Science Matters – Antarctica’s Unstable Ice Sheet

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Late last year scientists at the Halley VI British Antarctic Survey station in west Antarctica were forced to abandon their premises.

An advancing chasm in the Brunt Ice Shelf nearby was threatening to cast them adrift on an iceberg roughly the size of Wales.  This chasm, undoubtedly caused by global warming, is an alarming indication of ice-sheet fragility.

But does it have implications for the whole of Antarctica?

Several computer models predict that ongoing melting of the Antarctic ice-sheet will cause sea levels to rise by at least a metre by 2100 and three metres by 2300.  So far predictions from different models disagree on the details of this future catastrophic scenario. With an area of 14 million square kilometres, Antarctica is huge – much larger than either the US or Europe.  So, like these continents, climatic conditions are likely to vary enormously between east and west coasts.

Now a team of US scientists has reconstructed the history of the Sabrina Coast continental shelf in Antarctica, some 5,000 kilometres east of Halley VI. Using a method called ‘seismic reflection profiling’, they studied 1,300 metre deep sediments deposited by glaciers that have drained east Antarctica since the ice-sheet formed some 34 million years ago.

The strata within these sediments reveal that for the past five million-years temperatures have been generally cooling, causing expansion of the ice-sheet. But in older, underlying strata, massive tunnel valleys appear, carved out by huge volumes of meltwater flowing along the ice-sheet and depositing large packages of sediment in open water. These features indicate periods of relative warmth and show that the ice sheet has undergone extensive advances and retreats at least eleven times since its formation.

When the tunnels and deposits were created, climatic conditions, including air and sea temperature and CO2 levels, resembled those predicted if today’s global warming continues. But previously these conditions have naturally reversed and the ice-sheet has extended again.

So clearly details of the response of Antarctica’s ice sheets to past warming cycles are relevant to the present-day situation.

PIC:

Antarctica is changing as a result of global warming. Photo: Jason Auch. no_a08Antarctic01