Listening in on oceanic warming.

Listening in on oceanic warming

Starting this fall, a new sound will be spread throughout the Pacific Ocean. The extremely faint hum will serve as the centerpiece of a major experiment designed to discern whether greenhouse gas pollution in the atmosphere is warming the world's oceans.

The experiment relies on a concept called acoustic thermometry, which involves repeatedly measuring the time it takes a sound pulse to travel thousands of kilometers through water. Because sound moves faster in warm water than in cold, this method can detect over a period of time whether the temperature of the ocean is gradually increasing. Walter Munk of the Scripps Institution of Oceanography in La Jolla, Calif., will direct the 30-month, $35 million project, funded by the Department of Defense.

While many scientists have experimented with sending sounds through the ocean, Munk and his colleagues performed the most ambitious test of this concept two years ago, when they emitted a low-frequency hum from Heard Island, in the southern Indian Ocean (SN: 1/26/91, p.53). Munk's group demonstrated that it could detect the faint sound up to 18,000 kilometers away, at stations on the east and west coasts of North America.

For the new study, Munk and his colleagues will set up sound sources on the US. West Coast and in the Hawaiian Islands. They plan to establish receiving stations in New Zealand, Japan, Tahiti, and the Kuril Islands in the North Pacific, says one of the experiment's participants, Robert Spindel of the University of Washington in Seattle. The sources will emit a modulated tone at a power of 250 to 1,000 watts.

Though not very loud, the hum can travel great distances because it moves within a sound channel - a layer of water in which sound travels slowest. Located hundreds of meters below the surface, this slow layer lies between the warm surface waters and the denser waters below. If sound traveling in the slow layer strays up or down, it is refracted back into the channel, an effect that allows the sound to cross the ocean.

Crude calculations suggest that greenhouse gases accumulating in the atmosphere should be warming the ocean by about 0.005 C [degrees] a year at a depth of 1,000 meters. Oceanographers would have trouble detecting such subtle warming with traditional methods because regional temperature variations would overshadow it, Munk says. But acoustic thermometry should enable them to discern such a trend, he explains, because the method provides a measure of average temperatures over large distances, removing the interference from local variations. If the Pacific thermometry experiment proves successful, Munk's group hopes to set up sources and receivers throughout all the world's oceans.