BOSTON –

Scientists were baffled last year after meltwater lakes atop Greenland's ice sheet suddenly drained out at rates rivaling Niagara Falls.

Now a team of U.S. researchers says it has figured out the bizarre phenomenon and that could help them forecast global sea-level rise.

Vertical shafts in the ice sheet, called moulins, can funnel melt water beneath parts of the glacier and lift them up. This causes cracks beneath the so-called supraglacial lakes that can empty them in days, according to scientists from the Massachusetts Institute of Technology-Woods Hole Oceanographic Institution (MIT/WHOI) Joint Program in Oceanography.

Draining lakes can accelerate sea-level rise by suddenly injecting large volumes of water into the ocean and lubricating the flow of ice offshore. However, the finding suggests that only lakes at lower, warmer altitudes on the ice sheet where moulins are more prevalent are vulnerable, according to the research published in the journal Nature.

"The trigger is less likely to occur at lakes at higher elevations on the ice sheet — even though water volumes in those lakes can be large," according to the research.

"Our discovery will help us predict more accurately how supraglacial lakes will affect ice sheet flow and sea level rise as the region warms in the future," lead author Laura Stevens wrote in a Woods Hole press release.

Scientists at Ohio State and Cornell University said last year that two lakes on the Greenland ice sheet that had previously held billions of gallons of water had mysteriously disappeared.

The Greenland ice sheet covers more than 600,000 square miles (1.6 million square kilometers) and is expected to be a significant contributor to sea-level rise as it melts.