columbia in Vancouver, Canada, the massive glaciers of Canada's Saint Elias region, now comprised of nearly 98 cubic miles of ice (453 cubic kilometers), are likely to be cut in half by 2100, even under middle-of-the-road climate-change scenarios.
"[And] that's the good news," Clarke said.
In parts of the Canadian Rockies, he said, today's glaciers will all but disappear completely, while others will shrink to remnants just 5 to 20 percent of their current size.
"We think that we will be witness over the next century mainly to the disappearance of the glaciers of western North America," he said.
Other disturbing finds are coming from the Himalayas, where Ulyana Horodyskyj, a graduate student at the University of Colorado's Cooperative Institute for Research in Environmental Sciences, has been monitoring supraglacial lakes—ponds of water that appear on the surface of melting glaciers.
"Most people think about glaciers receding," she said, "but they also shrink vertically. These lakes can lead to enhanced melting, and we see a lot of them forming throughout the Himalayas . . . You can think of these lakes as cancers that are consuming the glacier."
Even if all of the world's mountain glaciers were to melt, the effect on sea level rise would be small: Clarke, estimates, for example, that even if all of western Canada's glaciers were to completely melt away, the oceans would rise by only 6.6 millimeters (a quarter-inch), "not enough to worry anyone."
But glaciers act as natural reservoirs, storing water in the winter and doling it out in the summer as the ice slowly melts.
"If most of it disappears, there will be extreme consequences for most of these regions," Clarke said. "The stream flow will change, the timing of peak stream flow will change, and the temperature of streams will change."
Even the total volume of runoff will change, added Michel Baraer, of McGill University, Montreal, Canada, because glacial ice keeps the water locked away in a form in which it doesn’t easily evaporate.
Thus, even if precipitation remains the same in the high mountains, more of the water will be in liquid form, which evaporates more quickly.
Building dams also will not solve the problem of decreasing runoff. "Evaporation from reservoirs is much higher than sublimation [conversion of solid into gas] from glaciers," Baraer said. "Dams will never, ever, replace the [natural] hydrological systems that are in place today."
Already, Baraer said, Peru is on the verge of facing water shortages. That's because one of the largest rivers coming off the high Andes glaciers, the Rio Santa, is already running low on glacial melt, he said.
Previously, scientists had thought the problem lay several decades in the future.
But based on satellite measures of ice cover and water-flow at gauging stations in the river, his team has concluded that the Rio Santa has already hit "peak water"—the point at which glacial runoff plateaus and then begins to decline.
"What it means is that instead of having 10, 20, or 30 years' perspective in which to find some solution for water allocation, these years did not exist," he said.
And that's just the beginning. Much of South America, with its high mountains and tropical sunshine, appears to be particularly vulnerable to climate-induced glacial shrinking.
Thus, he said, the next step will be to turn to Peru’s neighbors, particularly Bolivia and northern Chile, to see if similar stream-flow changes are occurring there.