Antarctica (Bernd F. Laeschke – 29.06.2010): Melting ice in West Antarctica is contributing a substantial and increasing volume to the global sea level rise, a report by the British Antarctic Survey (BAS) concludes. The Pine Island Glacier (PIG) is the major source of fresh water pouring into the oceans, scientists say.
Researchers from the BAS, Lamont-Doherty Earth Observatory (LDEO) and the National Oceanography Centre used an autonomous underwater vehicle called Autosub to dive beneath the Glacier’s floating ice shelf and map the ocean floor. The measurements revealed a submarine ridge rising 900 feet (300 meters) above the sea floor. Scientists say that the Pine Island Glacier was once scraped across this underwater ridge, which slowed its flow.
However, in recent decades it has thinned and disconnected from the ridge, allowing the glacier to move ice more rapidly from the land into the sea. This also permitted relatively warm seawater to flow over the ridge and into a widening inner cavity, more than doubling the ice shelf area exposed to the corrosive ocean. As the melting increases the glacier’s flow accelerates, drawing down the inland ice and moving its vulnerable grounding line (where the ice begins to float) deeper into the interior of the West Antarctic Ice Sheet.
“The discovery of the ridge has raised new questions about whether the current loss of ice from Pine Island Glacier is caused by recent climate change or is a continuation of a longer-term process that began when the glacier disconnected from the ridge,” said lead author Adrian Jenkins, British Antarctic Survey, in a press release. “We do not know what kick-started the initial retreat from the ridge, but we do know that it started some time prior to 1970. Since detailed observations of Pine Island Glacier only began in the 1990s, we now need to use other techniques such as ice core analysis and computer modeling to look much further into the glacier’s history in order to understand if what we see now is part of a long term trend of ice sheet contraction. This work is vital for evaluating the risk of potential wide-spread collapse of West Antarctic glaciers.”
Scientists say that the West Arctic Peninsula is currently contributing about 10 percent of the previously frozen fresh water that causes global sea levels to rise. To put this in perspective, all the fresh water stored in the Arctic ice cap would only cause the oceans to rise 1 millimeter.
“Since our first measurements in the Amundsen Sea, estimates of Antarctica’s recent contributions to sea level rise have changed from near-zero to significant and increasing,” says co-author Stan Jacobs. “Now finding that the PIG’s grounding line has recently retreated more than 30 kilometers from a shallow ridge into deeper water, where it is pursued by a warming ocean, only adds to our concern that this region is indeed the ‘weak underbelly’ of the West Antarctic Ice Sheet. Increased melting of continental ice also appears to be the primary cause of persistent ocean freshening and other impacts, both locally and downstream in the Ross Sea.”