Computer models of Earth's climate support these observations, he says, and indicate more severe changes yet to come.

Even if societies successfully cap worldwide greenhouse gas emissions, the global climate will continue to warm into the 22nd century, though at a slower rate than if no attempt is made to control the emissions, Washington says.

Policy makers must prepare to adapt to a changing climate even while slowing the greenhouse gas buildup, he says. Washington will present these conclusions in his talk, "Predicting the Climate of the 21st Century," at the annual meeting of the American Association for the Advancement of Science in Denver on February 16.

"Scientific confidence in the ability of models to project future climate has increased," says Washington. "Meanwhile, recent experiments and routine monitoring have found evidence of global climate changes already occurring that are much larger than can be explained by the climate's natural variability."

In his talk, Washington explained how a computer model of global climate works and highlight what the most sophisticated models in the United States and elsewhere are now telling us about present and future climate.

A rise in the average global temperature is a convenient marker to indicate changes in actual climate extremes that affect our environment, says Washington.

For example, heat waves, as well as floods and droughts, will become more severe and occur more often. The first freeze dates will arrive later, and hard freezes will become less frequent in some areas. Rainfall and snowfall will increase or decrease, depending on the region.

The difference between daily high and low temperatures will shrink in many areas as average temperature rises. Such climate changes occurring around the globe are likely to affect disease and health patterns and threaten ecosystems, with important implications for forest survival, food supply, biodiversity, land use, and pollution.

"We are already seeing many of these changes," says Washington. "The debate on whether climate change is occurring has ended. The question is how much will take place and what are its impacts."

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