Melting ice sheets from Greenland and Antarctica has long been tied to rising sea levels. But these two sources are outpacing all others - including mountain glaciers and ice caps - to become the dominant feature in raising the seas, according to a new study slated for publication this month in the journal Geophysical Research Letters.

Part of the reason for the significance of these polar ice sheets is that the rate of melt is accelerating. Researchers at the University of California, Irvine and colleagues found that the two sheets lost a combined average of 36.3 gigatonnes more than they did the previous year. Mountain glaciers and ice caps are melting too, but at a rate three times smaller than the ice sheets.

"That ice sheets will dominate future sea level rise is not surprising - they hold a lot more ice mass than mountain glaciers," says lead author Eric Rignot, of NASA's Jet Propulsion Laboratory, Pasadena, California, and the University of California, Irvine in a press release. "What is surprising is this increased contribution by the ice sheets is already happening."

Rignot says if the trends continue, sea level is going to be significantly higher than what's been projected by the U.N. IPCC in 2007.

The 20-year study compared multiple measurement tools to estimate the decay of the ice sheet. Satellite data and radio echo soundings measured ice leaving the ice sheets, while regional atmospheric climate model data estimated ice sheet gain. Another set of satellite data measured minute changes in the Earth's gravity field due to changes in the distribution of the planet's mass, including ice movement.

The results matched up well, painting a more certain picture of how ice sheet loss would hike sea levels over time. Within the next four decades, melting ice sheets could raise sea level by 5.9 inches, bringing total sea level rise to 12.6 inches.

The stability of Antarctic ice has long concerned climate scientists. If the west Antarctic ice sheet's base were to collapse, global sea levels could shoot up by five meters.

But new research shows the ice could be a bit more tough than scientists thought. In a study published online last month in the geo-science journal Palaeogeograpy, Palaeoclimtology, Palaeoecology, University of Exeter-led geographer Christopher Fogwill and colleagues found that blue-ice moraines in West Antarctica fluctuated in thickness during the ups and downs of the Earth's glacial cycles. But for at least the past 200,000 years, and maybe as long as 400,000 years, they remained intact, even during warm, interglacial periods.

Moraines are a pile of rocks, often covered in ice in glaciated areas, that have been amassed by moving glaciers. The scientists analyzed the moraines in the Heritage mountain range near the central dome of the west Antarctic ice sheet for beryllium isotopes produce by cosmic radiation. When the rock is exposed, meaning it's absent of ice, the isotopes accumulate. Fogwill and his team found that the moraines have been covered in ice for at least 200,000 years.

Presumably, these areas might outlast this latest bout of climate change. At the very least, the findings could change scientists' understanding of how sensitive the ice sheet is to a warming world.