Seas may rise by 1.5m by 2100: experts

Sea levels may rise an average of as much as 1.5m by 2100, the latest figures show.

The range indicated by several new studies is between 50cm and 150cm, said Dr Tim Naish, director of the Antarctic Research Centre at Victoria University in Wellington, New Zealand.

A glaciologist who was chief scientist on a major Antarctic drill-core project, Naish said the latest "range of plausible sea level rise" was based on observations to calculate how much water would come from polar ice sheets.

The Intergovernmental Panel on Climate Change (IPCC) warned in its 4th Assessment Report in 2007 that the best estimate for sea level rise then was 18cm to 59cm by 2100 - but it did not take into account any "future rapid dynamical changes in ice flow" because more research was needed.

Naish was speaking at a briefing for media in advance of the climate change talks in Copenhagen later this month.

He noted that any sea level rise would not be evenly spread across the globe.

Satellite observations over 10 years of changes in the surface elevation, flow and mass of ice had started showing trends for loss of mass in both Greenland and West Antarctica.

"That trend is accelerating" he said.

Scientists were concerned about the West Antarctic ice sheet, which was melting from the bottom up, unlike Greenland which was melting from the top down.

The rate of ice loss from Greenland and Antarctica had more than doubled since 2003.

The rate of melt from the Antarctic ice sheet was contributing almost 1mm a year to sea level rise - equivalent to 8cm to 10cm in a century.

But there was potential for there to be a runaway retreat of the ice sheet, because ice equivalent to 3m of sea rise is actually below sea level, and there could be rises equivalent to 1m per century over 300 years.

Drill-cores from Antarctica indicated there had been a collapse of the West Antarctic ice sheet between three and four million years ago, when the Earth was as warm as it may be by 2100 - with atmospheric levels of carbon dioxide of about 400 parts per million.

Carbon dioxide concentrations in the atmosphere over Wellington, have reached 385 parts per million, according to measurements at Baring Head.

Naish said that surface melt in Greenland was percolating to the base of the ice sheet and lubricating its movement: "This is another dynamic process that can result in rather unpredictable and large-scale changes to the ice sheet, and we don't understand this process well."

A vice-chairman of the IPCC working group which assesses physical science, David Wratt, told the briefing that recent measurements of emissions had tended to be towards the higher end of IPCC scenarios, though it was not yet known how emissions had been affected by reductions in industrial activity because of economic recession.