The rate at which two major Greenland glaciers are moving has seen a dramatic acceleration, a study warns.
Swansea University researchers say the flow rate, or the speed at which the constantly travelling glaciers move, has doubled in two years.
Scientists have warned the findings could mean the ice cap is melting even more quickly than previously thought.
A report published by the UK government last week also expressed fears of the impact of climate change.
The Swansea-led study, which involved scientists from Leeds and Sheffield universities, looked at the Kangerdlugssuaq glacier in East Greenland and the Helheim glacier, 186 miles away.
The two glaciers had remained relatively stable throughout the 1990s despite changes in other Greenland glaciers. But both have now begun accelerating at very similar rates.
Dr Luckman said that Greenpeace activists have previously raised concerns at the speed at which the Kangerdlugssuaq was moving.
"The time of the disappearance of the Greenland ice sheet will be quicker than that suggested"
But he added: "This is the first time we have proved beyond doubt that it is happening."
There are two possible reasons for the acceleration, said Dr Luckman.
Either an increase in surface melt-water is lubricating the passage of the glaciers or rising ocean temperatures.
"We have no data on ocean temperatures but we don't find any correlation between the rate of surface melt and the acceleration," he said.
In the report, the Glaciology Group at Swansea's school of the environment and society has said: "The period of continued warming and thinning appears to have primed these glaciers for a step-change in dynamics not included in current models.
'Future stability'
"We should expect further Greenland outlet glaciers to follow suit."
Dr Luckman said that the results indicated that, "the time of the disappearance of the Greenland ice sheet will be quicker than that suggested."
The researchers studied images from satellite radar tracking technology.
Group leader, Professor Tavi Murray, added: "Results from this and our other work will feed into models that will predict the future stability of the Greenland ice sheet and its impacts.
"Greenland also has the potential to be a major net contributor to ocean freshening which could impact on ocean circulation and European climate over a relatively short time-scale."
A report published by the UK government last week expressed fears that the Greenland ice sheet is likely to melt, leading sea levels to rise by 7m (23ft) over 1,000 years.
The poorest countries would be most vulnerable to these effects, it added.
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