EVEN just a degree or two of greenhouse warming will have a dramatic impact
on water resources across western North America. Teams who have modelled the
climate in the area are warning of greatly reduced snowpacks and more intense
flooding as temperatures inch up during the 21st century.
It鈥檚 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 鈥渂usiness as usual鈥� 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鈥檛 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. 鈥淲e
realised that huge areas of the snowpack in the Sierra went down to 15 per cent
of today鈥檚 values,鈥� says Michael Dettinger, a research hydrologist at the
Scripps Institution of Oceanography in La Jolla, California. 鈥淭hat caught
everyone鈥檚 attention.鈥�
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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. 鈥淲e have to believe in these very warm,
very wet storms,鈥� says Andrew Wood, a water resources modeller at the University
of Washington, Seattle.
Since dams can鈥檛 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. 鈥淲e are taking this extremely seriously,鈥� 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鈥檚 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. 鈥淚t鈥檚 upon us,鈥� he says, 鈥渁nd it鈥檚 not clear what the fix is.鈥�