There are four major mechanisms: the thermal expansion of oceans in a warming world; the loss of ice from the Greenland and Antarctic ice sheets; the melting of mountain glaciers and ice caps (such as those in the Himalayas); and the extraction and discharge of groundwater.
The likely culprits are continental ice sheets. "[In IPCC models], the two big ice sheets in Greenland and Antarctica contribute nothing to future sea level rise, because they assume that the mass loss from Greenland is balanced by ice gain in Antarctica due to higher snowfall rates," says Rahmstorf.
But satellite data show that the ice sheets are losing ice to the oceans.
If the models have not accurately reproduced what happened in recent years, it is likely that their projections for the future are not correct either. Since 2007, the IPCC has recognised this. Its initial projection of a maximum sea level rise of 60 centimetres by 2100 has been upped to include an additional 20-centimetre rise due to ice sheets melting.
And the data are clear: from 1990 to 2000, the melting of Greenland and Antarctic ice sheets added about 0.25 millimetres a year to global sea level rise. For 2005-2010, that number has increased to about 1 millimetre a year
Greenland is losing the most ice, causing sea level to rise by about 0.75 millimetres per year. What's happening in Antarctica is more nuanced. East Antarctica is gaining mass because of increased snowfall, but this is more than offset by the loss of ice from West Antarctica, particularly along the Amundsen Coast, where warm water is melting ice shelves from beneath. This is leading to thinning and speed-up of glaciers, such the Pine Island and Thwaites glaciers.
Until now, sea level rise from the extraction of groundwater (which eventually ends up in the sea) has been countered by dams built on rivers over the last century, which hold water back on land. But the best sites for dams have now been utilised, so we can't expect to store more water on land.
As we extract more groundwater for irrigation – a trend that could increase as climate change causes droughts – it could add up to 10 centimetres to the sea level by 2100, according to Rahmstorf. "This will become a net contribution to sea level rise in the future," he says. "Not big, but not negligible."