The 2C figure is significant because this is level of warming that is likely to be adopted as the threshold to be avoided by the UN climate negotiations in Copenhagen – although small islands states and developing nations have argued that 1.5C would be a more appropriate target.
Nine metres of sea level rise is higher than anything predicted so far because the new study takes into account the potential that the Greenland and west Antarctic ice sheets start to melt as the Earth warms. This did not factor into the most recent assessment of the state of climate science by the Intergovernmental Panel on Climate Change (IPCC) in 2007. It forecast a sea-level rise of up to 59cm by 2100, and between 4-6m in the next few hundred years, if average global temperatures stabilised around 2C.
“Everybody’s known that the IPCC’s last numbers were underestimates because they didn’t include all the factors that can accelerate ice sheet melting,” said Robert Kopp of Princeton University, who led the latest study. “If the future models are limited, you want to look at other approaches to get at the question of sea-level rise one approach is to turn to the past record of sea-level rise.”
Kopp’s team reconstructed the sea levels in the last interglacial period, around 125,000 years ago. At the time, polar temperatures were around 3-5C warmer and equatorial sea-surface temperatures were around 2.5-3.5C warmer than today. “So you look at things like coral reef terraces and how high they grew and, if you know something about the ecology of corals, you can say how high sea level was relative to the top of the coral reef. Or you look at old beaches that are now stranded above the sea-line, or you look at sediments that have textures that indicate they were deposited inter-tidally.”
His results, published in the journal Nature, showed that sea levels around the world during the last interglacial were between 6.6m and 9m higher than today. “During this period when temperatures were 2-3C above pre-industrial levels, global sea level looks like it was very likely at least 6.6m higher than today, which implies significant melting of the Greenland and west Antarctic ice sheets.”
Kopp said the results could be used to infer what could happen to future sea levels over the next few hundred years, as a result of human-induced global warming. “The warming we’re on track to do now is more than enough to commit us to last-interglacial levels of sea-level rise.”
Kopp’s work echoes recent research by the Scientific Committee on Antarctic Research (SCAR) suggesting that sea-levels could rise much higher than predicted by the IPCC by the end of the century. The study by SCAR suggested that sea levels could rise by up to 1.4m by 2100 if the Antarctic ice began to melt.