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Scripps Scientist Discovers Warming Trend in Southern Ocean Over the Last 50 Years
Decline in Antarctic sea ice, carbon dioxide storage, possible implications

An armada of autonomous marine "robots" deployed in the 1990s has helped produce new evidence that the Southern Ocean is warming faster than the rest of the worldÕs oceans. Sarah Gille of Scripps Institution of Oceanography at the University of California, San Diego, has uncovered a warming trend over the last 50 years through a comprehensive comparison of temperature points throughout the Antarctic Ocean.

Gille will discuss these results at the 2002 Ocean Sciences meeting of the American Geophysical Union on Wednesday, Feb. 13. The study will be published in the Feb. 15 edition of the journal Science.

The Southern Ocean, the body of water surrounding Antarctica, has long been known by ocean voyagers as a harsh seagoing destination. However, it plays a key role in global climate conditions. With no continental barriers, the Southern Ocean serves as a conveyor belt, transmitting climatic signals between the Pacific, Atlantic, and Indian oceans.

"It's a very climatically sensitive region," said Gille, an assistant professor at Scripps. "We can think of it as a canary in a coal mine for telling us what may happen to the global climate. What's happening in the Southern Ocean can give us a picture of what could be exchanged into all of the ocean basins and into the latitudes where people live."

Gille's study uses information collected during the World Ocean Circulation Experiment (WOCE). In the 1990s, WOCE researchers deployed a series of Autonomous Lagrangian Circulation Explorer (ALACE) floats, instruments originated by Scripps Professor Russ Davis that sink to a predetermined depth and follow ocean currents for 10 to 25 days. They then rise to the surface to relay their position and temperature information via satellite (Davis has since developed a new generation of floats called Sounding Oceanographic Lagrangian Observers (SOLOs).

Using data from floats that captured roughly 50 cycles of data over two-and-a-half years, Gille paired thousands of ALACE data points geographically with temperature readings conventionally recorded in previous decades from ships.

Gille's comparison revealed no significant temperature changes between the 1930s and the 1950s and the most rapid warming in the 1950s and 1960s. A 0.17-degree Celsius warming she found in the 700- to 1,100-meter depth range after 1950 is nearly double the global trend. Thus, the results imply that the mid-depth Southern Ocean has heated more rapidly than the global ocean as a whole.

"We thought the ocean between 700 and 1,100 meters depth was pretty well insulated from whatÕs happening at the surface and that it should respond more slowly than the shallow depths," said Gille, of the Physical Oceanography Research Division at Scripps. "But these results suggest that the mid-depth Southern Ocean is responding and warming more rapidly than global ocean temperatures."

According to Gille, her study appears to suggest that the cold ocean current that moves around Antarctica, called the Antarctic circumpolar current, may have shifted southward around the continent as part of the warming.

While not a focus of her study, Gille says the implications of the warming include a potential depletion in the volume and stability of sea ice around the Antarctic continent, which carries ramifications for global sea levels, and, because cold water is capable of absorbing more carbon dioxide than warm water, a possible decline in the amount of carbon dioxide that can be stored in the oceans.

"This could mean that the ocean is less able to act as a carbon sink -- more of the carbon dioxide that comes from industrial activities could end up staying in the atmosphere," said Gille.

The study was supported by the National Science Foundation.