A rise of at least two metres in the world's sea levels is now almost unstoppable, according to experts.

"The crux of the sea level issue is that it starts very slowly but once it gets going it is practically unstoppable," said Stefan Rahmstorf, a scientist at Germany's Potsdam Institute and a widely recognised sea level expert.

"There is no way I can see to stop this rise, even if we have gone to zero emissions."

Dr Rahmstorf said the best outcome was that after temperatures stabilised, sea levels would only rise at a steady rate "for centuries to come," and not accelerate.

Most scientists expect at least 2˚C warming as a result of man-made greenhouse gas emissions, and probably more. The world warmed between 0.7 and 0.8˚C last century.

Dr Rahmstorf estimated that if the world limited warming to 1.5˚C then it would still see a two-metre sea level rise over centuries, which would see some island nations disappear.

His best guess was a one-metre rise this century, assuming 3˚C warming, and up to five metres over the next 300 years.

"There is nothing we can do to stop this unless we manage to cool the planet. That would require extracting the carbon dioxide from the atmosphere. There is no way of doing this on the sufficient scale known today," he said.

Scientists say that ice melt acquires a momentum of its own - for example warming the air as less ice reflects less heat, warming the local area.

"Once the ice is on the move, it's like a tipping point which reinforces itself," said Wageningen University's Pier Vellinga, citing various research.

"Even if you reduce all the emissions in the world once this has started it may be unstoppable. I conclude that beyond 2˚C global average temperature rise, the probability of the Greenland ice sheet disintegrating is 50 per cent or more."