An Antarctic glacier has come loose from the underwater ridge it was sitting on allowing ice to melt more rapidly according to findings from an automated underwater vehicle developed in the UK.

Thinning ice in West Antarctica is contributing nearly 10% of the global sea rise – with the Pine Island Glacier (PIG) thought to be a major source. Scientists sent the Autosub – which was developed and built at the National Oceanography Centre in Southampton – underneath the glacier’s floating ice shelf to take ocean and sea-floor measurements.

The Autosub revealed a 300m high ridge on the sea floor, which the Pine Island Glacier had been grounded on. This slowed the glacier’s flow into the sea, but since the ice has thinned, the glacier has become disconnected from the ridge, allowing ice to melt more rapidly.

“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 Dr Adrian Jenkins from the British Antarctic Survey, “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.”

Detailed observations of Pine Island Glacier began in the 1990s, so Jenkins believes other techniques like ice core analysis and computer modelling must be used to look further into the glacier’s history to understand if the ice sheet contraction is part of a long-term trend.

“Estimates of Antarctica’s recent contributions to sea level rise have changed from near-zero to significant and increasing,” co-author Stan Jacobs said, “Now finding that the PIG’s grounding line has recently retreated more than 30km 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.”