Mapping Ocean's Surface Yields Data On Global Warming

FOR the past 2-1/2 years, a radar-ranging satellite has performed a minor miracle for oceanographers. For the first time in the history of their science, they have a continuing global overview of the ocean's surface.

It's hard to chart the bumps and hollows in the sea surface caused by winds and currents using data from ships and instrumented buoys. And tide gauges are an uncertain guide to rising sea level. Now oceanographers can follow what happens to the ocean surface over most of the world as it changes from season to season and from year to year.

Among other things, this gives them a handle on how fast sea level is rising -- a key question in the debate over whether or not man-made global warming is already changing the climate. It also reveals circulation patterns that may threaten offshore installations.

These are the early fruits of a French-US collaboration called the Topex/Poseidon ocean-topography experiment. They are rich enough to justify extending the satellite's service beyond its nominal three-year mission.

For example, the satellite has already provided enough data to enable project scientists to predict tides over most of the deep ocean to an accuracy of 2 to 3 centimeters, according to team member Christian Le Provost at the Institute de Mecanique de Grenoble in France.

The satellite also has tracked large eddies that break off from a major current in the Gulf of Mexico. These giant swirls act like hurricanes in the sea, ranging up to 400 kilometers (248 miles) in diameter. Last summer, the satellite data helped oil-rig operators keep better tabs on a major eddy than was possible using their drifting buoys alone. Eventually, Topex/Poseidon data should help develop better methods to forecast the development and course of these dangerous swirls.

It is the sea-level observations, however, that get attention outside the scientific community. At last month's American Geophysical Union meeting in San Francisco, the Topex/Poseidon team reported a rise in average sea level of about 3 millimeters a year between December 1992 and September 1994. That's too short a period to draw any conclusions about global warming, but it does show the satellite's ability to deliver the kind of precision sea-level measurements the global warming debate has needed.

It works like this. Topex/Poseidon rode into orbit Aug. 10, 1992, on a French-supplied Ariane rocket. It now circles Earth in an orbit 1,336 kilometers (830 miles) high and inclined 66 degrees to the equator. From there, the satellite maps differences in sea-surface height to an accuracy of a few centimeters over distances of hundreds or even thousands of kilometers. Its data also allow absolute measurements of sea level.

Orbital tracking gives the satellite's radial distance from Earth's center. Radar ranging gives its distance from the sea surface. Subtracting that from the satellite's radial distance gives the height of the sea surface relative to Earth's center. That's something no tide gauge can do. Tide gauges measure sea level relative to a land site that may, itself, be rising or falling.

Replacing the uncertain tide-gauge data with precision sea- level measurements can be a valuable contribution to the global-warming debate. Sea-level rise is a mixture of natural effects and possible human factors. And the human factors are, themselves, a mix of possible global warming and additions to the ocean from the draining of underground aquifers and land-use changes. If satellite monitoring can be continued for the next couple of decades, it may be possible to sort out this mess.

The Topex/Poseidon team will request an extension of the current mission. It also is preparing plans for a successor satellite to be launched in 1999. Last August, the French space agency honored the team by awarding it the Centre National d'Etudes Spatiales medal. Both France and the United States should now honor team members by continuing to support their work.