The giant ice sheets of Antarctica and Greenland contain enough water to raise sea levels by about 70m if they all melted.
That would take many centuries to happen. But ice at the poles is already melting and the rate is speeding up as ocean and air temperatures keep rising.
Exactly how fast will be known next week, with the release of a special report by the Intergovernmental Panel on Climate Change (IPCC), which will give the most up-to-date estimates of sea-level rise, including the contributions from the major ice sheets.
Scientists and the United Nations know the threat from the ice sheets is grave. Even if just 10 per cent of polar ice melted, it would rewrite world maps by flooding coastal cities and submerging many small island nations.
"Antarctica and Greenland play a key role in sea-level rise globally, by discharging melting ice in the ocean," said Professor Michael Meredith, an oceanographer and science leader at the British Antarctic Survey - Britain's national Antarctic operation.
"This role is accelerating, and several recent studies have highlighted things like the extreme melt season that Greenland has just experienced, its accelerating melt, and the rapid thinning and acceleration of glaciers in West Antarctica."
Record heat this year triggered extreme melting in Greenland. Scientists estimate that by the end of the summer, enough ice will have melted or calved off its ice sheet to flood Pennsylvania or the country of Greece about 35cm deep, the Associated Press reported.
A study published in May in the journal Geophysical Research Letters found that up to a quarter of the West Antarctic ice sheet is thinning far inland.
The authors also found that the ice sheet is losing ice five times faster than in the 1990s. The warming Southern Ocean is melting the base and front of the ice sheet, allowing glaciers behind it to speed up and dump more ice into the sea.
Scientists had thought the much larger East Antarctic ice sheet was stable, but new research found that the ice sheet might also be melting at an accelerating rate.
The study, published in the Proceedings of the National Academy of Sciences, found that, overall, Antarctica now sends six times more ice plunging into the sea each year than it did in 1979, and that East Antarctica was responsible for more than 30 per cent of Antarctica's contribution to sea-level rise.
And that rate of melting could increase if global greenhouse gas emissions keep rising, accelerating sea-level rise that will last centuries.
Much, then, depends on actions now, Prof Meredith, one of the IPCC report's authors, told The Straits Times.
He said: "Different parts of the earth system respond to this imbalance on different timescales, and sea level responds over decades and longer. This means that a certain amount of sea-level rise is inevitable - but it's important to remember that this doesn't mean the worst changes are unavoidable.
"Decisions made now can greatly alter the amount of sea-level rise globally that the planet is likely to see by the end of this century and in the centuries beyond - so it is very much the case that, if policymakers are intent on avoiding the worst-case scenarios and limiting the harm done by sea-level rise to coastal communities and infrastructure, then taking action can be hugely beneficial, if taken early enough."