A new study has discovered what is behind a new phenomenon that is being seen on the Greenland ice sheet in recent years, due to it’s accelerated melting: the formation of large rivers of fresh meltwater flowing on the ice sheet’s surface.

Through previous climate models, scientists expected that a porous layer of ice that exists on the surface of Greenland’s ice sheet, called "firn", would absorb about 30 to 40 percent of the island’s surface meltwater, where the water would refreeze before it could flow into the ocean. Firn is an intermediate form of glacial ice, where it is partway between snow and the solid ice that forms the lower layers of glaciers. This effect was expected to last over the course of the next century, reducing Greenland’s impact on sea level rise.

However, during an unusually high period of melting experienced by Greenland in 2012, the firn layer re-froze into solid ice, making it unable to absorb surface meltwater. This resulted in the formation of large rivers of water flowing on the surface that were several yards wide, dumping massive amounts of fresh water directly into the ocean.

"That hadn’t been seen before," exclaimed study co-author William Colgan, a researcher with Toronto’s York University. "That was a very powerful visual, to see just how dramatically the firn had changed — to see no rivers one year, and the next year rivers extending an additional 20, maybe even 30 kilometres [13 to 19 miles] inland." Colgan estimates that due to this process, an extra 60 billion tons of fresh water poured into the ocean in 2012.

While firn layer is renewable, since it is formed from fresh snowfall, it can take 80 years for 40 meters (130′) to form, and the recent cycle of warmer temperatures that form a hard ice cap seal on the surface are becoming increasingly common. Colgan also warns that "Evidence is emerging to show Canadian Arctic firn is also capping off," further contributing to potential sea rise.