A major Kiwi-led study has painted a dire picture of Antarctica's future under climate change, with models simulating heavy ice loss under all but one optimistic scenario.
The study, published today in the major journal Nature and led by Dr Nicholas Golledge of Victoria University's Antarctic Research Centre and GNS Science, drew on state-of-the-art computer modelling to predict how the Antarctic ice sheet would respond to a range of greenhouse gas emission scenarios.
It showed how the warming would lead to the loss of large parts of the ice sheet - the only scenario in which this didn't happen was significantly reduced emissions beyond 2020 - which resulted in a substantial rise in global sea level.
"The long reaction time of the Antarctic ice-sheet - which can take thousands of years to fully manifest its response to changes in environmental conditions - coupled with the fact that CO2 lingers in the atmosphere for a very long time means that the warming we generate now will affect the ice-sheet in ways that will be incredibly hard to undo," Dr Golledge said.
In its Fifth Assessment Report, the UN's Intergovernmental Panel on Climate Change (IPCC) predicted that the Antarctic ice sheet would contribute only five centimetres to global sea-level rise by the end of this century even for its warmest emissions scenario.
But Victoria University's Professor Tim Naish, who worked with Dr Golledge on the new study, said that at the time that report was written there was insufficient scientific knowledge on how the Antarctic ice sheet might respond to future warming, meaning the IPCC sea-level projections could have been too modest.
"Our new models include processes that take place when ice sheets come into contact with the ocean," Dr Golledge said.
"Around 93 percent of the heat from anthropogenic global warming has gone into the ocean, and these warming ocean waters are now coming into contact with the floating margins of the Antarctic ice sheet, known as ice shelves."
If we lost these ice shelves, Dr Golledge said, the Antarctic contribution to sea-level rise by 2100 will be nearer 40 centimetres.
To avoid the loss of the Antarctic ice shelves, and an associated commitment to many metres of sea-level rise, the study showed atmospheric warming had to be kept below 2C above present levels.
"Missing the 2C target will result in an Antarctic contribution to sea-level rise that could be up to 10 metres above present day," Dr Golledge said.
"The stakes are obviously very high-10 percent of the world's population lives within 10 metres of present sea level.
"Without significant reduction in greenhouse gas emissions over the next couple of decades, we will commit the Antarctic ice sheet to ongoing and widespread melting for the next few thousand years. Is that something for which we really want to be responsible?"
Professor Naish felt it was therefore imperative that agreement was reached between nations at December's crucial UN climate talks in Paris to reduce global CO2 emissions to zero by the end of the century.
"To be on track this will require a global commitment to 30 percent reduction, below year 1990 levels, by the year 2030."
New Zealand is taking to Paris a new target of reducing its greenhouse gas emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels by 2030.
That commitment has been criticised by some climate scientists and environmental groups, and a recent Oxfam report described it as falling "well short" of a fair contribution towards limiting the global temperature rise to 2C.
Dr Golledge said the time had come for some serious questions to be answered.
"It becomes an issue of whether we choose to mitigate now for the benefit of future generations or adapt to a world in which shorelines are significantly re-drawn.
"In all likelihood we're going to have to do both, because we are already committed to 25 centimetres by 2050, and at least 50 centimetres of sea-level rise by 2100."
The last time CO2 concentrations in the atmosphere were similar to present levels was about 3 million years ago.
"At that time average global temperatures were two or three degrees warmer, large parts of the Antarctic ice-sheet had melted, and sea-levels were a staggering 20 metres higher than they are now," he said.
"We're currently on track for a global temperature rise of a couple of degrees which will take us into that ballpark, so there may well be a few scary surprises in store for us, possibly within just a few hundred years."
What climate change means for New Zealand
• Under present projections, the mean temperature in New Zealand could be 2C higher by the end of the century - and even between 3C and 4C higher if no action is taken to curb carbon emissions.
• Within the same period, sea level is expected to rise between 50cm and 100cm, leaving populations to adapt by either abandoning coasts and islands, changing infrastructure and coastal zones, or protecting areas with barriers or dykes.
• A recent report on sea level rise by Parliamentary Commissioner for the Environment Dr Jan Wright said the impact of even a small rise in sea level would be significant and costly for some landowners.
• Large storms occurring on top of a higher sea level - of which an increased number were predicted - would affect public infrastructure such as roads, railways and stormwater systems, as well as private homes.
• Horticulture production in some regions may become uneconomic due to a lack of winter chilling, while sub-tropical crops such as avocados and citrus may benefit from warmer conditions.
• Rising ocean temperatures and ocean acidification may alter marine life, moving fisheries