Ice shelves are thick plates of ice that float on the ocean around much of Antarctica. Snow, glaciers and ice flows feed these large plates in the colder months. In warmer periods, surface melting creates standing water that leaks into cracks and speeds the breaking off (calving) of icebergs, decreasing the continent’s mass in a natural cycle as old as Antarctica itself.

“Large icebergs calve off on a fairly regular basis from the larger ice shelves in Antarctica,” says Dr. Ted Scambos, a research associate at National Snow and Ice Data Center. “This is a part of their normal evolution.”

The only effect of such calving that scientists are sure about is that they are changing the outline of Antarctica. The break-up of the ice shelves, which account for about two percent of the continent’s landmass, does not have any measurable effect on sea levels. “Since an iceberg floats in ocean water, and much of it is below the surface, it is already displacing the same volume of water it will contribute when it eventually melts,” Scambos explains.

But while such calving activity may not be new, it has increased over the last 30 years, with larger and larger chunks breaking off from Antarctica where they float free in the ocean and break up into successively smaller pieces. One especially large iceberg, a chunk the size and shape of New York’s Long Island and dubbed “B15A” by researchers, broke off from Antarctica’s Ross Ice Shelf in 2000 and just last April collided with the continent’s Drygalski Ice Tongue (a long shelf of ice extending out to sea from the mainland). The iceberg itself remained intact, but a city-sized chunk of the ice tongue broke off and is now floating free.

Most researchers suspect that recent increases in calving are linked to warming surface air temperatures as a result of human-induced climate change. British glaciologist David Vaughan says, “There is no doubt that the climate on the Antarctic Peninsula has warmed significantly over the last few decades. What we're seeing now are changes only just working through to glaciers and ice sheets.” Scambos says that, as Antarctic summer temperatures continue to increase, the process can be expected to become more widespread, and could begin to significantly increase sea levels around the world.

Even a relatively small rise in sea level would make some densely settled coastal areas uninhabitable. The Intergovernmental Panel on Climate Change, an international group of climatologists, predicts a global sea level rise of less than three feet by 2100, but also warns that global warming during that time may lead to irreversible changes in the Earth’s glacial system and ultimately melt enough ice to raise sea levels many more feet in coming centuries. Some 200 million people inhabit low-lying areas in countries like Vietnam, Bangladesh, China, India and The Philippines and could be displaced, leading to a major international refugee crisis.