New results from an investigation into Antarctica's potential contribution to sea level rise are reported this week (Sunday 20 June) by scientists from the British Antarctic Survey (BAS), Lamont-Doherty Earth Observatory (LDEO) and the National Oceanography Centre in the journal Nature Geoscience.
Thinning ice in West Antarctica is currently contributing a substantial and increasing volume to global sea level rise, and scientists have identified Pine Island Glacier (PIG) as a major source. As part of a series of investigations to better understand the impact of melting ice on sea level, an exciting new discovery has been made in the Amundsen Sea. Using Autosub (an autonomous underwater vehicle) to dive deep and travel far beneath the PIG's floating ice shelf, scientists working on the research vessel NB Palmer made ocean and sea-floor measurements revealing a submarine ridge rising 300m above the sea floor.
Pine Island Glacier was once scraping 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 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.
Lead author Dr Adrian Jenkins of British Antarctic Survey said: "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."
"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 modelling 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."
Co-author Stan Jacobs adds: "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. Now finding that the PIG's grounding line has recently retreated more than 30 km 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' (T. Hughes, 1981) 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."