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Southern Ocean rise due to warming, not ice

Rises in the sea level around Antarctica in the past decade are almost entirely due to a warming ocean, not ice melting, says an Australian scientist leading a major international research program.

The finding comes after a US scientist warns that climate change could make Antarctica's ocean temperatures warm enough for sharks, which would threaten the region's unique marine life.

The 15-year study of temperature and salinity changes in the Southern Ocean found average temperatures warmed by about 0.3°C.

Satellites also measured a rise of about 2 centimetres in seas in the southern polar region over an area half the size of Australia, says Dr Steve Rintoul, leader of an Australian-French-US scientific program.

"The biggest contribution so far has been from warming of the oceans through expansion," says Rintoul, from CSIRO Marine and Atmospheric Research.

Melting sea ice or Antarctic ice shelves jutting into the ocean do not directly add to sea level rises.

Leaving Australia

Rintoul was speaking as French ship L'Astrolabe prepared to depart 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 metres along the 2700 kilometre, 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 says.

The project leader says sea level rise is not uniform in the Southern Ocean and that rises are not guaranteed to continue at the same rate in the future.

Seasonal changes

The study also shows that the Southern Ocean's uptake of carbon dioxide changes with the seasons.

In summer, an increase in phytoplankton brought about by the greater light causes the Southern Ocean to absorb more carbon dioxide from the atmosphere than in colder months, he says.

The study shows that as waters warm, some species of phytoplankton are extending further south, although more research is 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 says.

Warming luring sharks

In related news, a US biologist warns that global warming could bring sharks to Antarctic waters, threatening a unique marine life shielded from predators by frigid conditions for millions of years.

Biologists gathered for the annual conference of the American Association for the Advancement of Science warn that the return of predators to Antarctica could prove devastating to its underwater ecosystem.

Antarctica's surrounding waters remain too cold for sharks and other fish capable of crushing shellfish similar to the molluscs living in the vast continent's seas, says University of Rhode Island biology professor Cheryl Wilga.

"As a result, the Antarctic sea floor has been dominated by relatively soft-bodied, slow-moving invertebrates, just as in ancient oceans prior to the evolution of shell-crushing predators," she says.

But global warming has already pushed temperatures up by one to two degrees in the past 50 years, and the waters could become hospitable to sharks within the next 100 years, she says.