Climate test: hum heard 'round the world.' (worldwide sound wave experiment to test greenhouse effect)
From remote Heard Island far off the Antarctic coast, scientists next week will send sound waves throughout the world's oceans as part of an experiment to determine whether greenhouse gases are indeed warming the planet.
A simple physical fact underlies their technique: Sound travels faster through warm water than through cold water. By measuring changes in the speed of sound over a decade, scientists hope to obtain a clear answer about global warming.
Investigators will test the feasibility of this idea into early February. A dozen listening stations scattered around the globe will monitor the Heard Island transmissions in an attempt to measure the time it takes for the sound to cross the seas. If the experiment proves successful, scientists plan to begin routinely measuring sound speed in the ocean within the next few years. Walter H. Munk of the Scripps Institution of Oceanography in La Jolla, Calif., organized the Heard Island experiment.
This is the first attempt to establish a monitoring program over such vast distances, says Ted Birdsall of the University of Michigan in Ann Arbor, a principal investigator in the experiment. Researchers have used similar techniques to measure the speed of sound along a oceans. The channel exists in the region where sound travels the slowest--a layer sandwiched between the warm surface waters and the extremely dense layer below. When acoustic waves traveling through the sound channel begin to stray into the "faster" adjacent layers, refraction sends them back into the channel. In a 1960 experiment, sound waves from a single explosion off western Australia traveled through the sound channel and were clearly recorded at Bermuda.
In the Heard Island experiment, a transmitter lowered from a ship to a depth of 200 meters will create a low-frequency hum of about 57 hertz. The signal will be extremely faint by the time it reaches listening stations, so investigators will use complex processing techniques to isolate it from other noises.
Because marine mammals can hear sounds of the frequency, the experiment also involves biologists, who will observe the behavior of whales around the ship.
Computer climate models estimate that greenhouse gases should cause the world's oceans to warm by about 0.005[degrees]C per year at a depth of 1,000 meters. Direct measurements of ocean temperatures would not reveal such a small rise within a few years, but a program monitoring the speed of sound could detect clear signs of a greenhouse warming over five to 10 years, says project coordinator Robert Spindel of the University of Washington in Seattle. Over 10 years, the estimated warming would cut a few seconds off the signal's 3.5-hour trip from Heard Island to the Pacific coast of North America, he says.
In the past few years, climate experts have vigorously debated the meaning of a global warming trend apparent in records of continental and sea-surface temperatures. Because the planet's surface climate can fluctuate naturally, most scientists say they cannot tell whether the buildup of greenhouse pollutants in the atmosphere has indeed caused the observed warming trend.
The deep ocean, with its naturally stable climate, provides a means of side-stepping those problems, says Albert J. Semtner Jr. of the Naval Postgraduate School in Monterey, Calif., an oceanographer involved in the Heard Island experiment. "We think this is a clean way, an experimentally simple and clear way, to look for global warming," he says.
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|Date:||Jan 26, 1991|
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