A new study using seven decades of temperature data shows that mid-depth water around Antarctica has warmed nearly twice as much as the world ocean as a whole. That wasn't supposed to happen.
Geophysicists expect global warming to be strongest in polar regions. However, as Sarah Gille at the University of California, San Diego, explains: "We thought the ocean between 700 and 1,100 meters [2,300 and 3,600 feet] was pretty well insulated from what's happening at the surface. But these results suggest that the mid-depth Southern Ocean is responding and warming more rapidly than global ocean temperatures [generally]." How this unexpected finding fits into global-warming forecasts is unclear, but it could be significant.
Professor Gille notes that the Southern Ocean "is a very climatically sensitive region." It is at one end of the conveyor-belt circulations that carry heat poleward in upper-level currents and return cold water equaterward at great depths - a key part of the system that maintains Earth's present climate. Any change in Antarctic waters could directly affect circulations in the Atlantic, Indian, and Pacific Oceans.
The cold surface current that girdles Antarctica is a fixture of the Southern Ocean. It may already have been affected. Gille, who reports her findings in the Feb. 15 issue of the journal Science, says her study suggests that this current has moved closer to the polar continent as the mid-depth water has warmed.
Much of Gille's data was gathered by ships. However, free-floating robots also sent important information as part of the international World Ocean Circulation Experiment in the 1990s. The robots sink to a preset depth and drift with currents for 10 to 25 days. Then they bob to the surface to transmit data. Gille's analysis found that the mid-depth water had warmed 0.17 degrees C since 1950 - nearly double the global trend for ocean warming.
Gille's finding adds more complexity to the Antarctic climate puzzle in which changes are showing up in unexpected, even contradictory ways. Last month, for example, a research team reported that lakes on Signy Island off Antarctica have warmed by three to four times the global average air temperature rise. At the same time, another team reported that dry valleys on the continent itself are cooling. Likewise, a team reported in December that radar measurements show several West Antarctic glaciers are thinning rapidly. Yet, in January, another team reported that other West Antarctic ice streams are growing thicker.