A massive Greenland glacier holding enough ice to raise world sea levels around 18 inches is retreating from the frigid island's coastline, and its leading edge is tumbling into the North Atlantic, researchers report.

The Zachariae Isstrom Glacier has been in an accelerated retreat since 2012 and is now losing its ice at the rate of more than five billions tons a year, they say.

To make that determination, scientists used aerial surveys and satellite-based observations, a study published in Science reported.

"North Greenland glaciers are changing rapidly," says Jeremie Mouginot of the University of California, Irvine's Department of Earth System Science.

The last few years have seen dramatic changes in the shape and dynamics of Zachariae Isstrom, he says.

"The glacier is now breaking up and calving high volumes of icebergs into the ocean, which will result in rising sea levels for decades to come," he points out.

The glacier's melting and retreat are the result of a double whammy of global warming issues, researchers say.

"Zachariae Isstrom is being hit from above and below," says senior author and UCI professor of Earth system science Eric Rignot.

Decades of progressively-rising air temperatures are melting the glacier's top, while currents of warming ocean water are compromising its underside, he explains.

Located in northeastern Greenland, the glacier has lost 95 percent of the ice shelf that previously worked to stabilize it, researchers say.

Most of the reduction in Zachariae Isstrom's mass is from ice loss as the glacier calves off huge chunks of its forward end into the ocean, says researcher John Paden of the University of Kansas.

"Ice floating out into the ocean and melting is greater than the ice lost from surface melting," he notes.

Zachariae and the nearby Nioghalvfjerdsfjorden glacier, which is also in retreat, together contain enough ice — 12 percent of the entire island's ice sheet — to raise worldwide sea levels by more than three feet, the researches say.

A complete melting of the entire ice sheet would push up global sea levels by a catastrophic 20 feet, they note.

NASA scientists have estimated the Greenland ice sheet is losing several hundred billion tons of ice every year.

"Not long ago, we wondered about the effect on sea levels if Earth's major glaciers were to start retreating," Rignot says. "We no longer need to wonder; for a couple of decades now, we've been able to directly observe the results of climate warming on polar glaciers."