Large meltwater stream rushes across the surface of the Greenland Ice Sheet filling a supraglacial lake.

New research reveals that within the Greenland ice sheet there is a recently formed layer of ice that has been found where it should not be. It is this ice layer that will cause even greater rates of sea level rise than had previously been thought. Greenland is a moist environment and until recently most surface meltwater would percolate into the tightly packed snow on the surface absorbing vast quantities of water. But the discovery of this new ice layer, which formed during a very warm melt season in 2012, shows that the massive ice sheet can no longer absorb meltwater in some areas.

John R. Platt has a must read piece in Take Part that summarizes the recent research titled “Greenland meltwater storage in firn limited by near-surface ice formation” that was recently published in the journal Nature. (Firn is defined by the Free Dictionary as “Granular, partially consolidated snow that has passed through one summer melt season but is not yet glacial ice. Also called old snow.”)

Colgan said the “lion’s share” of that loss—about 5,000 tons per second—comes in the form of meltwater. The ice sheet, like a sponge, used to be able to absorb most of what melted each year because the uppermost layers are composed of tightly packed but permeable snow, as opposed to the impermeable layers of ice much farther below. That porous surface, called “firn,” normally would allow meltwater to sink downward, where it would refreeze and stay within the glacier.

A well-respected group of scientists noted that the percolation of meltwater through the firn began to change about 10 years ago. It was hard not to note that massive meltwater rivers had formed on the ice sheet and the meltwater that did not descend to the bedrock via a moulin (a vertical conduit that channels water downwards) became rushing torrents to the sea from distances of up to 30 miles. The study’s authors note that rivers had always been a part of the hydrology of Greenland, but they only were known to have traveled 15 miles. One of the study authors, William Colgan of York University in Toronto, noted: “That led us to speculate that the downward motion was being blocked somehow.”

It all stems from an extreme melt that took place in 2012. When that melted snow refroze, it formed a layer of ice several meters thick in the middle of the firn. Now melting water hits that thick ice layer and can sink no farther. Since it can’t go down, it goes sideways. “The rivers go downhill from the high interior of the ice sheet toward the coast,” Colgan said.

The researchers note that Greenland is extremely complicated but that every time new research on the hydrology of the ice sheet comes out, it turns out that the ice is melting more rapidly than was previously thought.

A ravine near the monitoring station shows the contrast between non-polluted ice and polluted ice. (Photo: Dark Snow Project)

“All the projections we made assumed the water would keep percolating vertically until it filled up all of that firn space,” Colgan said. “Now we can say that’s probably not going to happen over large areas of the ice sheet.” That, he said, means those earlier projections now underestimate Greenland’s current and future contributions to sea-level rise.

From the study abstract:

Approximately half of Greenland’s current annual mass loss is attributed to runoff from surface melt 1 . At higher elevations, however, melt does not necessarily equal runoff, because meltwater can refreeze in the porous near-surface snow and firn 2 . Two recent studies suggest that all 3 or most 3 , 4 of Greenland’s firn pore space is available for meltwater storage, making the firn an important buffer against contribution to sea level rise for decades to come 3 . Here, we employ in situ observations and historical legacy data to demonstrate that surface runoff begins to dominate over meltwater storage well before firn pore space has been completely filled. Our observations frame the recent exceptional melt summers in 2010 and 2012 (refs 5 , 6 ), revealing significant changes in firn structure at different elevations caused by successive intensive melt events. In the upper regions (more than ~1,900 m above sea level), firn has undergone substantial densification, while at lower elevations, where melt is most abundant, porous firn has lost most of its capability to retain meltwater. Here, the formation of near-surface ice layers renders deep pore space difficult to access, forcing meltwater to enter an efficient 7 surface discharge system and intensifying ice sheet mass loss earlier than previously suggested 3 .