Sea rose eight metres in warmer age: study

Sea levels were likely eight metres higher around 125,000 years ago when polar temperatures were 3-5 degrees C warmer, says a new study published Wednesday to show the effects of global warming.

The research by the US universities of Harvard and Princeton was released in the journal Nature as the world's nations met in Denmark to forge a strategy to head off harmful effects of global warming blamed on greenhouse gases.

To understand the potential effects of a rise in temperature, the researchers reexamined data about the last interglacial stage - a warmer period within an ice age - which climaxed about 125,000 years ago, they said.

At the time, polar temperatures were 3-5 degrees C (37-41 degrees F) higher than today, providing a comparison for current scenarios of future rises of 1-2 degrees C, they said.

"We find a 95 percent probability that global sea level peaked at least 6.6 metres (nearly 22 feet) higher than today during the last interglacial," the study said.

"It is likely (67 percent probability) to have exceeded 8.0 metres but is unlikely (33 percent probability) to have exceeded 9.4 metres," it said.

Previous estimates of sea-level rises for the same period had been at a lower 4-6 metres.

"The results highlight the long-term vulnerability of ice sheets to even relatively low levels of sustained global warming," the scientists said.

The researchers also calculated that during the last interglacial period the average sea level rose six-nine millimetres a year compared to around two millimetres a year during the 20th century.

That may have accelerated to around three millimetres a year between 1993 and 2003, at least partly because of the melting of the Greenland and Antarctic ice sheets.

US scientists Peter Clark and Peter Huybers said "the disconcerting message is that the equilibrium response of sea level to 1.5-2 C of global warming could be an increase of 7-9 metres."

"If their results are correct, the sea-level rise over the coming century will be followed by many more metres of rise over the ensuing centuries," they said in a commentary on the research also published in Nature.