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Where When How N/D 2017 - J/F 2018 : Page 94

Figure 5: The high-velocity sound layer at about 260 ft. (79 m) deflects sound, forming a “shadow zone” and obscuring any object in the shadow from sonar or other sound waves. The Sounds of Science Experiments with sound in the sea have progressed significantly since the discov-ery of the SOFAR channel; and with the end of the Cold War much of this research has turned to peaceful purposes. Research has also shown that the SOFAR channel is a truly amazing phenomenon, and may even be used by great whales to communi-cate with each other thousands of miles away. Today, sound is being used to study, of all things, climate change. It all began in 1991 with the first tests exploring what’s come to be known as “acoustic thermography.” Early that year, low-frequency sounds were generated by a ship in the southern Indian Ocean near Antarctica. The sounds — booming, low-frequency hums — were produced by powerful underwater loudspeakers emit-ted in all directions for an hour. This was followed by two hours of silence, and the pattern was repeated for five days. Trapped in the SOFAR channel, the sound was detected 11,160 miles (17,856 km) away off the California coast, near Point Con-ception. The transit time to travel nearly halfway around the world was about three hours. But how can sound be used to meas-ure ocean temperatures, especially — as scientists now do — to within a few thou-sandths of a degree? Recall that sound travels faster through warmer water. So, water temperature differences can be cal-culated by recording the precise time taken for sounds to reach receivers around the world. Still, what does this have to do with climate signal with 260 watts of acoustic power. These early experiments were forerun-ners to a much more ambitious program sponsored by the University of California’s Scripps Institution of Oceanography. In 1995, the Acoustic Thermography of Ocean Climate (ATOC) program began transmitting acoustic signals from the Pio-neer Seamount deep off the west coast of North America. The signals were recorded on U.S. Navy SOSUS receiving arrays, as well as stations near Hawaii and near the Kiritimati Islands (just north of the equator in the central Pacific). Signals were also re-ceived by a recorder off New Zealand. After more than two years of recording data, ATOC determined three key findings. First, the technology could, in fact, record sounds with more precision than originally thought possible (to within 20-30 millisec-onds at 1,860-3,100 mile [2,976-4,960 km] ranges). Secondly, the data could be clearly related to known ocean processes. And, lastly, that the ATOC data was consistent with and complementary to satellite data. As a result, by combining both acoustic and TOPEX/Poseidon satellite altimeter data, computer models of the ocean’s gen-eral circulation could be greatly refined, and the extent of ocean warming — and thus global warming — could be much bet-ter understood. In February 2005, Scripps researchers Drs. Tim Barnett and David Pierce used ATOC data and other sources in a report that many believe removed much of the uncertainty in the debate over global warming and — according to the authors — provided “the first clear evidence of human-produced warming in the world’s oceans.” With assistance from re-searchers at Lawrence Livermore Na-tional Laboratory, the study combined computer models and hard, observed evi-dence collected over the last 40 years in the world’s oceans. The conclusion: The data closely matched the results pre-dicted in computer models for warming caused by human activity. With a statisti-cal confidence exceeding 95 per cent, they found that warming in the upper 2,300 feet (697 m) predicted by the model corresponded to the measure-ments obtained at sea. This high degree of correlation between the model and real data confirmed that the ocean warm-ing — an increase of 0.5 C since the 1940s — is the product of human influ-ence. Although efforts were also made to explain the warming through naturally oc-curring variations in the climate or fac-tors such as solar and volcanic eruptions, this did not replicate the observed ocean warming data. As one of Barnett’s col-leagues at Scripps, Dr. Jeffrey Severing-haus, said, “There is no doubt that humans are warming the planet. That’s very clear now; the data is beautiful and 94 • • • • • NOV/DEC/JAN/FEB 2017/2018 “Where When How -Turks & Caicos Islands”

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