Warming of 1.5 to 2 degrees Celsius above current levels could lead to "unstoppable" sea level rise that would last for thousands of years, according to a new model of Antarctic ice sheets.
The new model, published today in Nature, shows that such temperatures would result in 80 to 85 per cent loss of major Antarctic ice shelves, something that is possible by the end of the century under existing IPCC scenarios.
Collapse of the ice shelves would trigger a rapid melt of the Antarctic ice sheets, releasing vast amounts of Earth's freshwater stores into the ocean, said the researchers.
The IPCC reports have said the collapse of the ice shelves was unlikely whereas we're showing it's actually a very likely scenario.
Dr Chris Fogwill, University of New South Wales
By 2100 this would add up to 40 centimetres to sea levels, melt rate would continue to accelerate until 2300, and sea levels would continue to rise after that for thousands of years.
The good news, said the scientists, is that their research suggests it's not too late to stop this, if we're prepared to take tough action to reduce greenhouse emissions.
"A lot of people are out there saying there's no point -- we're in that world now where it's all going to happen," said researcher Dr Chris Fogwill of the Climate Change Research Centre at the University of New South Wales.
"But actually, what this modelling shows is there is still an opportunity, even now, to keep below these thresholds, where we avoid that long-term commitment."
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Antarctic contribution to sea level rise underestimated
The Earth is currently experiencing one of the highest rates of sea level rise for thousands of years, linked to global warming.
According to the IPCC global sea levels could rise over current levels by about 30 to 100 centimetres by 2100, depending on emission scenarios, with the main contributors being expansion of the warming oceans and melting of the Greenland icesheet and other land glaciers.
YouTube: Animation of Antarctic melt model with a high emission scenario (Courtesy Golledge et al.)
IPCC emission scenarios
The IPCC gives four scenarios, called RCPs, of how the planet will heat by 2100, according to the amount of greenhouse gases emitted into the atmosphere.
RCP 2.6 - Low emissions
Emissions peak at 2020; are halved by 2050 and are zero by 2100. Temperatures do not exceed 2 degrees C above pre-industrial levels.
RCP 4.5 - Intermediate emissions
Emissions peak 2040. Temperatures reach 2.5 degrees C above pre-industrial levels by 2100.
RCP 6 - Intermediate emissions
Emissions peak in 2060. Temperatures reach 3 degrees C above pre-industrial levels by 2100.
RCP 8.5 - High emissions
Business as usual - emissions continue to increase. Temperatures reach 4.3 degrees C above pre-industrial levels by 2100.
To date it has been thought that melting of Antarctic ice sheet would contribute very little to future sea-level rise -- just 4 to 5 centimetres at most.
But, said Dr Fogwill, these conclusions were reached using models that were not sophisticated enough to show major ice shelf collapse.
Dr Fogwill and a team led, by Dr Nicholas Golledge, from Victoria University in New Zealand, have developed the best model yet of the response of Antarctica to different scenarios of global warming. They have found the IPCC has been underestimating the contribution of Antarctica to sea level rise.
"The IPCC reports have said the collapse of the ice shelves was unlikely whereas we're showing it's actually a very likely scenario."
Unlike most climate models, which are run over centuries at most, the new model was run over 5,000 years to estimate the full impact of global warming on ice sheets.
The model showed that Antarctic ice would remain "incredibly stable" over 5,000 years at the lowest IPCC emission scenario, but ice shelves would collapse under all the other scenarios.
"In those higher concentration pathways we destabilise those ice shelves," said Dr Fogwill.
Collapse of the ice shelves under such scenarios would lead to Antarctic ice sheets contributing around 40 centimetres rather than 4 centimetres to sea level by 2100, said Dr Fogwill.
But, he said, it would not be until 2300 that the Antarctic ice melt rate would peak. By that stage the not-so-frozen continent would be contributing as much as 3 metres to sea level rise.
Unstoppable sea-level rise under high emission scenarios
Due to the very slow response of the massive Antarctic ice sheets to global warming, Antarctic contribution to sea level rise would be "unstoppable" for thousands of years, and could be as much as 10 metres by 5000, according to the model.
These estimates are most conservative, said Dr Fogwill, not taking into account "polar amplification", which is the extra warming occurring at the poles.
The last time Earth experienced CO2 levels similar to today's was 3 million years ago and at that time sea levels at that time were a staggering 20 metres higher, said the researchers.
The emission controls required to prevent Antarctic ice shelf collapse are tougher than most are willing to consider, said Dr Fogwill.
But he and colleagues point to socioeconomic and ethical implications for future generations given the number of people who live within metres of sea level.
The findings will inform discussions at the upcoming Paris climate negotiations.
http://www.abc.net.au/news/2015-10-15/antarctic-ice-shelf-sea-level-rise-warning/6853780