How will these changes affect our water supply in general?

GCMs simulating a climate that is based on a doubling of CO₂ suggest a global mean increase in precipitation and evaporation of between 3 and 15 percent. Yet the more useful information, i.e., the location and timing of these changes, is still uncertain.

There are general indications that:

• the present mid-latitude rain belt would shift northward;
• snowmelt and spring runoff would occur earlier than at present;
• evapotranspiration would be greater, as it would start earlier and continue longer;
• the interior continental region in the Northern Hemisphere will, in general, experience drier summers.

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