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No. 489:
Heard Round the World

Today, we fire a shot, and it's heard round the world. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

If you're my age, you'll remember an old song lyric:

How deep is the ocean? How high is the sky?

Well, I'll tell you how deep and how high. Relative to your 16-inch globe, the ocean is no deeper than the layer of blue paint that represents it. The sky reaches only 1/32 of an inch above the surface. Our water and air are so thin and fragile!

That brings us to the question of global warming. When the composition of our thin atmosphere changes, the flow of solar energy through it changes just a little. Yet it's enough to affect the ocean -- that mere layer of paint upon our Earth.

So if we could measure overall changes in the ocean's temperature, we'd know how much warming has occured. People at the Scripps Institution in La Jolla are trying to do that. And they're using the thinness of the ocean.

The ocean averages a little over two miles deep. The density of its water varies with depth. The salinity, temperature, and pressure all change. And they change the density. Finally, the density of water determines how sound travels through it.

Those density variations do a strange thing to the spread of sound in the ocean. Sounds go out in all directions in air. But sound that originates at a particular ocean depth spreads only outward. Sound stays in a plane, if it has a certain density pattern. Sound also travels five times faster in water than in air. And it doesn't attenuate as fast.

So they'll lower a loudspeaker near an island in the South Indian Ocean. They'll broadcast a low-frequency tone. Listening posts will pick up the sound three hours later, when it reaches all the way to America. It'll be very weak but still perceivable. Understand: this is no radio signal. It's pure acoustic sound. We'll literally hear halfway round the world.

The precise speed of the sound reflects a global average temperature in the ocean. By doing the experiment each year, we'll find out how much Earth is really warming.

The experiment bends my mind. It stretches my perception of both size and sound. The oddest wrinkle of all is that this stratum in the ocean is a communications channel for whales. They use it to sing to each other over huge distances. We've already garbled the channel with submarine noise and geophysical soundings. Now this! There's a danger that this strange experiment will interfere with the whales' party line.

Finally, you won't believe the name of the island that's going to be heard all the way to America. It's name is H-E-A-R-D -- Heard Island.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Browne, M.W., Ocean Loudspeakers to Sound Off for Data on Global Warming. The Environment, The New York TImes, Tuesday, Nov. 5, 1990, p. B9.