A sound way to take the sea's temperature?

Oceanographers in 1992 thought they had a good strategy for seeing whether the Pacific Ocean was warming. The team of researchers planned to lower transmitters into the deep sea and measure how long it took pulses of sound to reach submerged receivers far away. Because sound travels faster in warm water than in cold, this method, called acoustic thermometry, could provide a precise method of taking the ocean's temperature, they reasoned. The proposal, however, raised a howl of protests from environmentalists who worried that the loud noises would harm marine mammals.

After years of delay and substantial changes in the plans, the acoustic thermometry team now reports the first results from its $40 million research program. "This technique works and seems to work well," says Peter E Worcester, principal investigator for the Acoustic Thermometry of Ocean Climate (ATOC) project and a researcher at the Scripps Institution of Oceanography in La Jolla, Calif. "There is no question that we've shown it is a good technique for measuring large-scale ocean temperatures."

In separate experiments, biologists say that the sound transmissions have had little effect on nearby whales and elephant seals.

The ATOC team installed its first transmitter on the Pioneer Seamount, about 100 kilometers southwest of San Francisco, at a depth of 939 meters. The device emits a 195-decibel rumble that gets picked up by a network of Navy sound receivers positioned around the Pacific and by underwater microphones near Hawaii, Christmas Island, and New Zealand.

Under an agreement with environmental groups, the ATOC team gave control of the California transmitter to marine biologists so they could run experiments to determine whether the sounds alter animal behavior. Although the transmissions were not ideal for measuring ocean temperature, the data collected over a 15-month period show that the thermometry technique is even better than expected, says Worcester

The system measured the hour-long journey of sound traveling 5,000 km across the Pacific with a precision of 20 milliseconds, says Worcester. That translates into temperature measurements with a precision of .005 [degrees]C to .01 [degrees]C along the path of the sound, enough to discern a greenhouse warming signal within a decade, he says.

In the Aug. 28 SCIENCE, the researchers report that the technique can also yield important information in short-term experiments. By analyzing seasonal changes in sound travel times, the ATOC group calculated how much the ocean warms in summer and cools in winter

Others have estimated this variation using a satellite instrument that tracks changes in the height of the ocean surface. As water warms, it expands. The satellite team assumed that temperature changes accounted for most variation in ocean height. The ATOC team, however, found that temperature could explain only about half of the height fluctuation. Other factors--such as ocean currents--also play major roles in forcing sea level up and down, says the ATOC team.

Despite their success, the thermometry researchers end their California experiment this year They will not try to extend the test because they lack the resources to go through another contentious permitting process, says Worcester A second transmitter, located off the island of Kauai, started transmitting thermometry signals last month and will operate through the end of 1999. The ATOC group is currently deciding whether to try to extend that experiment.

Adam S. Frankel, a bioacoustic researcher at Cornell University, has studied humpback whales near the ATOC transmitter. Researchers have detected only minor changes in behavior, he says. "Nobody that I know of working with the ATOC program has had any result that would cause concern."