A recent study says that the sea level is rising faster than expected and may cross one metre mark by the end of this century.
That is double that of the estimates made by the Intergovernmental Panel on Climate Change in 2007, says a study.
"What's missing from the models used to forecast sea-level rise are critical feedbacks that speed everything up," says Bill Hay, a geologist at the University of Colorado, US. The feedbacks include data on the Arctic Sea ice, the Greenland ice cap, soil moisture and groundwater mining.
"There is an Arctic Sea ice connection," says Hay, adding that melting sea ice -- which is already in the ocean -- does not itself raise sea level. Instead, it plays a role in the overall warming of the Arctic, which leads to ice losses in nearby Greenland and northern Canada.
When sea ice melts, Hay explains, there is an oceanographic effect of releasing more fresh water from the Arctic, which is then replaced by inflows of brinier, warmer water from the south, according to a Colorado statement.
"It's a big heat pump that brings heat to the Arctic but that's not in any of the models," says the geologist.
The warmer water pushes the Arctic toward more ice-free waters, which absorb sunlight rather than reflect it back into space like sea ice does. The more open water there is, the more heat is trapped in the Arctic waters, and the warmer things can get.
Then there are gigantic stores of ice in Greenland and Antarctica. During the last interglacial period, sea level rose 10 metres due to the melting of all that ice -- without any help from humans, says Hay.
New data suggests that the sea-level rise in the oceans took place over a few centuries, according to Hay.
"Ten years ago we didn't know much about water under the Antarctic ice cap," points out Hay. But it is there, and it allows the ice to move -- in some places even uphill due to the weight of the ice above it.
Another missing feedback is the groundwater being mined all over the world to mitigate droughts. That water is ultimately added to the oceans. All of these are positive feedbacks speeding up the changes in climate and sea-level rise.
Hay will be presenting some of these feedbacks on Sunday at the meeting of the Geological Society of America in Charlotte, North Carolina, US.