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Floods flow from small climatic shifts.

Day after day, currents of carbon dioxide and other greenhouse gases waft over the planet, threatening to bring about potentially disastrous shifts in the global climate. Given a shifting climate, what sorts of changes will people actually experience?

Geologist James C. Knox of the University of Wisconsin-Madison has probed the geologic record of past floods to provide one answer. And in the Feb. 4 NATURE, Knox reports that relatively modest shifts in the globe's average annual temperature and precipitation may have dramatic regional effects on the frequency of catastrophic floods. The new study is one of a handful that link climatic change directly to local flooding, Knox says.

Computer simulations of global climate change emphasize long-term, gradual trends. However, "we should not assume that things will always be gradual," Knox points out. "The stratigraphic record shows many examples where things have changed rather quickly and abruptly"

Knox probed the sedimentary records of 68 floods along tributaries of the Mississippi River. During the past 7,000 years, he discovered, the largest floods carried 3-foot-wide boulders in their torrents and covered surrounding floodplains with over 16 feet of water.

However, Knox emphasizes the large effect that relatively minor climatic change can have on flood size. After the shift to a cooler, wetter climate some 3,300 years ago, the largest floods in the upper Midwest - equivalent to those now seen about every 500 years-occurred more frequently

The changes in precipitation and temperature that apparently brought on these floods are significantly less dramatic than those predicted for the future by global climate models. Based on indirect fossil evidence, Knox determined that the increase in flood size came with temperature shifts of 1 degree C to 2 degree C. In contrast, some models of global change predict future temperature increases of 4 degrees C to 5 degrees C.

Geologist Victor R. Baker of the University of Arizona in Tucson has also conducted research on ancient floods. These historical studies, he notes, show nature in action, whereas today's sophisticated, yet theoretical, global climate models show how we think nature works. Thus, Knox' findings are valuable because "they provide a complement- that is, they fill in what the other studies leave out."
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Title Annotation:moderate changes in temperature and precipitation could change frequency of floods
Author:Pendick, Daniel
Publication:Science News
Article Type:Brief Article
Date:Feb 6, 1993
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