Icebergs grounded near Adelaide Island, Antarctic Peninsula. The Antarctic Peninsula is one of the most rapidly warming parts of the continent. Photo: Nicholas Golledge
The world will be locked into thousands of years of unstoppable sea level rise from melting Antarctic ice if it doesn't cut emissions and prevent another two degrees of global warming, scientists have warned.
New projections by Australian and New Zealand-based scientists, which were published in the journal Nature on Thursday, find the additional warming of the planet and oceans would trigger a collapse of crucial natural barriers that protect Antarctica's massive 14 million square kilometre ice sheet.
Those barriers - known as ice shelves - are floating extensions of Antarctic land ice that stretch out across the sea and buttress the ice sheet from the ocean.
The study found that an extra 1.5 to 2 degree rise in global average temperatures above today's levels, along with 0.5 degree rise in ocean warming, would lead to the loss of 85 per cent of this floating ice barrier.
That would expose the massive Antarctic ice sheet and trigger centuries, and even millennia, of runaway sea level rise from its melting that would be almost impossible to stop.
Using complex computer modelling, the researchers projected that if the ice shelf did collapse then melting Antarctic ice could alone contribute as much as three metres of sea level rise across the planet by 2300, rising up to nine metres by the year 5000, under the worst future scenario.
One of the authors, Dr Nicholas Golledge, from Victoria University in New Zealand, said the reaction of the ice sheet could take thousands of years to play out and global warming caused by humans today would effect it in ways that would be "incredibly hard to undo".
It comes as US researchers this week forecasted in a separate study that at current warming rates surface melting on Antarctica's ice shelves would double by 2050, and could lead to their collapse by 2100.
The 1.5 to 2 degrees of warming needed to trigger the collapse of the Antarctic ice shelf would come on top of the almost 0.9 degrees of global warming the world has experience since pre-industrial times.
Exactly how much sea level rise that the melting Antarctica ice sheet could cause would depend on how much greenhouse gas emissions are allowed in coming decades.
The Australian study used four scenarios of future emissions prepared for the United Nation's Intergovernmental Panel on Climate Change (IPCC). The only scenario that did not result in the collapse of the ice shelves saw the world significantly reduced greenhouse gas emissions beyond 2020.
Another of the study's authors, Dr Chris Fogwill from the University of NSW, said it showed that there was still a path the world could take to avoid a runaway sea level rise from melting Antarctica ice.
The findings also suggests the estimates of expected global sea level rise contained in the IPCC's last major report could have been too modest. Under the highest future emissions scenario the panel in 2013 found up to 0.82 metres of sea level rise could be expected by 2100
But the IPCC only included a five centimetre contribution from melting Antarctic ice because of uncertainty over how the Antarctic ice sheet would respond to future warming.
The Nature study suggests that if the ice shelves collapse then a further 35 centimetres of sea-level rise could be expected on top of the IPCC estimates.
Dr Fogwill added that the estimates contained in the Nature study were conservative, because they did not take into account that part of the polar regions are warming much more rapidly than the global average.
