Scientists meeting in Wellington say new research shows hundreds of coastal cities could be in danger from rising sea levels.
They say the West Antarctic Ice Sheet has melted and re-set often in the past, and is likely to melt again over the next 1,000 to 2,000 years.
This would raise sea levels by five metres. If ice also melts in Greenland the increase would be 13 metres.
The scientists, from New Zealand, the United States, Italy and Germany, base their findings on several sources including a 1,300 metre core sample taken from rock below Antarctica.
Current forecasts are that there will be a half-metre rise in sea levels by the end of the century.