The birth of a big iceberg, Antarctica

Rift developing across the pine Island Glacier
From bbc.co.uk

About every ten years the Pine Island Glacier (PIG) on the West Antarctica coast sheds a giant iceberg. The last event was in 2001, and so it's time. Sure enough, "right on schedule" a 30 km long crack is developing across the tongue of the glacier, and a 880 sq. kilometer (300 sq. mile) iceberg is expected to break loose within a few months.  Although scientists are not attributing this to climate change because of the history of such events, there has been a significant thinning of the PIG, which may be.  It is about 60-100 m down to the water surface, but the crack may extend underwater to a depth of 500 m.

Typically glaciers like this end in ice shelves that are grounded against the sea floor near the coast.  However warm seawater has been eating away at the underside of the PIG shelf, so that it has been floating free. This allows a feedback in which more and more melting occurs.  A floating ice shelf is much more vulnerable to breaking off than a grounded one.

The Pine Island glacier is a major outlet for the West Antarctic Ice Sheet, and a potential significant contributor to long-term sea-level rise. It drains about 10% of the West Antarctic Ice Sheet. It is a fast-moving glacier, and its motion has been accelerating recently. Between 1974 and 2007 the speed increased by 73%. By the end of 2007, the PIG had a negative mass balance of 46 gigatonnes per year (that is, more water was going into the sea than was being replaced by snow), which would translate to 0.13 mm global sea level rise per year.  Once this piece breaks off, the snout of the PIG will be the furthest back since monitoring of it started in the 1940's. It will be interesting for scientists to monitor it for the next decade to see if it follows its old pattern of growing back out, or stalls.