Up to four times more coastal glaciers in Greenland are at risk of accelerated melting than previously thought, say scientists who have mapped the region's coastal seafloor and bedrock beneath its massive ice sheet.

Researchers from NASA and the University of California Irvine in the US created the most comprehensive, accurate and high-resolution relief maps ever made of Greenland's bedrock and coastal seafloor.

Among the many data sources incorporated into the new maps is data from NASA's Ocean Melting Greenland campaign.

The OMG campaign surveyed large sections of the Greenland coast for the first time ever.

The new maps reveal that two to four times more oceanfront glaciers extend deeper than 200 metres below sea level than earlier maps showed.

That is bad news, because the top 182 metres of water around Greenland comes from the Arctic and is relatively cold, according to the study published in the journal Geophysical Research Letters.

The water below it comes from farther south and is three to four degrees Celsius warmer than the water above. Deeper- seated glaciers are exposed to this warmer water, which melts them more rapidly.

The team used the maps to refine their estimate of Greenland's total volume of ice and its potential to add to global sea level rise if the ice were to melt completely, which is not expected to occur within the next few hundred years.

The new estimate is higher by seven centimetres for a total of 7.42 metres.

"These results suggest that Greenland's ice is more threatened by changing climate than we had anticipated," said OMG principal investigator Josh Willis of NASA's Jet Propulsion Laboratory (JPL).