Two papers in Nature today shed light on the possible future behaviour of the West Antarctic ice sheet (WAIS), complete loss of which would produce a worldwide rise in sea level of around 5 metres.
The two teams – one using a high-resolution ice sheet model, the other looking at glacial records contained in seafloor sediment – independently arrive at similar conclusions: The WAIS has intermittently melted during the past five million years or so, and its oscillations follow a 40,000 year cycle in the Earth’s axial tilt. Small variations in tilt – called the obliquity of the ecliptic – result in reduced or increased amounts of sunlight reaching the poles, thus pacing the succession of ice ages and warm periods.
During the warmest interglacial phases the WAIS has in the past episodically collapsed entirely, the studies suggest (Editor’s Summary) Global temperatures around 3 degrees Celsius warmer than today seem to have sufficed to initiate the transition from grounded ice to open waters in the Ross Bay, reports the team led by Tim Naish of Victoria University in Wellington, New Zealand, who analysed a sediment core recovered from beneath the Ross ice shelf by the ANDRILL programme. Model simulations suggest the transition from full glacial to intermediate state (such as today’s) to nearly ice-free conditions can proceed rapidly. In the warmest ‘super-interglacials’, such as one around 1.07 million years ago, it took only around thousand years for the WAIS to collapse, report David Pollard and Robert DeConto of Pennsylvania State University in the second study.
The currently observed retreat of the WAIS began in fact some 15,000 years ago. Is it likely then that it will come to a full collapse at some point during the rest of the current interglacial, which, not least because of our own activities’ impact on climate, might last longer and get even warmer than usual?
Alas, it is. At the current rate of greenhouse gas accumulation, three degrees warming could well materialize by the end of the century. It might take longer for the ocean surrounding Antarctica to warm by 5 degrees, which Pollard and DeCanto reckon is required to generate enough sub-ice melting to trigger collapse. Even so, the WAIS is in peril.
The consequences of a full demise would be rather unpleasant. A 5 metre rise in global sea level would flood lands and cities home to hundreds of millions of people. Add Greenland, whose ice cap is even more vulnerable to climate warming, and global sea levels could rise up to 12 metres: What this would mean for human civilization is everybody’s guess.
Now, apocalypse is fortunately not yet round the corner. Neither our children nor our grandchildren will probably have to abandon London, Tokyo or New York, or will need to rebuild them further inland. As things stand, global seal level rise exceeding one metre by the end of the century – which would be a lot, though – is unlikely.
But we’d better start thinking anyway. The West-side story of Antarctic ice – as Philippe Huybrechts explains in his so-entitled News and Views article – is a strong reminder that “the prospect of sea-level rise is probably the most serious long-term threat from unabated climate warming.”