Earth’s polar ice sheets — Greenland and Antarctica — are melting at a quickening pace, overtaking mountain glaciers and ice caps as sources of sea-level rise much sooner than cryosphere scientists expected.

Glaciologists for a few years now have been zeroing in on one of the biggest subjects of uncertainty in the 2007 United Nations climate report — the rate of future sea level rise and the melting of the planet’s ice. Now an 18-year study published in Geophysical Research Letters provides some answers.

Together, Greenland and Antarctica, are adding an average of 475 billion tons (2.2 trillion pounds) of water to the oceans, compared to 402 billion tons by mountain glaciers, the research team reports, and the ice sheet pace is accelerating at the rate of 36.3 billion tons a year.

News: Greenland Ice Sheet’s Disappearing Act

Here is how lead author Eric Rignot of NASA’s Jet Propulsion Laboratory and the University of California in Irvine, describes the findings in a statement released by JPL:

“That ice sheets will dominate future sea level rise is not surprising — they hold a lot more ice mass than mountain glaciers. What is surprising is this increased contribution by the ice sheets is already happening. If present trends continue, sea level is likely to be significantly higher than levels projected by the United Nations Intergovernmental Panel on Climate Change in 2007. Our study helps reduce uncertainties in near-term projections of sea level rise.”

For the first time, the new ice sheet mass-loss estimate combines data from two different international satellite-borne measurements with advanced regional atmospheric climate model data to produce what JPL describes as “a consistent record of ice mass changes since 1992.” Co-author Isabella Velicogna, also of JPL and UC Irvine, called it “a major achievement that the results agree so well.”

Over the 18 years of study, the team found that Greenland lost ice faster each year than the year before, by an average of 21.9 billion tons of ice. Antarctica’s year-over-year acceleration rate was measured at 14.5 billion tons.

If these rates continue for the next four decades, the researchers say, seas will rise a total of 12.6 inches by 2050 — 5.9 inches from ice sheets, 3.1 inches from mountain glaciers, and, because warmer water takes up more space than colder water, 3.5 inches is expected due to the thermal expansion of the warming ocean.

The numbers however are not indicative of where the effects will be seen. The ocean is warming more in some areas than it is in other areas and is already creating warm hills and cool valleys over the sea surface. Beaches along the cool valleys experience a drop in sea level rise, while other areas are inundated, and could experience rises higher than the total estimated sea level rise.

IMAGE 1: Store Glacier, western Greenland. CREDIT: Eric Rignot, NASA.

IMAGE 2-4: From animation created with data from NASA’s Gravity Recovery and Climate Experiment Satellite. CREDIT: NASA/JPL.