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How man may be helping to keep down the level of the seas

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ONE of the more alarming news stories that continues to recur is the threat that a rise in sea level might pose to beachfront property. This is not the future inundation that climatic prophets link to the melting of the Antarctic ice cap as Earth warms up over the next century. It's a slow, present rise of about a millimeter per year which some scientists find in tide gauge data gathered over the past century or so along many of the world's coastlines.

The question is whether or not these scientists are seeing a true rise in global sea level -- that is, an increase in ocean volume -- as opposed to local effects. Moreover, if the ocean is indeed growing, it's not at all clear how much growth is taking place.

Sea level change is a complex phenomenon. The more scientists look into it, the more subtle are the factors they find they have to take into account.

Recently, for example, Walter S. Newman of the City University of New York and Rhodes W. Fairbridge of Columbia University pointed out that people are probably a previously unsuspected factor. Estimating the vast amounts of water stored in reservoirs and used in irrigation, they conclude that this has diverted enough water to modify sea level significantly.

As they put it, summarizing their work in Nature magazine: ``. . . reservoir and irrigation storage have caused sea-level rise to lag about 26 years behind its projected uncontrolled rate of rise. Mankind has thus unwittingly been exercising an appreciable control on sea-level. . . .'' They figure that, without this control, sea level might be rising an extra 0.75 mm -- an extra 75 percent -- a year in addition to the 1 mm per year rise indicated by the tide gauges.

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