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Antarctic glacier - Source: Reuters -
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A lot of Antarctic ice looks set to melt over the next 100 years, with the entire West Antarctic ice sheet likely to melt within 1,000 years, scientists say.
The meltwaters, and thermal expansion as the world's oceans warm, are likely to trigger sea level rises measured in metres, rather than the 59 centimetres currently predicted by many climate scientists.
When similar melting already under way on the Greenland ice shelf is taken in to account, average sea levels are likely to be 12 metres higher than today - over a long period - with 5m of that rise coming from West Antarctica.
The latest research is based on analysis of a 1280m-long core taken from the sea floor under the Ross Ice Shelf by New Zealand drillers working for the $30 million Antarctic Geological Drilling (Andrill) research programme. It was published today in the Nature scientific journal.
A team of researchers led by Professor Tim Naish, director of Victoria University's Antarctic research centre, has shown that when atmospheric carbon dioxide concentrations reached 400 parts per million (ppm) - around 4 million years ago - the globe warmed up enough to enhance an existing effect caused by variations in the Earth's tilt towards the Sun.
This was enough to trigger sheet fluctuations in the size of the West Antarctic ice sheet. Now the carbon dioxide concentration in the atmosphere is again approaching 400 ppm.
Even carbon dioxide concentrations in the atmosphere only slightly higher than today could affect the stability of the West Antarctic ice sheet, according to the 50 researchers, who come from the USA, Italy and Germany, as well as New Zealand.
The findings refine previous understanding about the linkages between atmospheric carbon dioxide concentration, ocean temperature, sea-level rise and the natural cycles in Earth's orbit.
"Changes in the tilt of Earth's axis has played a major role in ocean warming that has driven repeated cycles of growth and retreat of the West Antarctic ice sheet for the period in Earth's history between three and five million years ago," said Naish.
For hundreds of thousands of years during the Pliocene era, between 1.65 million years and 5.2 million years ago, the West Antarctic was too warm to retain its ice cover, and now there is a risk that a significant body of permanent Antarctic ice could be lost within the next century.
Andrill data combined with computer models of ice sheet behaviour shows collapse of the entire West Antarctic Ice Sheet is likely to occur over the next 1000 years.
Other recent studies show that melting has already begun.
A related study led by David Pollard of Pennsylvania State University in the United States, also published today in Nature used a computer model of the ice sheets to show that when the West Antarctic ice sheet collapsed, the margin of the East Antarctic Ice Sheet also melted and the meltwater from both lifted sea levels an average of 7m.
The UN's Intergovernmental Panel of Climate Change (IPCC) has said its most optimistic model shows CO2 could reach 549 ppm by 2100, or roughly 50% more than today.
The IPPC has made projections of sea level rises ranging from 18cm to 59cm this century, but has noted that if the Greenland ice sheet melts proportionally to warming rates, then sea levels would rise by up to 79cm this century, and eventually by 7m.
Now the climate scientists have hard data on how the West Antarctic ice has previously melted.
Though meltwater from the West Antarctic will eventually lift global sea levels by an average of 5m - the impacts will vary: levels may actually fall close to Antarctica but increase by 7m in the northern hemisphere.
The bigger ice sheet in East Antarctica will likely remain, though there is a debate around how stable that will be, according to Naish.
