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.
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."