Jonathan Bamber, an ice sheet expert at the University of Bristol, told the conference that previous studies had misjudged the so-called Greenland tipping point, at which the ice sheet is certain to melt completely. "We're talking about the point at which it is 100% doomed. It seems quite an important number to get right." Such catastrophic melting would produce enough water to raise world sea levels by more than 6m.

"We found that the threshold is about double what was previously published," Bamber told the Copenhagen Climate Congress, a special three-day summit aimed at updating the latest climate science ahead of global political negotiations in December over a successor to the Kyoto treaty. It would take an average global temperature rise of 6C to push Greenland into irreversible melting, the new study found.

Previous estimates, including those in the recent reports from the Intergovernmental Panel on Climate Change, said the critical threshold was about 3C - which many climate scientists expect to be reached in the coming decades.

"The threshold temperature has been substantially underestimated in previous studies. Our results have profound implications for predictions of sea level rise from Greenland over the coming century," the scientists said.

There is time to make sure the earth doesn't exceed that threshold. 6C - that's a lot.

Some rather major bodies of ice lying on the sea floor are already considered doomed in the next few years in Antarctica.

As for 2 metres, no it needs absolutely no underestimate of the recoverable fossil fuels. On the contrary, the already discovered reserves are about twice as big as needed to get +2-3°C on the current models, and so far they have always proved to underestimate the ice melting.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi

The rate of sea-level rise has increased in the period from 1993 to the
present (Figure 1), largely due to the growing contribution of ice loss from
Greenland (Box 1) and Antarctica. However, models of the behaviour of
these polar ice sheets are still in their infancy, so projections of sea-level
rise to 2100 based on such "process models" are highly uncertain. An
alternative approach is to base projections on the observed relationship
between global average temperature rise and sea-level rise over the
past 120 years, assuming that this observed relationship will continue
into the future. New estimates based on this approach suggest a sealevel
rise of around a metre or more by 2100

The above recently doubling of the estimate to around 1 meter by 2100 is based on a linear extrapolation from previous data. Hansen argued a couple of years ago that the linearity assumption should be questioned, and that if rapid collapse of the arctic and antarctic glaciers were to occur, quick rises of sea level beyond the 1 meter range could result.
http://www.newscientist.com/article/mg19526141.600-huge-sea-level-rises-are-coming--unless-we-act-no w.html?page=1

Many civic projects related to flood control are financed and designed with 100 year time frames, so large uncertainty in this area is difficult from both the technical and the political viewpoints...