Unpredictable pace
But the speed of sea level rise is an open question.
"Ice sheets are contributing to sea level rise sooner, and more than anticipated," said Eric Rignot, a glaciologist at the University of California, Irvine, and NASA's Jet Propulsion Laboratory in Pasadena, California.
That's because people have never seen the collapse of a huge ice sheet and therefore don't have good models of the effects, Rignot said.
The Intergovernmental Panel on Climate Change, an international organization created by the United Nations that produces climate change models, has predicted that sea levels could rise as much as 21 feet (6.4 meters) in the next century if global warming continues unabated.