THE good news is that it would take more than 1,000 years to melt the massive ice sheet in West Antarctica that could raise sea levels by 16 feet.

The bad news is this event could become unstoppable this century if carbon dioxide concentrations keep rising as predicted, a study has found.

An investigation into the stability of the giant West Antarctic ice sheet has found that it has collapsed before when carbon dioxide concentrations in the atmosphere have reached about 400 parts per million, a level expected to be reached by 2050, if temperatures continue to rise.

It will almost certainly collapse when nearby ocean temperatures rise by about 5C, the likely result of global warming this century.

Two groups of researchers say the ice sheet has collapsed regularly, most recently about 400,000 years ago and one million years ago, resulting in large increases in sea levels.

David Pollard of Pennsylvania State University, and Robert DeConto of the University of Massachusetts used a computer model to simulate the behavior of the ice sheet over the past five million years.

They focused on a period called the Pliocene, some five to three million years ago, when temperatures were similar to those expected in the coming centuries.

The scientists found that the West Antarctic ice sheet melted and reformed several times. Each switch took just a few thousand years.

The results are bolstered by a separate analysis of sediment dug from underneath the Antarctic Ross ice shelf, which also indicate periodic large-scale melting during that period.

Pollard said: “The modeling shows it (the ice sheet collapse) has happened with regularity in the past and will happen again, driven by ocean warming.”

He said more studies were needed to work out what level of ocean warming in the region would provoke another collapse. The 5C figure in the new paper is a “rough number” he said. “It could be 3C or it could be 6C.”

Warmer oceans would melt the floating ice sheets around Antarctica, which currently block the sea’s access to larger, ground-based ice sheets further toward the continent’s interior.

With the floating ice sheets gone, the land-based ice would be free to melt and so raise sea levels. Glaciologists call such an event a collapse, but Pollard said it would not be rapid, and would take thousands of years to unfold: “We had a bit of a debate whether to use the word collapse in the paper. It’s not something like an avalanche.” How quickly the West Antarctic and Greenland ice sheets could break up and melt has become a hot topic in climate science.

The 2007 report from the Intergovernmental Panel on Climate Change said not enough was known to make any predictions.

Since then some scientists have warned that the ice sheets are more unstable than realized — and that sea levels could rise faster than expected.

The two studies are published in the journal Nature.