Now German researchers may have an answer for the pickup in speed – the ice sheet is melting underwater as well as at the surface.
A team from the Alfred Wegener Institute surveyed the ocean floor near the 79° North Glacier in northeast Greenland and discovered a two-kilometre-wide trough which, they say, channels comparatively warm water from the Atlantic directly towards the glacier.
More detailed analysis revealed a bathymetric sill – a barrier that the water flowing over the seafloor has to overcome.
Once it does, the water rushes down the back of the sill and under the glacier’s ice tongue – a strip of ice that has slid down into the ocean and floats on the water without breaking off from the land ice.
Thanks to this acceleration of the warm water mass, says oceanographer Janin Schaffer, large amounts of heat from the ocean flow past the tongue every second, melting it from beneath.
To make matters worse, the layer of warm water that flows toward the glacier has grown larger: measured from the seafloor, it now extends 15 metres higher than it did just a few years ago.
“The reason for the intensified melting is now clear,” says Schaffer. “Because the warm water current is larger, substantially more warmth now makes its way under the ice tongue, second for second.”
The ice tongue of 79° North Glacier is 80 kilometres long. It is the largest in northeast Greenland, but far from the only one – and the researchers fear others are being affected in a similar way.
Schaffer and her colleagues also investigated a region on Greenland’s eastern coast where another glacier, the Zachariæ Isstrøm, juts out into the sea, and where a large ice tongue had recently broken off from the mainland. Working from the surface of an ice floe, they measured water temperatures near the ocean floor.
“The readings indicate that here, too, a bathymetric sill near the seafloor accelerates warm water toward the glacier,” says Schaffer.
“Apparently, the intensive melting on the underside of the ice at several sites throughout Greenland is largely produced by the form of the seafloor.”
The new paper is published in the journal Nature Geoscience.