The researchers report that the world's total ocean volume is less than the most recent estimates by a volume equivalent to about five times the Gulf of Mexico, or about 0.3% lower than the estimates of 30 years ago.
The results reveal how accurate scientists were in the past, using cruder techniques to measure ocean depth. As long ago as 1888, for example, John Murray dangled lead weights from a rope off a ship to calculate an ocean volume—the product of ocean area and mean ocean depth—just 1.2% greater than the new figure reported in Oceanography.
Starting in the 1920s, researchers using echosounders improved depth estimates significantly. Most recently, satellites have been used to calculate ocean volume.
The trend toward a progressive lowering of volume estimates is not because the world's oceans are losing water. Rather, it reflects a greater ability to locate undersea mountain ranges and other formations, which take up space that would otherwise be occupied by water.
Satellite measurements reveal that ocean bottoms "are bumpier and more mountainous than had been imagined," says Matthew Charette, an associate scientist in WHOI's Department of Marine Chemistry and Geochemistry. As measurements improve, ocean-volume values are lowering, he notes, emphasizing that this does not reflect an actual lessening of water but a more accurate accounting of undersea formations.
Satellite-based radar cannot "see" the ocean bottom, he explains. Rather, it measures the ocean surface, which reflects what lies beneath. For instance, if a mountain range lurks under a certain part of the ocean, the surface above it will bulge outward.
The satellite project has covered virtually all the world's oceans, except for some areas of the Arctic that are covered with ice. The result is a "new world map" of the oceans.
But satellite measurements have their shortcomings. "There is a problem of spatial resolution, like an out-of-focus camera," says Walter H.F. Smith, a geophysicist at the National Oceanic and Atmospheric Administration. "We're measuring the sea surface that is affected by mountains," he says, "but we're seeing only really big mountains, and in a blurry way. The resolution is 15 times worse than our maps of Mars and the moon."
Consequently, the researchers say, more ship-based measurements are needed to augment and "fine tune" the satellite data. And so far, ship-based sonar and other instrumentation have mapped only 10% of the Earth's seafloor. "We have gaps in echosounding measurements as wide as New Jersey," says Smith.
It would take a single ship 200 years (or 10 ships 20 years) to measure all the ocean-floor depths with an echsounder, according to published U.S. Navy estimates. "That would come to about $2 billion," Smith says. "NASA is spending more than that on a probe to [the Jupiter moon] Europa."
Charette and Smith are not sure why so little ship-based ocean mapping has been done throughout history. It may be because ocean depth and volume seem to have few direct, practical implications.
However, Charette notes, accurate estimates of ocean depth and volume could tie in with the growing field of ocean observation and exploration as well as, perhaps, climate change models and estimates of salt in the oceans. The study's calculation of the ocean's mean depth is 3,682.2 meters — 21-to-51 meters less than previous estimates.
Citation: Matthew A. Charette, Walter H. F. Smith, 'The Volume of Earth’s Ocean', Oceanography, May 2010