New maps of Greenland's coastal seafloor and bedrock beneath its massive ice sheet show that two to four times as many coastal glaciers are at risk of accelerated melting as had previously been thought. Researchers at the University of California at Irvine (UCI), NASA and 30 other institutions have published 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 are data from NASA's Ocean Melting Greenland (OMG) campaign.

The Greenlandic ice shield is the second largest freshwater reserve in the world. Its enormous weight presses the island deep into the Earth’s crust and thus leaves many areas flooded if the ice should vanish. Credit: Michael Wenger
The Greenlandic ice shield is the second largest freshwater reserve in the world. Its enormous weight presses the island deep into the Earth’s crust and thus leaves many areas flooded if the ice should vanish. Credit: Michael Wenger

Lead author of the study Mathieu Morlighem of UCI had demonstrated in an earlier paper that data from OMG's survey of the shape and depth, or bathymetry, of the seafloor in Greenland's fjords improved scientists' understanding not only of the coastline, but also of the inland bedrock beneath glaciers that flow into the ocean. That is because the bathymetry where a glacier meets the ocean limits the possibilities for the shape of bedrock farther upstream. The nearer to the shoreline, the more valuable the bathymetry data are for understanding on-shore topography, Morlighem said. "What made OMG unique compared to other campaigns is that they got right into the fjords, as close as possible to the glacier fronts. That's a big help for bedrock mapping." Additionally, the OMG campaign surveyed large sections of the Greenland coast for the first time ever. In fjords for which there are no data, it is difficult to estimate how deep the glaciers extend below sea level.

The map shows a stretch of Greenland's coastline as created by BedMachine before and after the inclusion of new OMG data. Especially noteworthy is the level of details (right side) compared to the previous map (left side). Credit: University of California Irvine, UCI
The map shows a stretch of Greenland's coastline as created by BedMachine before and after the inclusion of new OMG data. Especially noteworthy is the level of details (right side) compared to the previous map (left side). Credit: University of California Irvine, UCI

The OMG data are only one of many datasets Morlighem and his team used in the ice sheet mapper, which is named BedMachine. Another comprehensive source is NASA's Operation IceBridge airborne surveys. IceBridge measures the ice sheet thickness directly along a plane’s flight path. This creates a set of long, narrow strips of data rather than a complete map of the ice sheet. Besides NASA, nearly 40 other international collaborators also contributed various types of survey data on different parts of Greenland. No survey, not even OMG, covers every glacier on Greenland's long, convoluted coastline. To infer the bed topography in sparsely studied areas, BedMachine averages between existing data points using physical principles such as the conservation of mass. The new maps reveal that two to four times more oceanfront glaciers extend deeper than 200 meters below sea level than earlier maps showed. That's bad news, because the top 200 meters of water around Greenland comes from the Arctic and is relatively cold. The water below it comes from farther south and is 3 to 4 degrees Celsius warmer than the water above. Deeper-seated glaciers are exposed to this warmer water, which melts them more rapidly.

Left: Greenland topography color-coded from 1,500 below sea level (dark blue) to 1,500 m above sea level (brown). Right: regions below sea level connected to the ocean, either shallower than 200 m, light pink; between 200 - 300 m, dark pink; or continuously deeper than 300 m below sea level (dark red). The thin white line shows the current extent of the ice sheet. Credit: NASA
Left: Greenland topography color-coded from 1,500 below sea level (dark blue) to 1,500 m above sea level (brown). Right: regions below sea level connected to the ocean, either shallower than 200 m, light pink; between 200 - 300 m, dark pink; or continuously deeper than 300 m below sea level (dark red). The thin white line shows the current extent of the ice sheet. Credit: NASA

Morlighem's 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 7 centimeters for a total of 7.42 meters. OMG Principal Investigator Josh Willis of JPL, who was not involved in producing the maps, said, "These results suggest that Greenland's ice is more threatened by changing climate than we had anticipated."

Source: Carol Rasmussen, NASA