How much water on Earth? Exactly 1.33 bn cubic km

For the romantics, love is as deep as the ocean. And now, researchers at the Woods Hole Oceanographic Institution (WHOI) have actually tried to quantify the age-old query— How deep is the ocean?

They’re also tackling an even more intriguing—if less romantic—question— What is the volume of the Earth’s oceans?

And the answer is 1.332 billion cubic kilometres, says Matthew Charette, an associate scientist in WHOI’s Department of Marine Chemistry and Geochemistry who is part of a research effort to audit all the water on the planet.

“A lot of water values are taken for granted. If you want to know the water volume on the planet, you Google it and you get five different numbers, most of them 30- or 40-year-old values,” he said.

Using satellite measurements, the researchers have come with up the new ocean volume figure.

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 500 times the Great Lakes.

While that might seem a lot at first glance, it is only about 0.3 percent lower than the estimates of 30 years ago.

Most recently, researchers have pioneered the use of satellites to calculate ocean volume.

The trend toward a progressive lowering of volume estimates is not because the world’s oceans are losing water.

Instead, 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,” said co-investigator Walter H.F. Smith.

He noted that as measurements improve, ocean-volume values are lowering, 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.

The satellite project has covered virtually all the world’s oceans, except for some areas of the Arctic that are covered with ice, he said.

The result is a “new world map” of the oceans, he added.

“Matt and I are seeing a better picture of the shape and volume of oceans,” said Smith

But satellite measurements have their shortcomings.

"There is a problem of spatial resolution, like an out-of-focus camera. We’re measuring the sea surface that is affected by mountains, 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,” he said.

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 percent of the Earth’s seafloor. ...

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