US researchers recently discovered that two sub-glacial Greenland lakes of meltwater have drained away.
What’s left is two significantly depressed areas of ice on the surface, which have sunk because the lakes previously hidden beneath them flowed away.
By flying over Greenland with cameras in tow, researchers spotted the huge drops in ice, marking a significant discovery.
Indeed one of the lakes has revealed the natural plumbing of the ice sheets, showing how melted ice on the surface flows down below the ice sheet and gets trapped above the bedrock, creating these sub-glacial lakes.
As significant as a drop can be
One lake was found after billions of gallons of meltwater, which carries latent heat, flooded away a couple of years ago. The other has actually filled and emptied twice in the past two years.
The discovery of this meltwater below the ice is significant because, unlike Antarctica, Greenland has never been successfully mapped for glacier differences and topographical nuances.
What seems to be happening now is the increased meltwater below is overflowing the ice sheet’s natural plumbing system, creating “blowouts” beneath the surface which drains the lakes away, as reported by Science Daily.
“The fact that our lake appears to have been stable for at least several decades, and then drained in a matter of weeks – or less – after a few very hot summers, may signal a fundamental change happening in the ice sheet,” said Ian Howat, who lead the team that discovered one of these lakes.
The lake’s return offers new insights
While Howat’s report has been logged in the open-access journal The Cryosphere, the second discovery of a lake that has refilled twice recently, led by Michael Willis and co-authored by Michael Bevis, is documented in Nature.
Every time this lake fills up, the meltwater brings latent heat that compromises the structure of the ice sheets around it.
“If enough water is pouring down into the Greenland Ice Sheet for us to see the same sub-glacial lake empty and refill itself over and over, then there must be so much latent heat being released under the ice that we’d have to expect it to change the large-scale behaviour of the ice sheet,” said Bevis.
An example of a supraglacial lake and rivers on the surface of the Greenland Ice Sheet. Photo via Thomas Nylen
Each summer bright blue streams are visible from above, which is melted ice. What happened to this water when it disappeared into cracks in the ice had remained a mystery.
“We’re seeing surface meltwater make its way to the base of the ice where it can get trapped and stored at the boundary between the bedrock beneath the ice sheet and the ice itself. As the lake beneath the ice fills with surface meltwater, the heat released by this trapped meltwater can soften surrounding ice, which may eventually cause an increase in ice flow,” said Willis.
A milestone in ice loss, a milestone in global warming?
The fact that these lakes, and the refilling of one, is visible from above, signals that Greenland’s ice loss has reached some sort of milestone.
A major problem for researchers, due to the lack of knowledge of the subterranean network of meltwater flows, is nobody knows how many lakes lay below the surface, and where the “blowouts” are.
“Until we get a good map of the bed topography where this lake was, we have no idea whatsoever how many lakes could be out there,” Howat said.
"There may be something really weird in the bed in this particular spot that caused water to accumulate. But, if all you need is a bumpy surface a bit inland from the coast, then there could be thousands of little lakes.”