The threat of the Greenland ice sheet slipping ever faster into the sea because of warmer summers has been ruled out by a scientific study.
Until now, it was thought that increased melting could lubricate the ice sheet, causing it to sink ever faster into the sea. The issue was a key unknown in the landmark 2007 report from the Intergovernmental Panel on Climate Change, which pinned the blame for climate change firmly on greenhouse gas emissions from human activities.
However, the impact of rising sea temperatures on melting ice sheets is still uncertain, meaning it remains difficult to put an upper limit on potential sea level rises. Understanding the risk is crucial because about 70% of the world's population live in coastal regions, which host many of the world's biggest cities, such as London, New York and Bangkok.
"The Greenland ice sheet is safer than we thought," said Professor Andrew Shepherd of the University of Leeds, who led the research published tomorrow in Nature.
Shepherd's team used satellite imagery to track the progress of the west Greenland ice sheet as it slipped towards the sea each summer, over five years.
Researchers had feared that more melting from the surface of the ice in hotter years would in turn provide more meltwater for a slippery film at the sheet's base. More melting would mean more slippage and a greater rise in the sea level.
But they discovered that, above a certain threshold, the slipping began to slow. On-the-ground studies and work done on alpine glaciers suggest that higher volumes of meltwater form distinct channels under the ice, draining the water more efficiently and reducing the formation of a lubricating film.
The Greenland ice sheet studied by Shepherd's team is up to 1,000m (3,280ft) thick. If the entire ice sheet melted, sea levels would rise by a catastrophic seven metres, but this is likely to take 3,000 years if warm air blowing over the ice is the only way in which the ice melts.
Shepherd said most of the Greenland ice cap was on land and not in contact with the sea, unlike the west Antarctic ice sheet. That ice sheet contains enough water to push up sea level by six metres if it all melted.
He said the next scientific question to answer was whether warmer oceans would erode the edges of ice caps, causing them to fall rapidly into the ocean. "The real threat now is from the oceans melting the west Antarctic ice sheet, which is 3km-4km thick, of which 1km-2km is below sea level."
Shepherd said his work was helping to reduce uncertainties about the consequences of climate change. Asked if he thought his work suggested the wider risks of global warming could be discounted, he said: "Not at all."