Antarctica's massive ice shelves are shrinking because they are being eaten away from below by warm water, a new study finds. That suggests future sea levels could rise faster than many scientists have been predicting.

The western chunk of Antarctica is losing seven metres of its floating ice sheet each year. Until now, scientists were not exactly sure how it was happening and whether or how man-made global warming might be a factor. The answer, according to a study published Wednesday in the journal Nature, is that climate change plays an indirect role – but one that has larger repercussions than if Antarctic ice merely were melting from warmer air.

Hamish Pritchard, a glaciologist at the British Antarctic Survey, said research using an ice-gazing NASA satellite showed that warmer air alone could not explain what was happening to Antarctica. A more detailed examination found a chain of events that explained the shrinking ice shelves.

Twenty ice shelves showed signs that they were melting from warm water below. Changes in wind currents pushed that relatively warmer water closer to and beneath the floating ice shelves. The wind change probably is caused by a combination of factors, including natural weather variation, the ozone hole and man-made greenhouse gases, Pritchard said in a phone interview.

As the floating ice shelves melt and thin, that in turn triggers snow and ice on land glaciers to slide down to the floating shelves and eventually into the sea, causing sea level rise, Pritchard said.

Rising sea level

Thicker floating ice shelves usually keep much of the land snow and ice from shedding to sea, but that is not happening now. That whole process causes larger and faster sea level rise than simply warmer air melting snow on land-locked glaciers, Pritchard said.

"It means the ice sheets are highly sensitive to relatively subtle changes in climate through the effects of the wind," he said. What's happening in Antarctica "may have already triggered a period of unstable glacier retreat," the study concludes.

If the entire Western Antarctic Ice Sheet were to melt, which would take many decades if not centuries, scientists have estimated it would lift global sea levels by about three metres.

NASA chief scientist Waleed Abdalati, an expert in Earth's ice systems who was not involved in the research, said Pritchard's study "makes an important advance" and provides crucial information about how Antarctica will contribute to global sea level rise.

Another outside expert, Ted Scambos of the National Snow and Ice Data Center, said the paper will change the way scientists think about melt in Antarctica. Seeing more warm water encircling the continent, he worries that with "a further push from the wind" newer areas could start shrinking.