(CNN) Scientists have long known that higher air temperatures are contributing to the surface melting on Greenland's ice sheet.

But a new study has found another threat that has begun attacking the ice from below: Warm ocean water moving underneath the vast glaciers is causing them to melt even more quickly.

Nioghalvfjerdsfjorden Glacier -- also known as the 79° North Glacier -- in northeast Greenland. The findings were published Monday in the journal Nature Geoscience by researchers who studied one of the many "ice tongues" of theNioghalvfjerdsfjorden Glacier -- also known as the 79° North Glacier -- in northeast Greenland.

Another view of the 50-mile-long glacier tongue shows torrential meltwater streams making their way toward the ocean.

An ice tongue is a strip of ice that floats on the water without breaking off from the ice on land. The massive one these scientists studied is nearly 50 miles long.

The survey revealed an underwater current more than a mile wide where warm water from the Atlantic Ocean is able to flow directly towards the glacier, bringing large amounts of heat into contact with the ice and accelerating the glacier's melting.