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

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.

However, and whatever the human influence may be, it still isn't clear what the tide gauges are telling us. A panel from the International Association for Physical Sciences of the Ocean (IAPSO) surveyed this question last August and noted that ``the available [tide gauge] data do not provide a truly global coverage.''

The panel warned that ``considerable risk is accepted'' in assuming that these data represent global conditions.

The heat-trapping gas carbon dioxide, released when we burn coal and oil, is accumulating in the atmosphere. This may already have warmed Earth a little. If the ocean as a whole is growing, it could be gaining water from the melting of northern hemisphere glaciers. Also, ocean water would expand as it warms so that even a slight increase in average temperature of upper ocean layers would raise global sea level a little.

On the other hand, changes in major sea water circulation patterns, such as those associated with the Gulf Stream, and in the way wind blows across large stretches of water, can change apparent sea level along a coastline without the ocean growing at all.

Likewise, changes in land elevation can affect tide gauge readings, as when a slow rise of coastal land acts like a lowering of sea level. These effects have to be taken into account in interpreting tide gauge readings.

Thus, the IAPSO panel said that it is premature to draw general conclusions about global sea level rise. A much more extensive tide gauge network, plus extensive study of all factors that affect apparent sea level, are needed to resolve this question.

Meanwhile, as Newman and Fairbridge point out, if sea level is indeed found to be a threat, a global network of reservoirs could help regulate it.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.