Catherine Brahic, New Scientist 14 Jul 09;
The biggest glacier in the Arctic is on the verge of losing a chunk of ice the size of Manhattan. A group of scientists and climate change activists who are closely monitoring the Petermann glacier's ice tongue believe the rapid flow of ice is in part due to warm ocean currents moving up along the coast of Greenland, fuelled by global warming.
During the summer of last year, Jason Box of Ohio State University in Columbus noticed a huge crack in the glacier's floating ice tongue, which acts as a conveyor belt, pushing the glacier's ice through a fjord and out to sea. The crack extended almost completely from one side of the fjord to the other, 16 kilometres away.
This prompted Box and colleagues to return this year on the Arctic Sunrise, a Greenpeace vessel. The researchers are equipped with an arsenal of cameras and sensors, which they have been setting up on surrounding cliffs as well as on the ice itself.
Break imminent
Stitched together, the pictures they are taking will provide a blow-by-blow animation of the event. "We're looking on a minute by minute basis at what it's doing, how it's moving in relation to the rest of the glacier, and looking for that critical point where it fractures and breaks off," says Alun Hubbard, a glaciologist at the University Of Wales, UK.
The team believes this will happen within weeks. Only yesterday, a 3-square-kilometre chunk broke away. There are now more than 10 cracks in the ice, some 500 metres wide. The researchers expect the ice tongue to break up within the coming weeks.
When this happens, an island of ice the size of Manhattan, spanning 100 km2 holding 5 billion tonnes of ice, will break free and drift out to sea.
Melt concerns
As with all glaciers that terminate over water, big chunks of ice regularly break off the Petermann ice tongue, a process which is normally compensated for by the snow that falls on the upper reaches of the glacier. But the sheer amount of ice that could break away in a single event is concerning the scientists – five billion tonnes of ice is equivalent to nearly half of the glacier's usual annual flow.
The researchers are unsure what exactly is causing the break-up. A chunk of 1 million tonnes of ice broke off last year and there has been an acceleration in the flow of ice over the past few years. They think a number of factors are involved including warmer ocean currents that are melting the ice from below and warmer air temperatures that are melting it from above.
"Ocean warming currents are circulating around the fjord and eroding the underbelly of Petermann glacier at an incredible rate," says Hubbard.
Driller thriller
Melting at the surface of the ice forms huge whirlpools of relatively warm fresh water that bore holes into the floating sheet. The scientists believe this process is accelerating the ice's demise.
In places, the meltwater bores holes through the ice right down to the bottom of the ice tongue. Surfacing seals are proof that some of the holes – called moulins – pierce to bottom of the ice.
Box, who in addition to posting his instruments on the ground is surveying the ice by helicopter, says the view from above is one of "innumerable turquoise pools, from puddles to lakes – thousands of them".
When the huge section of ice breaks off, it could be like uncorking a bottle. A smaller ice tongue will provide less resistance for the glacier as it flows out to sea, which ultimately will accelerate sea level rise.
