BOULDER -- Expect hotter days, warmer nights, heavier rain and snowfallevents, and more floods over the next century, says a new studypublished September 22 in the journal Science. The article reviewsobservations, impacts, and results from some 20 global climate modelscurrently in use worldwide. It sizes up extreme events that haveintensified during the past century and are expected to escalate overthe next as carbon dioxide and other greenhouse gases shake up theearth's climate. The paper's lead authors are David Easterling ofNOAA's National Climatic Data Center and Gerald Meehl, a climateexpert at the National Center for Atmospheric Research (NCAR).
"A climate model is like a huge wok with a lot of stir-fryingredients," says Meehl. "We throw in solar variability, ozonechanges, greenhouse gases, and many other items in the form ofequations. If the model's past climate matches fairly well what'salready happened in the real world, we get some confidence in therecipe." Meehl's research was funded by the U.S. Department of Energyand the National Science Foundation. NCAR's primary sponsor is theNational 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. Thetrend is most obvious in higher daily minimum temperatures. Duringthe same period precipitation has increased over land in the mid- tohigh latitudes and decreased in the tropics. These two temperatureand precipitation trends together can lead to changes in extremeweather, say the scientists.
Some changes have already been observed over the last century and areexpected to escalate. These include an increase in very hot days insome areas, higher minimum temperatures with fewer frost days, andheavier short-term rainfall (lasting one or several days), especiallyin the midlatitudes. In the United States, incidents of heavyrainfall over several days increased most noticeably in the southernMississippi River Valley, Southwest, Midwest, and Great Lakes.
Other changes are expected to appear later in this century asgreenhouse gases continue to accumulate, trapping more heat in theatmosphere. Among them is a worldwide drying out of midcontinentalareas during summer, with an increased chance of drought. Enhancedevaporation and higher temperatures would eventually outweighprecipitation increases in those inland areas. In North America, thecentral and southeast regions will get the hottest and driest, withthe West Coast probably less affected because of its heavier rainfalland more moist soil.
There is no consensus yet on future trends for El Nino, hurricanes,and midlatitude storms. Scientists have recently embedded fine-scaleregional models into global climate models to predict trends inhurricane number, intensity, and track shifts. Early results show atendency for a future increase in hurricane intensity. Scientistsexpect this technique to yield better estimates in the future asmodel resolution improves.
Some models show slightly warmer El Ninos in the future, but a moreconsistent result is a trend toward a generally warmer ocean surfacein the central and eastern equatorial Pacific, says Meehl. Future ElNinos would then be superimposed on a warmer surface, bringing evenheavier rainfall to the central and eastern Pacific and lighterrainfall over Southeast Asia than occurred during the 1997 El Nino.But results vary from model to model and the jury is still out on thefuture of El Nino.
Growth in both population and wealth, along with demographic shiftsto storm-prone areas, has made the United States more vulnerable toweather assaults. Total federal relief payments for weather-causeddisasters from 1990 through 1997 hit $12 billion. Costs are expectedto 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 seasonalbehavior changes. Some gradual biological changes may be responses tochanges in extreme weather and climate events.
Materials provided by National Center For Atmospheric Research (NCAR). Note: Content may be edited for style and length.
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