Study suggests future sea-level rises may be even higher than predicted

A new study of past sea levels shows that they rose by an average of 1.6 metres every one hundred years the last time the Earth was as warm as it is predicted to be later this century, with levels reaching up to six metres above those seen today. The findings suggest that current predictions of sea-level rises may be too low.

The study by a consortium of scientists from the National Oceanography Centre, Southampton and research centres in Tübingen (Germany), Cambridge and New York, is published this week in the new journal Nature Geoscience.

The rate of future sea level rise is one of the crucial uncertainties in projections of future climate warming. During the last interglacial (124 to 119 thousand years ago), also known as the Eemian or Marine Isotope Stage 5e, the Earth's climate was warmer than it is today, due to a different configuration of the planet's orbit around the Sun.

It was also the most recent period in which sea levels reached around six metres (20 feet) above the present, due to melt-back of ice sheets on Greenland and Antarctica. The new results provide the first robust documentation of the rates at which sea level rose to these high positions.

Lead author, Professor Eelco Rohling of the University of Southampton's School of Ocean and Earth Science, based at the National Oceanography Centre, said: 'There is currently much debate about how fast future sea level rise might be. Several researchers have made strong theoretical cases that the rates of rise projected from models in the recent IPCC Fourth Assessment are too low. This is because the IPCC estimates mainly concern thermal expansion and surface ice melting, while not quantifying the impact of dynamic ice-sheet processes. Until now, there have been no data that sufficiently constrain the full rate of past sea level rises above the present level.

"We have exploited a new method for sea level reconstruction, which we have pioneered since 1998, to look at rates of rise during the last interglacial. At that time, Greenland was 3 to 5°C warmer than today, similar to the warming expected 50 to 100 years from now. Our analysis suggests that the accompanying rates of sea level rise due to ice volume loss on Greenland and Antarctica were very high indeed. The average rate of rise of 1.6 metres per century that we find is roughly twice as high as the maximum estimates in the IPCC Fourth Assessment report, and so offers the first potential constraint on the dynamic ice sheet component that was not included in the headline IPCC values."

The researchers' findings offer a sound observational basis for recent suggestions about the potential for very high rates of sea-level rise in the near future, which may exceed one metre per century. Current ice-sheet models do not predict rates of change this large, but they do not include many of the dynamic processes already being observed. The new results highlight the need for further development of a better understanding of ice-sheet dynamics in a changing climate.