The main principle behind the Acoustic Thermometry of Ocean Climate (Atoc) project is that sound travels more quickly in warmer water. A transmission from California at current temperatures will take almost two hours to reach New Zealand, but each degree Celsius of warming will speed it up by about 17 seconds. The plan is to track the long-distance hum in order to measure the temperature of the ocean and look for signs of change.
The 'demonstration' phase of Atoc, designed to see if the system works, will use two transmitters built about half a mile down in the ocean off California and Hawaii respectively. They will transmit a low-frequency hum at about 190 decibels - or 1,000 times as loud as a Tube train or noisy factory machinery. But the vibrations that make the noise are at much lower frequencies, between 60 and 90 hertz (vibrations per second), similar to a rumble of thunder. The hum will last for 20 minutes and be repeated every four hours for two years. It will be monitored at 13 listening posts around the Pacific, many of them run by the US Navy.
According to the Scripps Institution of Oceanography in San Diego, California, which is running the project, the data will be 'vital to provide firm evidence of ocean warming'. But, says Andrew Forbes, deputy director of Atoc, to be able to detect long-term trends in ocean temperature and distinguish them from natural variability, 'we will want to monitor all the world's oceans for several decades'. The demonstration phase has a budget of dollars 35m (pounds 23.5m) but the eventual bill could approach a billion dollars over a decade, making Atoc one of the biggest and most expensive scientific experiments ever attempted.
John Gould of Britain's Institute of Oceanographic Sciences in Wormley, Surrey, which has joined in the project, says: 'The beauty of this method of measuring is that it evens out the natural variability of temperature across the oceans. To attempt the same thing by measuring from ships would take a long time and a lot of measurements. And satellites can only look at the temperature at the surface, whereas this experiment could monitor warming in the ocean's interior.'
The experiment was due to get under way this spring, with transmissions from within California's Monterey Bay National Marine Sanctuary. But an electronic protest movement conducted over the marine science bulletin board run by Internet - the information 'super-highway' used by academics around the world - has halted the project in its tracks. After questions were raised in the US Congress, the US Government's National Marine Fisheries Services, which must issue a permit for the project to go ahead, last month got cold feet. It demanded that Scripps produce a new environmental impact assessment and it announced public hearings to be held this month.
The two main questions in the debate are these: could the experiment deafen marine mammals such as whales, or prevent them communicating with one another? And is the risk justified by the likely scientific gain?
Scripps, in a public relations blitz aimed at securing the project, says that the noise will be 'about the same intensity as that generated by a large merchant ship'. It argues that most marine mammals cannot hear in the frequency range of 60-90 hertz that the transmitters will use. The transmitters, at a depth of 3,000 ft, will be 'well below where these mammals are found'. And beyond a mile or two of the transmitters, the hum will be indistinguishable from the background noise of the oceans.
Some scientists are not convinced. A statement signed last month by 15 marine biologists from the US and Europe said that the project should not go ahead. It claimed that 'our information on the hearing sensitivities and on the diving abilities of most marine mammals is limited, and we do not yet know what the subtle long-term effects of noise might be'.
The bulletin-board furore was begun by Lindy Weilgart, a biologist at Cornell University, New York. She says that 'the effects could range from gradual deafening and extinction of whole whale species to virtually nothing at all. The difficulty is what we do in the face of that lack of knowledge. I argue the precautionary principle, but sadly some scientists see that as an extreme view.'
She points out that the first two transmission areas, where the noise will be greatest, both have large populations of whales. Point Sur in California, she says, is 'probably one of the richest areas for marine mammals in the world'. Kauai in Hawaii, she maintains, is a 'critical breeding area for humpback whales'.
Part of Weilgart's concern is the frequency of the hum. 'The comparisons with supertankers are ridiculous,' she argues. 'These vessels make most noise at frequencies that no mammal can hear, right down at two hertz (human hearing ceases at about 30 hertz). The transmitters will put out as much noise as the world's entire fleet of supertankers does, but at 70 hertz. Some species can certainly hear at those sort of frequencies.'
The Atoc team has promised that if it discovers any harmful effects on whales swimming near the transmitters, the project will be stopped. But another opponent, Hal Whitehead of Dalhousie University in Nova Scotia, Canada, is sceptical. 'Who decides?' he asks. 'They have so much invested in this project that it will be very difficult to stop. And anyway, harmful effects might only show up many years down the road.'
Lindy Weilgart agrees: 'The project is a big gamble, given how little we know. What I don't understand is what's the hurry? It will be 50 years before they get any conclusive results about global warming, anyhow.' She complains that the full weight of the academic establishment has been assembled in an attempt to frighten her into silence. 'They play real hard ball. I worry about my career. What is so unfair is that they pretend I am alone, and they question my motives. And yet it is they who have a conflict of interests.'
The project has certainly polarised marine scientists. Many biologists are attracted by the dollars 3m ( pounds 2m) that the project intends to spend on research into the behaviour of marine mammals exposed to the noise of the transmitters. But several working at the US's leading research centre, the National Oceanic and Atmospheric Administration (Noaa) spoke of their disquiet at the experiment and their scepticism about it yielding useful results in time to make decisions about global warming.
The row was heightened by the publication earlier this year of a report from the US National Research Council, underlining the lack of knowledge about what marine mammals hear, and how. The panel did not take a formal view on the Atoc project, but it did say that 'the data (on hearing in marine mammals) are extremely limited and cannot constitute the basis for informed prediction or evaluation of the effects of intense low-frequency sounds'.
David M Green of the University of Florida in Gainsville, who chaired the panel, says it was most worried about the sensitivity of the baleen whale. 'There is good reason to believe that the baleen is more sensitive to the low frequencies,' he says. 'Its hearing may be most sensitive at about 100 hertz. But there is considerable uncertainty.'
Britain's leading researcher into whales is Margaret Klintowka of Cambridge University's Physiology Laboratory, who angered animal rights activists by saying that whales are no more intelligent than cows. She dismisses the fears of the project's critics, saying: 'The uncertainty level is pretty low. There is unlikely to be any harm done. The decision to put off the experiment is a political one.'
The major environment groups, caught between their desires to protect whales and to push for action on global warming, have been reluctant to join the debate. Greenpeace's specialist on whales, Kieran Mulvaney, said: 'We have not taken an active position on this.'
Atoc is the first phase of a global programme of acoustic monitoring that will form a large part of the Global Climate Observing System, which is run by UN agencies and was endorsed at the Earth Summit in Rio two years ago. The next step, says Atoc's Andrew Forbes, is likely to be monitoring the Atlantic.
John Gould at the Institute of Oceanographic Sciences has joined the project committee on behalf of Britain to prepare for the Atlantic phase. He says that, 'from a climate point of view, the Pacific is a backwater. The Atlantic is far more important for monitoring global warming.'
The world's oceans absorb much of the carbon dioxide, the main gas responsible for global warming in the atmosphere. Currents then take the gas to the ocean bed for hundreds of years. Most of this burial process happens in the north Atlantic, and virtually none in the Pacific.
Enthusiasm for monitoring the temperature of the Atlantic increased early this month when Gould's colleague, Harry Bryden of the Chilworth Research Centre in Southampton, reported that research ships had found a sharp rise in the temperature of the north Atlantic over the past 35 years. But this raised again the question of whether Atoc was essential to taking the temperature of the oceans.
'Taking data from ships may not be so much fun as sending shock waves through the ocean,' Lindy Weilgart says, 'but surely it is better than running the risk of damaging whales - at least until we can be sure what will happen. Didn't global warming get to be a crisis because we took exactly that approach, refusing to accept that we might have a problem until it was too late?'-
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