Global warming and melting glaciers

Retreating glaciers in Antarctica and rising sea levels are a trap we have fallen into by pumping greenhouse gases into the atmosphere, writes Stan Jacobs

A joint US-UK expedition to the Amundsen Sea has revealed that the floating ice shelf extension of Antarctica's accelerating Pine Island Glacier was melting 50 per cent faster in 2009, than in 1994. The authors, from the Lamont-Doherty Earth Observatory and the British Antarctic Survey, found that a stronger circulation of warmer ocean currents thinned the ice - which has recently lifted off a previously unknown submarine ridge, reducing friction with the sea floor and allowing the grounded PIG to slip more rapidly into the sea.

We frequently see pictures of large icebergs calving, ice shelves disintegrating and ice caps melting, but this study documents a process hidden from sight - hundreds of meters below the sea surface. The PIG ice shelf is now larger than it was in 1994, and experiences little topside melting. But repeated measurements of ocean properties and transports, using ship-deployed instruments, showed increasing basal melting and an evolving sub-ice cavity. This work required persistence by the investigators, along with the National Science Foundation and National Environmental Research Council, as several attempts to reach the PIG - after 1994 - were unsuccessful.

Changes in the PIG are important because it is viscerally connected to the interior of the West Antarctic Ice Sheet. Unlike most of East Antarctica and Greenland, West Antarctica is anchored in the ocean - which is absorbing more than 80 per cent of "global warming". While that warming seems to have played only a supporting role to stronger currents above the sub-ice ridge, other ice shelves are similarly exposed to the depth-dependent melting potential of corrosive seawater. Even without ocean erosion, the PIG and adjacent Thwaites Glacier are theoretically vulnerable to sustained retreat since most of their underlying beds - where the ice meets the floor - slope downward toward the centre of the ice sheet.

The Antarctic Ice Sheet budget has long been close to equilibrium, with net snowfall roughly balancing peripheral melting and iceberg calving. It was also expected to grow under a warmer atmosphere, which can carry more moisture and deposit it over vast areas that would remain below freezing year-round. Attrition appears to be increasing faster than accumulation, although, especially in the South East Pacific sector - where slowly warming deep water intrudes onto the continental shelf. This has tipped the balance, with Antarctica now acknowledged as a significant contributor to global sea level rise, strongest in the Amundsen region.

Changes have also been observed and modelled in the large-scale atmospheric circulation over the Southern Ocean, during recent decades. Sea surface pressures have decreased along the Antarctic coastline, deepening migratory lows and increasing the strength of winds around and off the ice sheet. That increases the extent of sea ice and flow of surface water away from the white continent, in turn enhancing the upwelling of warmer deep water, which flows into sea floor troughs that extend beneath the ice shelves. Ice age glaciers that calved those deep troughs have, therefore, set a trap for their eventual demise – a trap we may be baiting by pumping greenhouse gases into the atmosphere.

Stan Jacobs is a senior research scientist at Columbia University's Earth Institute, in the US