Melting moments put ice sheets in a new light

Jo Chandler 

The Age, March 15, 2012

SOME 400,000 years ago, in the long, warm interglacial of the Middle Pleistocene, the world's oceans rose dramatically, but not, it seems, quite as dramatically as first thought.

The finding, published in the journal Nature today, provides new insight into some of the most contentious and crucial questions in modern science - the stability of the great ice sheets in a warmer world, and the implications for sea levels when the slumbering giants of the Arctic and the Antarctic are disturbed.

Fresh analysis of readings taken from ancient shorelines around Bermuda and the Bahamas now peg sea level peaks in that era at somewhere between six and 13 metres above the present shorelines. Previous estimates had put it at more than 20 metres higher.

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The revised findings, published by scientists from Columbia and Harvard universities, come as a result of adjustments taking into account post-glacial subsidence of the sites over time.

What the lower level of sea-level rise suggests, they say, is that both the Greenland Ice Sheet and the West Antarctic Ice Sheet collapsed during the protracted warm of the Pleistocene, when the dance of the Earth's orbit increased the solar radiation that the planet received.

But the revised level indicates that the East Antarctic Ice Sheet, the behemoth slab of ice spread over 11 million square kilometres and an average two kilometres thick, did not melt significantly through that long period of warming.

This greatly interests glaciologists investigating the future stability of Greater Antarctica in response to human-induced warming.

It is the second Nature paper to emerge this week throwing new light on the mysteries of the ice sheet dynamics.

On Monday, researchers from Spain and Germany revealed findings that Greenland's ice sheet is much more sensitive to global warming than previously thought, and may already be approaching a critical threshold.

They calculated that if global average temperatures reach 1.6 degrees above pre-industrial levels - and they have already warmed 0.8 degrees - the Arctic ice would likely tip towards irreversible loss.

The significance of the two papers, says Dr John Church, a CSIRO specialist researcher in climate and sea level change, was the reinforcement of the idea that in interglacial periods, the Greenland ice and perhaps also the West Antarctic Ice Sheet were susceptible to decay. This had now been borne out in several investigations.

Nonetheless history cannot help anticipate the ice's response to a unique twist of human-induced atmospheric conditions.

Carbon dioxide levels are much higher now - and rising - than they were in historic warm periods. Then, they were in the realm of 280-300 parts per million. Today they are heading past 390ppm.