The Year in Science: Oceanography

Greenhouse gases turn oceans acidic, and the secret of milky seas.

Sunday, January 08, 2006

Greenhouse Gas Makes Oceans Acidic and Dissolves Marine Life

Increasing levels of carbon dioxide in the atmosphere are turning the oceans more acidic and may endanger marine life, according to a report released in September. An international team of 27 oceanographers churned through 13 global models and concluded that carbon dioxide emissions could cause pH levels in the ocean to drop from an average of 8.1 today to 7.7 by the end of the century. That may doom small but essential sea creatures.

As seawater becomes more acidic, it dissolves carbonate minerals, which many plankton require to build their shells. The researchers found that if carbon emissions continue to rise at the current rate, the entire Southern Ocean and part of the North Pacific will be so corrosive by the year 2100 that many calcifying plankton will be unable to grow properly; others may dissolve outright. The worst-case scenarios move up the timetable to the year 2050.

"We know with certainty what's going to happen to the seawater chemistry," says Victoria Fabry, a biological oceanographer at California State University at San Marcos. "What we don't know for certain is the impact that will have on the biology." In an experiment, Fabry exposed planktonic snails called pteropods to seawater with a level of acidity matching that predicted for the Southern Ocean in 2100. "The shells were dissolving before my very eyes," Fabry says.

Conditions may only worsen after 2100, warns Ken Caldeira, a chemical oceanographer at the Carnegie Institution. "Our model predicts that if we don't do anything, within a few centuries we will produce ocean conditions that will prevent coral reefs from existing." —Anne Casselman

Secret of Milky Seas Uncovered

Milky seas have excited seafarers for centuries. Jules Verne, in his 1870 underwater adventure novel, Twenty Thousand Leagues Under the Sea, wrote about a "lactified" ocean. Since 1915 there have been 235 documented cases of vast expanses of water—from horizon to horizon—emitting a uniform white glow in the dark of night.

With so many sightings, scientists have not doubted the existence of luminescent waters. But given their unpredictable nature, such seas have remained mysterious and elusive. In October Steven Miller, a researcher in the marine meteorology division of the U.S. Naval Research Laboratory, announced that he and his team had matched 1995 satellite data to a firsthand account from a British merchant ship, the SS Lima, and found a section of the Indian Ocean that glowed for three nights in a row. The surprise? Its size. The glow covered 5,945 square miles off the Horn of Africa, an area about the size of Connecticut.

Although no one is certain what causes the water to glow, scientists have a good idea. In 1985 a research vessel happened through a luminous part of the Arabian Sea and took samples. The researchers ruled out dinoflagellates, copepods, and other plankton known for luminescence, but their samples were taken at a depth of about three yards. The glow, according to reports, appears to be confined to the surface, like an oil slick. Some samples taken from a small net provided a significant clue, says David Lapota, a marine biologist at the Space and Naval Warfare Systems Command who was on board. "We're pretty sure that the bacteria we brought back were the causative guys, which was the Vibrio harveyi bacteria." The next step: real-time satellite images that can be used to dispatch research vessels immediately. —Anne Casselman

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