Scientists blame man-made global warming for the melting. Burning fossil fuels, such as coal and oil, emits carbon dioxide and other greenhouse gases that trap heat, warming the atmosphere and oceans. Bit-by-bit, that erodes the ice sheets from above and below. Snowfall replenishes the ice sheets, but hasn't kept pace with the rate of melting.
Because the world's oceans are so big, it takes a lot of ice melting — about 10 trillion tons — to raise sea levels 1 inch (2.5 centimeters). Since 1992, ice sheets at the poles have lost nearly 5 trillion tons of ice, the study says, raising sea levels by about a half inch.
That seemingly tiny extra bit probably worsened the flooding from an already devastating Superstorm Sandy last month, said NASA ice scientist Erik Ivins, another co-author of the study. He said the extra weight gives each wave a little more energy.
"The more energy there is in a wave, the further the water can get inland," Ivins said.
Globally, the world's oceans rose about half a foot (15 centimeters) on average in the 20th Century. Melting ice sheets accounts for about one-fifth of sea level rise. Warmer water expands, contributing to the rise along with water from melting glaciers outside the polar regions.
Just how much ice is melting at the two poles has been difficult for scientists to answer. In 2007, the Intergovernmental Panel on Climate Change did not include ice sheet melt in its calculations of future sea level rise because numbers were so uncertain.
It's an important factor because if all the polar ice sheets somehow melted — something that would take centuries — global sea levels would jump by more than 200 feet, said Pennsylvania State University ice scientist Richard Alley, who wasn't part of the research.