Global warming is expanding the extent of sea ice around Antarctica in winter in a paradoxical shift caused by cold plumes of summer melt water that re-freeze fast when temperatures drop, a study showed on Sunday.

An increasing summer thaw of ice on the edges of Antarctica, twinned with less than expected snowfall on the frozen continent, is also adding slightly to sea level rise in a threat to low-lying areas around the world, it said.Climate scientists have been struggling to explain why sea ice around Antarctica has been growing, reaching a record extent in the winter of 2010, when ice on the Arctic Ocean at the other end of the planet shrank to a record low in 2012.

"Sea ice around Antarctica is increasing despite the warming global climate," said Richard Bintanja, lead author of the study at the Royal Netherlands Meteorological Institute.This is caused by melting of the ice sheets from below," he told Reuters of the findings in the journal Nature Geoscience.Ice is made of fresh water and, when ice shelves on the fringes of Antarctica thaw in summer because of upwellings of warming sea water, the meltwater forms a cool layer that floats on the denser, warmer salty sea water below, the study said.

In winter, the melt water readily turns to ice because it freezes at zero degrees Celsius, above sea water at -2C (28.4F).At a winter maximum in September, ice on the sea around Antarctica covers about 19 million sq kms (7.3 million sq miles), bigger than Antarctica's land area. It then melts away into the ocean as summer approaches.