Radio Script
1998

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Sound Travels
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STORY: Last October off the coast of Franz Josef Land--a rocky, windswept outpost high above Scandinavia, Russian scientists lowered a device called an acoustic source into the Arctic Ocean. Every four days since then, it's been sending out short bursts of low-frequency sound waves. Twelve hundred kilometers away in the Lincoln Sea north of Canada, underwater microphones record the sounds as they pass.

Odd as it seems, sending sound through the ocean is helping scientists take the temperature of the Arctic Ocean. Peter Mikhalevsky is a researcher with Science Applications International, a private company under contract with the U.S. Navy. He heads a joint U.S.-Russian experiment to measure the temperature of the entire Arctic Ocean. The experiment is nicknamed ACOUS, the Greek word meaning "listen."

MIKHALEVSKY: "The overall objective of the program is to use a network of several sources and receivers that would be installed in the periphery of the Arctic Ocean and they would send out acoustic signals, these low rumbles. We then process and measure the travel time of the signals. And we use that information to determine the temperature of the ocean. So what we would like to do is in real time determine the general features of the change in temperature of the Arctic Ocean. This is extremely important when we're trying to understand cyclical phenomenon that may be occurring in the Arctic Ocean, or trends in global warming."

Saltwater has always been a good conductor of sound, but it was only recently that scientists began using sound to calculate the ocean's temperature. To make the calculation, scientists apply a sort of reverse logic. Since they already know how fast sound should travel through saltwater at different temperatures, they're able to deduce the temperature of the ocean merely by knowing how fast it took the sound to get from point A to point B.

Mark Johnson is an associate professor of marine science at the University of Alaska Fairbanks. He says the method lets scientists measure the temperature of large tracts of ocean in a short period of time.

JOHNSON: "And that average sound speed is a function of temperature and salinity. So that's how we get the temperature of the ocean. Except it's across an entire ocean rather than a single point measurement, which is what usually happens when you take a ship out or you deploy a mooring. This technique integrates the temperature entirely across the ocean basin. It's a much more satisfying measurement of the real ocean temperature."

Ultimately, the goal of the project is to develop realistic computer models of the Arctic Ocean that will be used for such things as understanding the climate and how much the climate might be changing. Mark Johnson is one of the researchers developing these complex computer models at the University of Alaska Fairbanks.

JOHNSON: "We have already models that do reasonably well showing us the way the Arctic ocean flows and its interannual variability. But this experiment will help us validate those models. Most importantly, they'll help us determine exactly where those models are working well and where they are not working well, where they can be improved. So that will be one wonderful way to test whether our models are accurate."

There's reason to believe the work will have real payoffs in improving scientific understanding of the Arctic Ocean. Peter Mikhalevsky says that in field trials of the technology done in 1994, researchers discovered that the Arctic Ocean had warmed more than climatologists expected.

MIKHALEVSKY: "When we compared the travel time results that we measured during the experiment with those that were computed from historical climatology--which was based on previous measurements of temperature and salinity in the Arctic Ocean--we discovered in fact that the signal had arrived several seconds sooner than had been predicted in the climatology. This was an indication to us that the Atlantic layer of the Arctic Ocean had warmed by an average of 0.4 degrees Celsius, which is a huge amount."

Using sound to take the ocean's temperature isn't without controversy, however. The idea got off to a rocky start several years ago when scientists proposed sending sound waves across the Pacific Ocean, from near Monterey, California. Environmentalists, concerned that high frequency sounds would damage the hearing of marine mammals, forced numerous changes to the experiment. Peter Mikhalevsky says bad publicity hasn't been a problem for this Arctic experiment.

MIKHALEVSKY: "We initiated studies back in the early 1990s with our Russian colleagues on the densities and locations of marine mammals in the Arctic. We completed an environmental assessment report. Basically it was determined there was no significant impact, given the levels of the sound in the water and the numbers of animals that might be affected, which is extremely small because there is a very low density of marine mammals where we have the source."

Although the experiment right now consists of just one acoustic source and a receiver, scientists will create a network of sound sources and microphones across the Arctic. Next year, they'll install a microphone array off the coast of Barrow, Alaska. Soon after, they'll be measuring the ocean's temperature beneath the Arctic ice pack.

OUTRO: For Arctic Science Journeys, this is Doug Schneider, reporting from Fairbanks, Alaska.