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Global Water Supply Takes Some Heat

A substantial portion of the world’s population relies on water that comes down from mountains in seasonally predictable patterns. But rising temperatures are changing those patterns, making it more difficult to predict how much water will be available. Some researchers are now saying that under future climate conditions, demand for water may not be met in many parts of the world.

Researchers from the National Resources Conservation Service survey the snowpack near Mount Hood in Oregon. New research is suggesting that global warming may significantly decrease snowpack, especially in coastal mountain ranges, which would decrease summer water availability. Image courtesty of Ron Nichols, USDA NRCS.


By 2100, Earth will warm by between 1.4 and 5.8 degrees Celsius, according to the Intergovernmental Panel on Climate Change’s (IPCC) Third Assessment Report. And over the past century, global surface temperatures have increased about 0.6 degrees Celsius (1 degree Fahrenheit). Temperatures are inexorably linked to precipitation in a complex relationship that changes depending on the location. Generally, scientists think that warming temperatures will cause the global water cycle to intensify, meaning a possible exacerbation of extremes such as droughts and floods. Warming temperatures will also affect high-altitude water resources, according to Tim Barnett of the Scripps Institution of Oceanography in La Jolla, Calif., and colleagues with the Climate Impacts Group at the University of Washington in Seattle, who published a report on such impacts in the Nov. 17 Nature.

Traditionally in high altitudes, much of the precipitation falls in the winter as snow, where cold temperatures trap it in a snowpack. Then, when temperatures warm in the spring, the snowpack melts, and water slowly drains from the mountains and can be captured in reservoirs for later use.

In mountainous regions that hover around the freezing point, however, a small rise in temperature can cause the precipitation to fall as rain rather than snow, significantly reducing the snowpack, says Philip Mote, state climatologist of Washington and a member of the Climate Impacts Group and IPCC. A smaller snowpack means that less water is available later in the year, as the rain that falls drains straight out to sea, he says.

As a result of these processes, the largest declines in snowpack are already occurring in mountainous regions with milder climates, Barnett and colleagues report, such as western North America and Europe. Other colder areas, however, are also at risk due to glacial processes, such as northern China, northwestern India, areas south of the Hindu Kush in Asia, and basins downstream of the southern Andes in South America. In these high-risk areas, researchers are examining the water resources — including snow and ice accumulation and runoff in rivers.

Looking at the resources under a number of different emissions scenarios from IPCC, Ruby Leung of the Pacific Northwest National Laboratory in Richland, Wash., and colleagues ran regional climate simulations to see what changes might be expected over the next 50 to 100 years — the timeframe most useful for policy-makers. Combining climate models with observed trends, regional patterns begin to emerge.

The western United States, for example, where temperatures are expected to warm at least 0.8 to 1.7 degrees Celsius by 2050 on average, will likely see a substantial reduction in mountain snowpack, Leung says. Mountain snowpack currently supplies about 90 percent of the spring, summer and fall water for Western states, she says.

Even stronger effects will occur locally, such as over the Cascades or Sierra Nevada, which may see 2- to 3-degrees Celsius warming by 2050, leading to reductions in snowpack of 50 to 70 percent. Such warming would also lead to the spring streamflow peaking at least one month earlier than it does currently.

Most reservoirs, Leung says, are not built to withstand such changes. Reservoirs can only accommodate certain amounts of runoff at certain times of the year for flood control, hydropower generation and other water uses, so such shifts in runoff patterns could create substantial water-management challenges, including possible water shortages.

Warming portends even greater problems, however, for areas that depend heavily on glacial meltwater, Barnett says. As the glaciers melt, the “fossil” water that is usually frozen and incrementally melts out in the summer will become depleted too quickly. Glaciers are retreating throughout the world, but the situation is most critical for the Hindu Kush/Himalaya region of Asia and the South American Andes, where most of the dry-season water comes from glaciers (see story, this issue).

In the Hindu Kush, glaciers provide 50 to 70 percent of the summer water flow, supplying water to half of the world’s population. Virtually all of the glaciers there show “substantial melting,” Barnett and colleagues wrote, with a particularly “marked retreat” by the glacier that provides most of the water to the Yangtze, the largest river in China. And in Peru, 25 percent of the country’s glaciers have disappeared in the past 30 years, potentially spelling disaster for a country where glacial meltwater provides electricity and drinking water to almost all of the residents, Barnett says.

In the near term, however, these glacial-melt-dominated river systems will actually see increased streamflows, Barnett says, compounding an already complex planning situation. When people in those locations see more water than they are used to, it will no be easy to convince them that they will be facing shortages in coming decades. But “we don’t have 50 years to wait and see what happens,” he says.

In the long run, people “need to be prepared to live in the environment in which we choose to live,” says Brian Hurd, an economist at New Mexico State University in Las Cruces. People everywhere will have to adapt to having less water and lower their consumption of water, especially as populations grow and cities expand.

“The bad news,” says Alan Hamlet, a research scientist with the Climate Impacts Group, is that many people will probably have to learn to live with less water. The good news, though, is that “this is not very difficult to accomplish.”

Increasing the storage capacities of reservoirs is a possibility in some, albeit few, places, and desalination of seawater is another option in coastal areas, he says. Using reclaimed water for irrigation is another “attractive option.” Still, Hurd says, “the best strategy for coping with an uncertain and changing climate is to plan to grow responsibly.”

As policy-makers and planners make long-term resource management decisions, they need to take into account climate change, Mote says. California, for example, has been working on the water-climate issue for more than a decade, he says, and the Pacific Northwest is quickly jumping on board.

Whatever “solutions” are chosen, Mote says, “they are best addressed in the proactive, rather than the reactive.” He points to experts’ warnings in advance of Hurricane Katrina that New Orleans’ levees were not strong enough, and how they were proved right too late — after the levees broke. “We don’t want be in an ‘I told you so’ mode” about climate change and water. “The real crisis isn’t here yet,” Mote says. “There’s still time to come up with thoughtful solutions.”

Megan Sever

Links:
"Ice Hunter: Q&A With Lonnie Thompson," Geotimes, March 2006
Climate Impacts Group
Intergovernmental Panel on Climate Change

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