Sound Wave Tracked Under Ocean : Science: Signal traveled 11,160 miles. Researchers hope test will help in detecting the greenhouse effect.

An underwater acoustic signal was heard across the globe this week, marking a milestone in an experiment that researchers hope will make it easier to detect warming of the world’s oceans from the greenhouse effect.

Sounding like an underwater foghorn, five transmitters suspended 600 feet below a U.S. Navy ship in the southern Indian Ocean began booming every three hours, for one hour at a time, last Saturday.

As had been predicted in 1988 by UC San Diego oceanographer Walter Munk, scientists using electronic ears could detect the sound waves as far as 11,160 miles away, on the West Coast. It took about 3 1/2 hours for the sound to travel that far.

This proof that the technology works was the first, most basic test of an experiment that will be repeated for several years as researchers try to detect warming of ocean waters.

Since sound travels more quickly in warmer water, temperature differences can be calculated based on how long it takes the sound to reach various receivers around the world.

Analysis over the next few months is expected to reveal whether the signals sent from near Heard Island in the Indian Ocean, southwest of Australia, are clear enough to calculate the average temperature of the ocean to within a few thousandths of a degree.

Because water gains heat more slowly than air, small changes in ocean temperatures are considered a more reliable indicator of global change. In theory, the experiment could settle the argument over whether carbon dioxide and other greenhouse gases are warming the planet artificially.

“We don’t know yet whether it’s good enough to do the global warming experiment, but we’re very encouraged because the signals are very strong in some locations--much stronger than we had expected,” said Robert C. Spindel, who heads the experiment’s command center at the University of Washington.

“It’s really astonishing that they can drop a simple hydrophone in the water in the south Indian Ocean and pick it up in Coos Bay, Ore. That’s a stunning result,” said Robert Park, a physicist at the University of Maryland and an official of the American Physical Society.

Richard C.J. Somerville, director of climate research at UC San Diego’s Scripps Institution of Oceanography, was more cautious.

“In global warming we don’t yet have a smoking gun,” Somerville said.

“The Heard Island experiment is the start of an effort that after several years of observing may tell us whether the ocean is warming up,” he said. “If it shows the fingerprint of the greenhouse theory, then we’ll have a very big step towards convincing evidence that we are changing the Earth’s climate.”

Although many scientists believe rising average air temperatures are sufficient proof, the Bush Administration has declined to act on their conclusions, saying more evidence is needed.

In contrast, policy-makers throughout the world have moved to cut emissions of chlorofluorocarbon gases because they believe the ozone hole provides clear proof of an atmospheric problem.

With assistance from the Office of Naval Research and other federal agencies, the experimenters are using acoustic techniques developed during the last decade over the longest distances ever tried.

The sound signals are emitted near Heard Island, and monitored at receiving stations on ships and offshore stations operated by scientists from several countries, including the Soviet Union, France, Canada, New Zealand, South Africa, India, Australia and Japan.