Seawater Saltiness Seen from Space
The ocean, by some estimates, holds enough salt to cover the entire surface of the Earth, layered to the height of a 40-story office building.
Much of the salt in the ocean comes from rock that gets eroded by slightly-acidic rainwater. Carbon dioxide in the rainwater chemically breaks down the rocks, flushing dissolved salts — lots of them — into riverbeds, streams and eventually the ocean. Geothermal vents that churn water with dissolved minerals from the Earth’s hot crust are another source of salt. A cubic mile of seawater contains about 120 million tons of salt, according to the U.S. Geological Survey.
In June 2011, NASA launched its first satellite designed to measure ocean saltiness. From more than 400 miles above Earth, the Aquarius mission scans and maps salt in the Earth’s oceans with unprecedented precision. The equipment on the satellite is sensitive enough to detect the equivalent of a pinch of salt in a one-gallon bucket of water.
Last week, the Aquarius team released dazzling new maps that chronicle changes in sea surface saltiness. It showed wide variations in different ocean regions. In the North Pacific, for example, heavy rainfall dilutes the seawater’s saltiness. And in South America, a wonderful plume of freshwater pulses from the Amazon River and into the sea. The saltiest patch of water stretches across the North Atlantic Ocean, where little rainfall and lots of evaporation occurs.
Freshwater is constantly getting sucked out of the ocean by evaporation and rising as water vapor into the atmosphere, where it condenses and then gets dumped back into the ocean as rainfall. And as changes in climate cause the rates of evaporation and rainfall to change and intensify, so do variations in ocean saltiness.
Measuring how salt is distributed throughout the ocean can tell us a great deal about climate: By tracking salinity, scientists can measure changes in the freezing and melting of sea ice, rainfall, evaporation, ocean temperature and circulation — all crucial factors for understanding the planet’s water cycle.
“When we think about ocean circulation, most people are familiar with currents that move horizontally,” said Gene Carl Feldman, an oceanographer and project manager for the Aquarius mission. “But the oceans are over seven miles deep, and there’s also a three-dimensional circulation of ocean water, which is very critical for helping regulate many things on the planet — most importantly temperature.”
Ocean temperature is critical to understanding global temperatures. In fact, the upper three meters of ocean holds as much heat as the entire atmosphere, Feldman said.
“The ocean is an incredibly efficient heat sink — storing heat and moving it around the planet,” he added. “That’s why Sioux Falls, South Dakota is a hell of a lot colder than Seattle, Washington. The ocean waters around Seattle help moderate the temperature.”
One other thing: Carbon dioxide levels in the Earth’s atmosphere are influenced by ocean salinity and the water cycle. Surface waters that are saltier or colder than surrounding waters are more dense and tend to sink. Elements like carbon dioxide that may be dissolved in those waters will sink into the ocean depths, sometimes staying underwater and out of the atmosphere for thousands, even tens of thousands of years, William Large, director of the climate and global dynamics division of the National Center for Atmospheric Research, told us in an interview shortly before the launch in 2011. Had the ocean not been been taking up tremendous amounts of carbon dioxide, carbon dioxide levels in the atmosphere would be much higher, he said.
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Rebecca Jacobson, Patti Parson and David Pelcyger contributed to this report.
*Correction: An earlier version of this story incorrectly identified the location of the Amazon River.