Australasian Science: Australia's authority on science since 1938

Ancient Sea Levels Surged

By Stephen Luntz

Sea levels experienced a dramatic surge during the Earth’s last warm period, spelling a warning about what could soon be in store for the world.

“During the last interglacial there was an extended period where sea levels were 3–4 metres higher than today, with climate a little warmer,” says Dr Mick O’Leary of Curtin University’s Department of Environment and Agriculture. “At the end of that period they suddenly jumped to 8.5 metres.”

O’Leary says measurements are not precise enough to determine the period over which this happened: “It could have been 1000 years, but it could have been less than 100”. He hopes better techniques for dating fossil corals will answer this question.

“During the first part of the interglacial the Northern Hemisphere was receiving more sunlight than now. We think Greenland’s ice cap was reduced, and this explains the 3–4 metre rise,” O’Leary says. “At the end, the insolation curve shifted and the majority of the heat moved to the Southern Hemisphere, which we think melted the West Antarctic ice sheet.”

The West Antarctic ice sheet is considered more unstable than its larger neighbour on the east of the continent, as it is grounded below sea level and thus a more likely source for most of the sea level rise.

Although similar patterns for sea level rise have been observed elsewhere, these have been subject to considerable dispute. “While global temperatures and CO2 levels are fairly well-understood for the last interglacial period, the level of the sea during this time remained a highly controversial question,” O’Leary says. “This is because tectonic movements, and the isostatic response of coastlines to shifting ice-water masses over glacial and interglacial periods, can artificially raise or lower the original shoreline position.”

O’Leary and American colleagues surveyed the fossil reefs along the length of Western Australia’s west-facing coastline from Augusta to Exmouth. “Tectonics have a purely local effect – in cases like the Sumatra and Japanese earthquake the majority of the land movement was local,” O’Leary says. Consequently such effects could not explain consistent measurements over thousands of kilometres.

Moreover, Western Australia makes a good study site. “It has a carbonate shoreline with good preservation of reefs,” O’Leary says. “It is also distant from the glaciated regions that bulge out as they are pushed down by the weight of ice and rebound as the ice melts.”