Southern Ocean rise due to warming, not ice melts

SYDNEY - Rises in the sea level around Antarctica in the past decade are almost entirely due a warming ocean, not ice melting, an Australian scientist leading a major international research program said. The 15-year study of temperature and salinity changes in the Southern Ocean found average temperatures warmed by about three-tenths of a degree Celsius. Satellites also measured a rise of about 2 centimeters (about an inch) in seas in the southern polar region over an area half the size of Australia, Rintoul told Reuters.

"The biggest contribution so far has been from warming of the oceans through expansion," said Steve Rintoul, Australian leader of an Australian-French-U.S. scientific program. Melting sea ice or Antarctic ice shelves jutting into the ocean do not directly add to sea level rises.

Rintoul was speaking as French ship L'Astrolabe prepared to depart on Monday from Hobart, on Australia's southern island of Tasmania, for its fifth voyage of the current summer season for the Surveillance of the Ocean Astral (Survostral) program.

The research program has been taking temperature and salinity readings for 15 years to a depth of 700 meters along the 2,700 km, six-day route between Hobart and the Antarctic. This has produced the longest continuous record of temperature and salinity changes in the Southern Ocean for scientists studying how the ocean contributes to global climate.

"Survostral has given us a foundation for much of what is known about the way the ocean in this inhospitable and difficult-to-access region controls the global climate," Rintoul said. The project leader said sea level rise was not uniform in the Southern Ocean and that rises were not guaranteed to continue at the same rate in the future.

The study had also shown that the Southern Ocean's uptake of carbon dioxide changed with the seasons. In summer, an increase in phytoplankton brought about by the greater light caused the Southern Ocean to absorb more carbon dioxide from the atmosphere than in colder months, he said. The study showed that as waters warmed, some species of phytoplankton were extending further south, although more research was needed to determine the importance of this finding.

"What's significant is that we've detected changes in the physical environment and now we're also detecting changes in the biology in response to those physical changes. The next challenge is to figure out what these biological changes mean for carbon uptake and for higher levels of the food chain," he said.

Tiny phytoplankton are at the bottom of the food chain and are a crucial food source for a number of species. Investigations by the L'Astrolabe in the world's largest ocean current between Tasmania and Antarctica had shown that deep streams of water were taking warming deep into the ocean.

"The program started as just measuring temperature and salinity. We've now recently begun a much more comprehensive chemistry and biology program of measurements," Rintoul said. This would widen the scientific investigation to the impact of climate change on biology and on the carbon cycle, he said.

(By Michael Byrnes, Reuters, 17/02/2008)