Canberra, Oct 14 (IANS) Australian scientists have made an alarming discovery that sea levels have risen more in the past century than at any other comparable period in the past 6,000 years.

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

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 industrialisation.

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

A 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 picked submerged sediments that may include tree roots, suggesting a previously lower sea level, or molluscs, whose fossil records were measured to determine the previous sea level.

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

Kurt Lambeck, a professor in the ANU, said sea levels have been oscillating by no more than 20 cm 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 cm, 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 industrialisation.

"For the last 150 years we have been seeing this increase, this rising sea level, at a rate of a few mm per year and in our older records we do not see similar behaviour," 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.