It's the first time that global climate modellers have worked so closely with teams running detailed regional models of snowfall, rain and stream flows to predict exactly what warming will do to the area. The researchers, from the National Center for Atmospheric Research in Boulder, Colorado, and elsewhere, were surprised by the size of the effect generated by only a small rise in temperature.
Assuming "business as usual" emissions, greenhouse gases will warm the west coast of North America by just one or two degrees Celsius over the next century, and average precipitation won't change much. But in the model, warmer winters raised the snowline, drastically reducing the crucial mountain snowpack, the researchers told the American Geophysical Union in San Francisco last week. "We realised that huge areas of the snowpack in the Sierra went down to 15 per cent of today's values," says Michael Dettinger, a research hydrologist at the Scripps Institution of Oceanography in La Jolla, California. "That caught everyone's attention."
The researchers also predict that by the middle of the century, melting snow will cause streams to reach their annual peak flow up to a month earlier. And with warm rains melting snow or drenching already saturated ground, the risk of extreme floods will rise dramatically. "We have to believe in these very warm, very wet storms," says Andrew Wood, a water resources modeller at the University of Washington, Seattle.
Since dams can't be filled until the risk of flooding is past, the models predict they will trap just 70 to 85 per cent as much run-off as they do now. This is a particular problem for California, where agriculture, industry, a burgeoning population and environmental needs already clash over limited water supplies. "We are taking this extremely seriously," says Jonas Minton, deputy director of the California Department of Water Resources.
And observations certainly back up the models. Minton points out that an increasing percentage of California's precipitation over recent decades is falling as rain rather than snow. And Iris Stewart, a climate researcher at the University of California, San Diego, has found that in the last 50 years, run-off peaks in the western US and Canada have been happening earlier and earlier. The cause seems to be a region-wide trend towards warmer winters and springs.
Dettinger has little doubt that the models point to a real and immediate problem. "It's upon us," he says, "and it's not clear what the fix is."
Author: Robert Adler
New Scientist issue: 22/29 December 2001
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