Thousands of new rivers of meltwater are forming on Greenland's ice sheet far inland and dumping gigatonnes of water into the ocean. That could boost sea levels faster than expected, a new study suggests.

The massive ice sheet covering much of Greenland is on average 1.5 to 2 kilometres thick and contains enough ice to raise world sea levels by about six metres if it all melted. That process could take thousands of years to complete, but has been accelerating due to climate change.

Scientists previously thought that melting process might be slowed down by a porous layer on the surface of the ice called firn, which is partway between ice and snow.

That was a very powerful visual... to see no rivers one year, and the next year rivers extending an additional 20, maybe even 30 kilometres inland. - William Colgan , York University

Climate models expected the firn layer to absorb up to 30 to 40 per cent of any meltwater that travels across the ice sheet, allowing it to refreeze instead of pouring into the ocean.

But a new international study has found that during 2012, an unusual amount of melting caused the top of the firn to freeze into solid ice. That meant it could no longer absorb the meltwater, which instead formed thousands of new rivers snaking across the surface of the ice sheet to the ocean.

"That hadn't been seen before," said William Colgan, a researcher at York University in Toronto who co-authored the new study.

York University researcher William Colgan took part in a five-week expedition to the Greenland ice sheet in 2013. The evening after the plane dropped the researchers off, their whiskey had already frozen solid. (Courtesy William Colgan)

"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 inland."

The rivers could be several metres wide and many appeared farther inland than had ever been seen before.

The Intergovernmental Panel on Climate Change's predictions of sea level rise are based on models that assume the firn would fill up gradually over the course of a century, reducing the amount of meltwater that Greenland pours into the oceans.

The new study, led by Horst Machguth at the Geological Survey of Denmark and Greenland and published this week in Nature Climate Change, suggest those predictions are underestimates.

"In a year like 2012... an extra 60 gigatonnes of water or thereabouts went into the ocean," Colgan said.

Rising sea levels are already causing flooding in some coastal areas, and could become a more serious problem more quickly than anticipated.

While firn is theoretically renewable — it reforms from fresh snowfall — the top 40 metres of firn take 80 years to develop, and warmer years like 2012 are becoming increasingly common, Colgan said.

"Now the firn is being capped off much faster than it's being recreated."

And the phenomenon may not be restricted to Greenland — many other ice caps like those in Canada are covered in firn too.

"Evidence is emerging to show Canadian Arctic firn is also capping off," Colgan said. He added that relative to their size, glaciers in Canada contribute more to sea level rise than the Greenland ice sheet.

Freezing for science

In order to find out how the firn was changing with climate change, the researchers made four expeditions to the Greenland ice sheet between 2009 and 2015. They drilled into the firn, extracting long cylinders that they analyzed while braving temperatures that hovered around -40 C.

"It's pretty grim," said Colgan, who was part of a five-week expedition in 2013. The evening after the plane dropped the researchers off, their whiskey had already frozen solid.

The researchers slept in unheated, two-person tents, each huddled inside two nested sleeping bags while wearing many layers of clothing. They worked long days drilling through the ice and then cutting the ice cores up to measure the volume, mass and density of each layer.

"All of that core logging is very tedious work and you're not moving around, and it's so cold out that you can just feel the heat leaving your toes as you work late into the night," Colgan recalled.

But that's what was needed to get the data about detailed year-to-year changes in the firn, he said.

One of the next things he wants to do is confirm that the surface of the firn in Canada is also freezing up and unable to absorb meltwater. Based on the Greenland measurements, that may be possible to do using aerial surveys that measure the reflectance of the surface of the ice surface.