Everyone knows the melting of Antarctic ice sheets, leading to substantial rises in sea levels, is perhaps the scariest threat posed by climate change. But even the brainiest scientists can't predict how big the threat is. They just don't know enough about what drives the big melt.

But now they know a lot more, with the results of a huge research project being published tonight in the top scientific journal 'Nature'.

Led by scientists from the British Antarctic Survey the project used over 4 million measurements of ice thickness made by a laser instrument on NASA's ICESat.

They now believe warm ocean currents are the dominant cause of recent ice loss from Antarctica. They say changing wind patterns have brought warmer water under the ice shelves that surround the continent.

In itself that has no effect on sea levels because the ice shelves float on the sea surface. So when they melt, the water displaces the same volume as the ice that melted.

BUT - and this is crucial - the ice shelves act as a dam holding back the flow of ice through glaciers from the main ice sheet covering the land mass of Antarctica. And when THAT melts, sea levels WILL rise.

So what's happening is that warmer seas are melting the ice shelf from below, and that's speeding up the flow of glaciers (which are frozen rivers), and that's bringing more ice, and that's increasing sea levels.

Some ice shelves are thinning by a few metres a year and, in response, the glaciers drain billions of tons of ice into the sea.

The ICESat in orbit. Credit: Nasa Goddard Space Flight Centre/Chris Meaney

Why is this research so important? One because it confirms the fear that Antarctic ice is melting because of changing climate. And two because it gives scientists the tools they need to predict how big a threat this is.

Top ice man at BAS, Professor David Vaughan, said: “The key to predicting how the ice sheet will change in the future is in understanding the oceans. Perhaps we should not only be looking to the skies above Antarctica, but also into the surrounding oceans.”