A major ice sheet in western Antarctica is melting irreversibly, and its collapse is predicted to raise global sea levels by nearly 61cm, according to scientists.

Theories of the ice sheet's impending doom have been circulating for some time, and a study in the journal Science said the process is now expected to take between 200 and 1,000 years.

The thinning of the ice is likely to be related to global warming, according to the paper which was funded by NASA and the National Science Foundation.

Airborne radar measurements of the West Antarctic ice sheet allowed scientists to map the underlying bedrock of Thwaites Glacier.

Other satellite measurements provided the height of the ice sheet as it thins out over time.

Scientists used predictive models based on this data to envision how the ice sheet will melt and when, and discovered that such a collapse may be inevitable.

"Previously, when we saw thinning we didn't necessarily know whether the glacier could slow down later, spontaneously or through some feedback," said study author Ian Joughin, a glaciologist at the University of Washington.

"In our model simulations it looks like all the feedbacks tend to point toward it actually accelerating over time; there's no real stabilizing mechanism we can see."

The fastest scenario scientists found for the melting was 200 years and the longest was more than 1,000 years.

Mr Joughin said the most likely melt times were between 200 and 500 years.

"All of our simulations show it will retreat at less than a millimeter of sea level rise per year for a couple of hundred years, and then, boom, it just starts to really go," Mr Joughin said.

The findings are echoed in a second study conducted by NASA, details of which are published in the journal Geophysical Research Letters.

Incorporating 40 years of measurements, it indicates the glaciers in the Amundsen Sea sector of West Antarctica "have passed the point of no return".

According to the study, these glaciers already contribute significantly to sea level rise, releasing almost as much ice into the ocean annually as the entire Greenland Ice Sheet.

They contain enough ice to raise global sea level by four feet and are melting faster than most scientists had expected.

The study's authors say the findings will require an upward revision to current predictions of sea level rise.