"A climate model is like a huge wok with a lot of stir-fry ingredients," says Meehl. "We throw in solar variability, ozone changes, greenhouse gases, and many other items in the form of equations. If the model's past climate matches fairly well what's already happened in the real world, we get some confidence in the recipe." Meehl's research was funded by the U.S. Department of Energy and the National Science Foundation. NCAR's primary sponsor is the National Science Foundation.

The earth's average temperature has risen about 0.6 degree Celsius (1.1 degree Fahrenheit) since the start of the 20th century. The trend is most obvious in higher daily minimum temperatures. During the same period precipitation has increased over land in the mid- to high latitudes and decreased in the tropics. These two temperature and precipitation trends together can lead to changes in extreme weather, say the scientists.

Some changes have already been observed over the last century and are expected to escalate. These include an increase in very hot days in some areas, higher minimum temperatures with fewer frost days, and heavier short-term rainfall (lasting one or several days), especially in the midlatitudes. In the United States, incidents of heavy rainfall over several days increased most noticeably in the southern Mississippi River Valley, Southwest, Midwest, and Great Lakes.

Other changes are expected to appear later in this century as greenhouse gases continue to accumulate, trapping more heat in the atmosphere. Among them is a worldwide drying out of midcontinental areas during summer, with an increased chance of drought. Enhanced evaporation and higher temperatures would eventually outweigh precipitation increases in those inland areas. In North America, the central and southeast regions will get the hottest and driest, with the West Coast probably less affected because of its heavier rainfall and more moist soil.

There is no consensus yet on future trends for El Nino, hurricanes, and midlatitude storms. Scientists have recently embedded fine-scale regional models into global climate models to predict trends in hurricane number, intensity, and track shifts. Early results show a tendency for a future increase in hurricane intensity. Scientists expect this technique to yield better estimates in the future as model resolution improves.

Some models show slightly warmer El Ninos in the future, but a more consistent result is a trend toward a generally warmer ocean surface in the central and eastern equatorial Pacific, says Meehl. Future El Ninos would then be superimposed on a warmer surface, bringing even heavier rainfall to the central and eastern Pacific and lighter rainfall over Southeast Asia than occurred during the 1997 El Nino. But results vary from model to model and the jury is still out on the future of El Nino.

Growth in both population and wealth, along with demographic shifts to storm-prone areas, has made the United States more vulnerable to weather assaults. Total federal relief payments for weather-caused disasters from 1990 through 1997 hit $12 billion. Costs are expected to soar if extreme weather intensifies over the next century.

Researchers studying wild plants and animals have documented climate- induced extinctions, shifts in species range, and other seasonal behavior changes. Some gradual biological changes may be responses to changes in extreme weather and climate events.

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