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Over The Horizon

Listening to Warm Water

New Zealand scientists are involved in a ten-year international experiment that will attempt to measure temperature change by monitoring the speed of sound through water. The unprecedented precision of the Acoustic Thermometry of Ocean Climate project may be able to finally confirm or refute the scenario of global warming.

Global warming remains a controversial concept, and it is by no means certain whether or not it is actually taking place. Detection of changes in the Earth's temperature is difficult and imprecise. To a large extent this is because atmospheric temperature varies greatly, depending on location, the time of day and the time of year.

The ATOC experiment will make use of two physical properties of water to check on the global warming hypothesis. Firstly, water has a much higher heat capacity than air, which means that the heat energy associated with the usual large daily fluctuations of air temperature causes a smaller change in the temperature of the sea. Deep in the oceans the temperature is basically constant -- about 4oC at 1,000 metres, but this temperature should gradually rise if global warming occurs. Secondly, the speed of sound through water is dependent on its temperature. The hotter the water, the quicker sound travels.

Bearing these two facts in mind, the ATOC experimenters plan to transmit sounds through the ocean, at a depth of 1,000 metres, from California to the East Coast of New Zealand. Over this kind of distance a water temperature increase as small as 0.004o C per year would shave a total of two seconds off the travel time after a period of ten years -- and this would be easily detectable. This sort of temperature rise at that depth is predicted in current global warming projections.

Of course it's not that simple. The Pacific Ocean is a big and noisy place, and at first glance it might seem that any sounds made in California would have to be loud indeed to be heard from New Zealand. A lot of work has gone into measuring the acoustic properties of the ocean, especially the background noise levels and the reduction in intensity associated with sound transmission. With this information the ATOC scientists have designed their signals so that they can be transmitted at a safe level. They will also be able to filter out the extraneous noise to positively identify received pulses.

Project member Dr Mike Guthrie of the Defense Scientific Establishment says that the experiment will give the researchers a chance to accurately detect an incredibly small trend lying under large-scale fluctuations. Using an Earth-scale path for such acoustical tests is not something that can be done in the atmosphere, he notes.

The ATOC experiment will actually involve several transmission sources and receiving stations, with transmitters to the north and, probably, the south of Hawaii, and at Point Sur, California. Receivers are to be located at Point Sur and, provided the necessary consents are granted, about forty nautical miles off the coast of Gisborne, with other, smaller, receivers dotted around the Pacific Rim.

The environmental impact of the ATOC experiment is to be carefully monitored and controlled, with extensive work being done to ensure the sound pulses won't affect marine life. Of special concern are whales and dolphins who themselves communicate with sound, sometimes over vast distances.

The project aims to ensure that the sound intensity at the New Zealand end is at least thirty decibels lower than that considered to be damaging to marine mammals, and Dr Guthrie said that a large part of the project's research work was devoted to ensuring Californian marine life would be similarly unharmed.

By using the technique of measuring sound speed through the ocean, and hence the oceanic temperature, it is hoped that long-term trends in the Earth's temperature can be identified much more quickly than would be the case if atmospheric temperature alone were to be measured.

ATOC could provide the solid data needed to determine just what is happening -- if anything.

Chris Smyth