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Satellites have confirmed mile-thick ice sheets covering Greenland and most of Antarctica are melting at a faster rate in a warming world, scientists say.

That's the conclusion supported by two decades of satellite readings, a study released Thursday by an international network of scientists found.

The melting of billions of tons of ice a year added almost half an inch to global average sea levels from 1992 to 2011, the researchers reported.

Although seemingly a small amount, it could have significant worldwide impact, they said.

"Small changes in sea levels in certain places mean very big changes in the kind of protection of infrastructure that you need to have in place," Erik Ivins, a geophysicist at NASA's Jet Propulsion Laboratory in California who took part in the study told CNN.

While the 19-year average worked out to about 20 percent of the rise of the oceans, "for recent years it goes up to about 30 or 40 percent," said Michiel van den Broeke, a professor of polar meteorology at Utrecht University in the Netherlands.

The rest comes from thermal expansion in a warming world, researchers said, explaining warmer water takes up more space.

The results are the clearest evidence to date the ice sheets are losing ground, study lead author Andrew Shepherd of Britain's University of Leeds said, and should be a benchmark for climate scientists to use for future calculations.