Undersea Ears Sound Out Data on Global Warming
An underwater acoustic signal was heard around the world this week, marking a milestone in an experiment that a UC San Diego researcher hopes will make it easier to detect warming of the world’s oceans from the greenhouse effect.
Sounding like an underwater foghorn, five transmitters suspended 600 feet below a Navy ship in the southern Indian Ocean began booming every three hours, for one hour at a time, last Saturday.
As had been predicted in 1988 by noted UCSD oceanographer Walter Munk, scientists using electronic ears could detect the sound waves as far as 18,000 kilometers (about 11,000 miles) away, on both U.S. coasts. It took about 3 1/2 hours for the sound to travel that far.
Over the next few months, further analysis will reveal whether the signals sent from near Heard Island are clear enough to calculate the average temperature of the ocean to within a few thousandths of a degree. Because sound travels more quickly in warmer water, temperature differences can be calculated based on how long it takes the sound to reach various receivers around the world.
In theory, the experiment could settle the argument over whether carbon dioxide and other greenhouse gases are warming the planet artificially.
“We don’t know yet whether it’s good enough to do the global warming experiment, but we’re very encouraged because the signals are very strong in some locations--much stronger than we had expected,” said Robert C. Spindel, who heads the experiment’s command center at the University of Washington.
Still, it proved an idea that, when Munk proposed it, seemed as fantastic as sending pictures from Mars would have seemed in the 1950s.
“That’s not a bad analogy at all. It’s really astonishing that they can drop a simple hydrophone in the water in the south Indian Ocean and pick it up in Coos Bay, Ore. That’s a stunning result,” said Robert Park, a physicist at the University of Maryland and an official of the American Physical Society.
Because water gains heat less quickly than air, small changes in ocean temperatures are considered a more reliable indicator of global change than air temperatures.
“In global warming, we don’t yet have a smoking gun,” said Richard C.J. Somerville, director of climate research at UCSD’s Scripps Institution of Oceanography. “The Heard Island experiment is the start of an effort that after several years of observing may tell us whether the ocean is warming up. If it shows the fingerprint of the greenhouse theory, then we’ll have a very big step toward convincing evidence that we are changing the Earth’s climate.”
Although NASA scientist James Hansen and others believe rising average air temperatures are sufficient proof, the Bush Administration has declined to act on their conclusions, saying more evidence is needed.
In contrast, policy-makers worldwide have moved to cut emissions of chlorofluorocarbon gases because the ozone hole provides clear proof of that atmospheric problem.
With assistance from the U.S office of Naval Research and other U.S. agencies, the experimenters are using acoustic techniques developed over the last decade over the longest distances ever tried.
“It wasn’t until we learned enough about how to do this over short distances that we could do it over long distances,” Spindel said.
The sound signals are emitted near Heard Island and monitored at receiving stations on ships and offshore stations operated by scientists from several countries. These include Soviet, French, Indian, Canadian, New Zealand, South African, Australian and Japanese researchers.
Although readings began Jan. 26, equipment trouble and an Antarctic storm over the last two days have limited observations to only five days so far. The experiment will continue for several more days before analysis of the data at the University of Michigan, Spindel said.
He noted that the Southwest Fisheries Science Center in La Jolla is closely monitoring the experiment for signs that the sounds are disturbing whales and other marine mammals. The experiment will be stopped if there are any problems noted, but none have been so far, he said.
“There are more marine mammal biologists down there at Heard Island right now than there are acoustics people, by a factor of almost 3 to 1. So we’re very serious about this,” Spindel said.
The marine mammal monitoring is under the supervision of Doug DeMaster at the La Jolla center, operated by the National Marine Fisheries Service.
Already highly regarded for his research, Munk, 73, regards the Heard Island experiment as his “last chance” to do something significant for science. In a 1988 interview, he laughed at the idea as a “romantic notion” that skeptics said probably wouldn’t work.
This week, he is one of two acoustic oceanographers bouncing from wave to wave in an Antarctic storm, and finding out that it wasn’t such a bad idea after all.
Measuring The Greenhouse Effect A sound signal broadcast underwater from Heard Island in the Indian Ocean was used to measurethe greenhouse effect. From a ship near that island, the sound reached in a straight line boththe east and the west coasts to North America.
