Antarctic ice threatens to crack: What does it mean?

A rift in an ice shelf in Antarctica is continuing to grow, with just 16km of ice preventing a massive iceberg from calving off.

The spectacular crack in the Larsen C ice shelf, located on the Antarctic Peninsula, risks splitting off an iceberg around 5000 square kilometres - about twice the size of Samoa.

It's been extending for years, but its growth has sped up in the last year, raising concerns for the future of the shelf.

Scientists aren't sure when the ice will give in and splinter, saying it could be days, months or even years.

"To be modelling these cracks is extremely challenging. It's a little bit like being able to predict the next earthquake," glaciologist Professor Eric Rignot said.

As ice shelves float on the water, the effect on global sea level rise if they do melt is minimal. A bigger concern is whether the shelves are holding back the greater ice sheets - land-based ice which, if it melted, would affect sea level.

"What we are seeing on Larsen C has implications for the big ice shelves farther south that hold considerable [sea level] potential," Prof Rignot said.

"The loss of these larger ice shelves and the resulting acceleration of glacial calving could amount to metres of sea level rise in the decades and centuries to come."

Previous ice shelves Larsen A and Larsen B fractured into nothingness, but Prof Rignot said even if this monster iceberg calves from Larsen C, it doesn't have to mean the end for the shelf.

"Icebergs detach every once in a while. It doesn't mean it's climate change."

When it comes to the destruction of the ice sheets, Prof Rignot said it's not all about melting.

"It's not going to melt away. It's going to fracture," he said.

"It's going to reach a limit beyond which it is not stable. It's going to fall apart like an eggshell that became too thin."

For the meantime, NASA and Project MIDAS, from the British Antarctic Survey, are monitoring the growing rift.