SINGAPORE - 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 already ice at the poles is melting and the rate is speeding up as ocean and air temperatures keep rising.
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. Keeping the poles on ice by making deep cuts to greenhouse gas emissions is vital, the UN says, but so far global efforts are falling short.
"Antarctica and Greenland play a key role in sea level rise globally, by discharging melting ice to the ocean," said Professor Michael Meredith, an oceanographer and science leader at the British Antarctic Survey.
"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. By the end of the summer, about 400 billion tonnes of ice - maybe more - will have melted or calved off Greenland's giant ice sheet, scientists estimate. That's enough water to flood Pennsylvania or Greece about 35cm deep, the Associated Press reported.
A study published in May in the journal Geophysical Research Letters, found 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 South Ocean is melting the base and front of the ice sheet, allowing glaciers behind to speed up and dump more ice into the sea.
Large parts of the West Antarctic ice sheet are below sea level and warming ocean waters are steadily melting the base, increasing the risks it will break apart.
Scientists had thought the much larger East Antarctic ice sheet was stable but new research published in January found that it 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.
The studies are part of a broader picture of the big polar ice sheets losing ice more rapidly, due to both atmospheric and oceanic warming, Prof Meredith told The Straits Times.
The most up-to-date estimates of sea level rise, including the contributions from the major ice sheets, will be released next week in the UN Intergovernmental Panel on Climate Change's special report on oceans and ice-covered areas of the planet. Prof Meredith is one of the authors.
So far, much of the 20cm of sea level rise since pre-industrial times has been from thermal expansion of the ocean - water expands as it warms. The oceans are a huge energy store: more than 90 per cent of the extra heat from global warming has gone into the ocean.
"But Antarctica and Greenland, and also smaller glacial systems, are key contributors to sea level rise also, and the changes we are seeing in those ice sheets and glaciers mean that those contributions are increasing," said Prof Meredith, who has made regular research trips to Antarctica and seen changes unfold over the years.
Apart from the retreat of glaciers and loss of ice shelves in West Antarctica, one of the most notable changes is the Southern Ocean that surrounds Antarctica is warming rapidly, including deep down, he said.
"This is doing us a climatic favour, in that it is storing heat that would otherwise stay in the atmosphere, but it can also impact negatively on marine ecosystems," he said, such as fish stocks and Antarctic krill, a key part of the food chain.
"Another big change is that the Southern Ocean is acidifying, because some of the extra carbon that we are putting in the atmosphere is being drawn down into the ocean - and this can be damaging to some animals that form hard shells and skeletons.
"There are many other changes also, some of which are more subtle and complex, but also concerning. Overall it is the whole changing environment, and the implications it has for life within it and across the whole planet, that concerns me most."
Just how much sea levels will rise depends on actions now, 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 really important to remember that this doesn't mean that 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."