Just imagine if the sea level rose by 20 metres. New Zealand's coastlines would be changed forever.

What would become of our beaches, coastal walkways, baches and homes?

New Zealand researchers have found that Antarctic ice sheets are capable of widespread melting, raising the sea level by up to 20 metres under current atmospheric carbon dioxide conditions.

Their new study, led by the Victoria University of Wellington, was published in a journal called Nature, on Thursday.

Estimates of global sea level rise can vary. This year the Intergovernmental Panel on Climate Change (IPCC) released a detailed report on oceans and the cryosphere (frozen areas of Earth).

It estimates that by 2100, there will be a global sea level rise of 43 centimetres even in a best-case scenario where work has been done to curb emissions.

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A future without serious mitigation could see as much as 84cm, and some estimates go as high as a metre.

For the new study published in Nature, Kiwi researchers looked at past sea levels to work out the potential for widespread melting of Antarctica.

Their research showed that three million years ago (during the Pliocene epoch) up to one third of Antarctica's ice sheets melted, causing the sea level to rise as much as 25m above present levels.

Levels of carbon dioxide in the Earth's atmosphere were similar to today's levels, and in response temperature was two to three degrees Celsius warmer.

The new study was led by Dr Georgia Grant, a recent Victoria University of Wellington PhD graduate, who's now at GNS Science.

For the study Grant used a new method of analysing marine geological sediments to construct a global sea level record.

She developed the new method of determining the magnitude of sea level change by analysing the size of particles moved by waves.

The method was applied to the geological archive from Whanganui Basin on the west coast of the North Island, which contains some of the best evidence anywhere in the world for global sea level change.

She was able to show that during the past warm period three million years ago, global sea levels regularly fluctuated between 5 to 25 metres

Professor Tim Naish, who was involved in the study, said it had implications for the stability of the Antarctic ice sheet and its potential to contribute to future sea level change.

"... If we do not keep our greenhouse gas emissions in line with the Paris Climate Agreement target of two degrees warming, then we may potentially lose not only the West Antarctic Ice Sheet, but also the vulnerable margins of the East Antarctic Ice Sheet."

Grant said of critical concern was that over 90 per cent of the heat from global warming to date had gone into the ocean, and much of it into the Southern Ocean which bathed the Antarctic ice sheet.

One third of Antarctica's ice sheet - equivalent to up to 20m sea level rise - sits below sea-level and is vulnerable to widespread and catastrophic collapse from ocean heating. It melted in the past when atmospheric carbon dioxide levels were 400 parts per million (ppm), as they are today.

"Our new study supports the idea that a tipping point may be crossed, if global temperatures are allowed to rise more than two degrees, which could result in large parts of the Antarctic ice sheet being committed to melt-down over the coming centuries. It reinforces the importance of the Paris target."

She said the study also had implications for computer-based ice sheet modelling.

"Our new sea level estimates provide a target for testing the results from computer models and improving their ability to make accurate projections of the Antarctic contribution to global sea level rise."

Dr Nick Golledge, from the Antarctic Research Centre, said the research was consistent with model results that showed long term ice sheet retreat under current carbon dioxide levels.

"The rate of sea level change estimated from this study is also in line with our understanding of climate sensitivity and supports future predictions of one metre of sea level rise by 2100."