How the Greenland ice sheet is fracturing
Drone images show rapid drainage of melted ice is causing chain reactions.
By Natalie Parletta
Fissures caused by melting of the Greenland ice sheet are threatening its stability, and now drone footage has shown the resulting water drainage is occurring more quickly than previously thought.
Melting of this ice sheet – the second largest in the world, with three million cubic kilometres of ice that is two kilometres thick on average – has the largest impact on rising sea levels globally.
“If it were to melt entirely, sea levels would rise by over seven metres,” says lead author Tom Chudley from the Scott Polar Research Institute at the University of Cambridge, UK, “although we don’t expect that to happen in the foreseeable future.”
Nonetheless, he adds, it is currently contributing to sea level rise of one millimetre per year – higher than predictions just a few years ago – and the rate is increasing faster than losses from other ice masses around the globe.
As the weather warms each summer, satellite images show increasing numbers of lakes have been forming on the sheet’s surface which flow within hours through more than a kilometre of ice to its base.
“Our previous work has shown that faster ice flow leads to more fractures,” says team leader Poul Christoffersen. This chain reaction can cause drainage of other lakes up to 80 kilometres away and accelerate the rate of sea level rise.
To get a closer look at how the fractures form, the researchers flew custom-made drones suited to extreme Arctic conditions on autopilot over a kilometre-wide lake that had formed on the surface of the ice sheet at Store Glacier in northwest Greenland.
Footage showed that as the lake expanded, a large fracture opened across virtually its whole diameter, causing a catastrophic drainage of five million cubic metres of water – around 2000 Olympic-sized swimming pools worth – to the base of the ice in just five hours and the ice level to rise by half a metre.
Using hundreds of photographs and GPS coordinates taken by the drone, the researchers created 3D models of the ice sheet surface before and after the event to investigate how and why the fracture formed, and the effect of the lake drainage on the ice sheet’s stability.
This showed that the ice flow increased from two to five metres per day as water drained to the sheet’s bed, causing new fractures and expansion of dormant ones.
“The acceleration, even if short-lived, caused other fractures to form, creating more pathways of water to the bed of the ice sheet,” says Chudley, and helps to explain the ripple effects of ice melt.
As melted water reaches the bed it loosens the bottom of the sheet, causing the ice to flow faster and break free. The resulting ice loss, combined with melting, nets one billion tonnes every day, or around 400 gigatonnes per year.
This adds new information to existing models that haven’t accounted for interactions between surface ice melt and ice flow dynamics. Together with the rapid lake drainage, it suggests current – as well as past – predictions are vastly underestimated.
“As recently as 25 years ago, glaciologists thought that the Greenland ice sheet wouldn’t be dramatically affected by climate change over human timescales,” says Christoffersen.
“Less than three decades later, it’s the largest single contributor to global sea level.
“Understanding how and why these dramatic changes have taken place is essential so that we can produce accurate numerical models of ice sheet behaviour in the twenty-first century, which will allow our society to act to adapt to the consequences of a warming planet.”
The findings are published in the journal Proceedings of the National Academy of Sciences.