Researchers from the Australian National University (ANU), based in Canberra, found that the 20-centimetre rise in sea levels since the start of the 20th century, caused largely by global warming and the melting of polar ice, was unprecedented.

The ANU study, a lengthy analysis of historical sea level trends, showed levels have remained steady for thousands of years before rapid rises over the past 150 years, since global industrialization.

The study looked at the fluctuation of ocean levels over the past 35,000 years, based on ice volume changes around the world.

unprecedentedA two-decade-long collection of about 1,000 ancient sediment samples off Britain, north America, Greenland and the Seychelles formed the basis of the research, which was recently published in Proceedings of the National Academy of Sciences (PNAS).

Researchers could pick submerged sediments that may include tree roots, suggesting a previously lower sea level, or molluscs, which can be measured against the fossil record to determine the previous sea level.

The study has been described as the most comprehensive paper of its kind looking at the period.

Professor Kurt Lambeck, from the ANU, said sea levels have been oscillating by no more than 20 centimeters over several millennia.

"In the last 6,000 years before the modern onset of sea level rise, the sea level has been quite stable," he said.

"We see no evidence for oscillations in sea level greater than say plus or minus 25 or 30 centimeters, on timescales of 100 years or longer, there's just no evidence for that."But he said there had been a rapid upward trend accompanying global industrialization. "For the last 150 years we've been seeing this increase, this rising sea level, at a rate of a few millimeters per year and in our older records we do not see similar behavior," he said.

Lambeck's team identified rising temperatures, which have caused polar ice to melt and thermal expansion of the sea, as a primary cause of the sea level increase.

"What we've seen is unusual, certainly unprecedented for these interglacial periods," he said.

"All the studies show that you can't just switch off this process. Sea levels will continue to rise for some centuries to come even if we keep carbon emissions at present day levels.""What level that will get to, we are less sure about. But it's clear we can't just reverse the process overnight," he said.