Gravity Data Measures How Much Tucson May Bathe

In about three years, a satellite named GRACE may intrude into Tucson, Ariz., politics. It will monitor small local changes in Earth's gravity field. These, in turn, reflect changes in subsurface masses, such as a drop in the local water table. The data could add fuel to an ongoing controversy over curbing water-hungry development to conserve the shrinking aquifer that nourishes semi-arid Tucson.

It's an example of scientists' increasing ability to measure small changes in our planet's properties that have significant implications.

GRACE stands for Gravity Recovery and Climate Experiment. It is designed to pick out details in the gravity field as small as 300 kilometers (190 miles) across. It should be able to detect changes in those details that happen as rapidly as 12 to 25 days. That's fine enough detail to show what we're "taking out of our precious [aquifer] reservoirs" right across the United States and elsewhere in the world, says GRACE scientist Jean Dickey at the Jet Propulsion Laboratory in Pasadena, Calif.

She adds that, after the satellite is orbited in 2001, it also will monitor glaciers, the great Greenland and Antarctic ice sheets, and the annual growth and melt of snow packs on the continents. The ability of fine-grained gravity data to keep tabs on how much water these frozen reservoirs hold will help scientists watch for subtle signs of global climate change.

Dr. Dickey joined several colleagues from various earth science fields in describing some of the new precision measurements during a recent meeting in Boston of the American Geophysical Union.

The satellite gravity data will refine measurements of sea-surface topography that already are accurate to within a few centimeters or less. Like "topo" maps that show a landscape's hills and valleys, maps of satellite altimeter data outline the seascape's undulations. They reflect the flow of ocean currents and surface temperature patterns. Where the surface layer is warmer, it expands and rises up a few centimeters above cooler regions.

Scientists trying to understand how the ocean affects climate need those marine topo maps to make their computer models more realistic. If they don't have a good map to start with, the computer run will likely turn out garbage. Chester Ropelewski of the International Research Institute for Climate Prediction in Palisades, N. Y., noted that gravity helps determine the sea topography. And, right now, the maps "are corrupted by gravity errors," he said. GRACE mission data should correct those errors. Dr. Ropelewski said he expects then to get "good" ocean topography and improved computer climate models.

One of the subtlest kinds of precision measurement connects Earth's spin and global winds. A line of radio telescopes called the Very Large Baseline Array (VLBA) runs from the Caribbean to Hawaii. When these telescopes work together, they simulate a radio dish with a diameter as large as the VLBA itself. This lets operators track distant astronomical objects very precisely. Working backward from such measurements, they can pin down Earth's rotation rate to within a tenth of a millisecond. That's how scientists detected that El Nio induced changes in the winds that added 0.8 milliseconds to the length of the day.

It works like this. Storms and their winds exert different pressures on the two sides of mountain ranges. In this way, the atmosphere puts a torque on the planet that slows its spin (lengthens the day) and adds momentum to the winds.

Likewise, friction between the winds and Earth's surface takes momentum out of the winds and speeds up the planet's rotation (shortens) the day. That's what happened during the recent El Nio warming of the equatorial central Pacific Ocean, which now appears to be winding down.