A cautionary picture of water supplies as Earth warms

New studies forecast declines for rivers in the US and elsewhere.

Mountain snows and alpine glaciers represent key reservoirs of fresh water for some 1.6 billion people worldwide. In 50 years, however, a warming planet is likely to disrupt many of these sources, leaving millions of people scrambling for additional supplies.

While conservation, additional reservoirs, and repairs to leaky water mains can help blunt the effects of these changes, efforts to adapt to shrinking snowpacks and vanishing glaciers are expected to require other changes in farming techniques, industrial practices, and lifestyles.

That's the warning a team of US scientists is issuing after reviewing field measurements and modeling studies that deal with the impact of global warming on alpine environments. Combined with a second, independent look at stream flows in key parts of the world, the studies are helping scientists fill in a picture of future freshwater supplies as the planet warms. (Both studies appear Thursday in the journal Nature.)

Efforts to anticipate the potential effects of global warming on water supplies have been under way for years. In the Western US, for example, some researchers have been saying for at least five years that despite uncertainties in model projections, the results are solid enough to be useful to regional water managers as they lay out long-term water-management plans.

But the US West is not the only region of the world that relies on winter snows to water crops, generate electricity at hydroelectric dams, or fill soup pots. So a team led by Timothy Barnett at the Scripps Institution of Oceanography looked at other regions that rely on snowmelt for at least 50 percent of their water and lack the ability to store a year's worth of runoff.

The principles involved are "exceedingly simple and uncontroversial," he says. "When it's warmer, you may have the same amount of precipitation, but more will be in the form of rain than snow. That's 'duh.' And if you have any snowpack in a warmer world, it's going to melt earlier." This can translate into less water in summer and fall.

The team looked at the effect of a shrinking snowpack on Europe's Rhine River and the Canadian prairie, as well as the Western US. They found a range of effects, from reduced freight shipments along the Rhine to increased farm vulnerability to drought.

As for glaciers, "they are fossil water," Dr. Barnett says. "They may melt right up to the end, and you don't think you have a problem. Then, hey, they're gone."

One of the areas the team sees as most crucial is the region whose thirst is slaked by glaciers in the Himalayas and Hindu Kush mountains. Collectively, these mountain ranges hold the third largest mass of ice after Antarctica and Greenland. The rivers they feed provide much of the water for 50 to 60 percent of the world's population. Yet China's latest survey of the mountains show that over the past 25 years, the glaciers are in wholesale retreat.

"These results are really robust," says Christopher Milly, a researcher with the US Geological Survey working at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory. Dr. Milly and two colleagues produced the second study, which compares the results of 12 climate models used to project future annual stream flows worldwide.

By 2050, the models projected a 10 to 40 percent increase in annual stream flows in eastern equatorial Africa, South America's La Plata basin, and the near-polar regions of North America and Eurasia. Southern Africa, southern Europe, the Middle East, and western North America saw 10 percent to 30 percent declines.

The impact on regions with more water coursing through rivers each year could be positive, Milly says. Earlier studies, however, suggest that in several regions showing an increase, precipitation is more likely to come in sudden bursts, making it more difficult to capture and store.

"It's critical we start thinking about this now," says Lara Binder, an outreach specialist with the University of Washington's Climate Impacts Group. "We can't be in a position where we're trying to play catch-up."