Sea levels will rise up to 22 metres even if the worst scenarios for global warming are avoided, researchers say.
An international team of scientists, which included a New Zealander, found that a 2C increase in global temperature would still cause the world's oceans to rise between 12m and 22m.
Two degrees is the recommended limit set by the Intergovernmental Panel on Climate Change. A bigger increase could lead to the melting of the West Antarctic ice sheet, some of the East Antarctic ice sheet and part of Greenland.
A 22m increase in sea level would dramatically transform New Zealand's coastal boundaries, with low-lying areas like Auckland and Wellington's harbours swamped by ocean currents.
But lead researcher Ken Miller said: "You don't need to sell your beach real estate yet, because melting of these large ice sheets will take from centuries to a few thousand years."
To reach their estimate, the scientists looked at sea levels in the period when the climate was similar to the present day - around 3 million years ago. At that time, the amount of carbon dioxide in the atmosphere was similar to present levels and atmospheric temperatures were 2C higher than now.
Victoria University sedimentologist Tim Naish investigated tiny fossils hidden in sediment in the Wanganui Basin. The fossils are known to exist at certain water depths, so they give clues to the sea levels millions of years ago.
He also calculated sea level changes by looking at old shorelines which had been exposed by tectonic uplift.
By combining these observations with international evidence, the team concluded with 95 per cent confidence that 3 million years ago the sea level peaked up to 30m above the present level, with a best estimate of 22m.
Dr Naish: "If you were standing at Marlborough looking north, you had a very big Cook Strait. Everything from Wellington, Manawatu, Palmerston North, southern Hawkes Bay would have been ocean. You wouldn't have hit the shoreline until you got up to southern Taranaki and central Hawkes Bay."
If the water was to rise 22m again, New Zealand would not face similar submersion, because the North Island has had significant tectonic uplift since then. But low-lying areas such as Petone, Christchurch and urban harbours would be affected.
"If you were sitting at the Auckland or Wellington waterfront, a 20m sea rise would have high-rise skyscrapers sticking right out of the ocean. It would go all the way up Lambton Quay and be flooding the Basin Reserve," Dr Naish said.
"You can engineer in some cities to avoid this, but in other areas you can't, and you have to move away."
Although Dr Naish was cautious about estimates for the long-term sea level rise, he said it was well-established that oceans would rise one metre this century.
3 MILLION YEARS AGO
* The last time the earth's climate was similar to the present day, sea levels were 20m above present levels.
* This was 3 million years ago, during the Pliocene epoch.
* At this time, Wanganui, Manawatu, Wellington, Palmerston North and lower Hawkes Bay were underwater.
* The Ruahines and Tararuas had not popped out of the water yet.
* The shoreline was at southern Taranaki and southern Hawkes Bay.
* The scientist described it like this: "The whole of the southern central North Island was shallow ocean, no deeper than 100-200 metres. You had a very big Cook Strait and there was nothing in the southern North Island."
* Nearly all of the South Island was above water.