IF SOME of the spectacular calving of ice shelves in Antarctica is down to global warming, then why did we not see break-ups on the same scale in Greenland, which is much warmer? It turns out that, counter-intuitively, it’s because Greenland is warmer.
When the ice sheets that blanket Antarctica and Greenland eventually meet the sea, they don’t immediately calve off and create icebergs. Instead, they extend out to sea as floating ice shelves while remaining joined to the ice sheets on land.
In 2002, a gigantic section of the Larsen B ice shelf in the Antarctic Peninsula suddenly broke off. It had been an unusually warm summer, with temperatures rising to a balmy 4 °C. As the ice melted, huge pools of meltwater formed on the surface of the ice, and as this water poured down crevasses it forced apart sections of the shelf. “It fell apart in a whole lot of little slivers,” says , a glaciologist at Pennsylvania State University in University Park.
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In contrast, while Greenland has experienced summer temperatures of up to 11 °C over the past half century, many of the ice shelves had held firm, despite some surface melting. “Greenland ice shelves are surviving,” says Alley.
Now Alley, along with Byron Parizek, also of Penn State, and colleagues have worked out how the warmer temperatures themselves could explain why. Mathematical models suggest the higher temperatures in Greenland cause lakes of meltwater to form on the ice sheet, rather than on the ice shelf as happens in Antarctica. This meltwater then pours down the glacier’s “plumbing” – its crevasses and moulins – to the ice sheet’s base, where it flows out to sea. Had the meltwater pooled on an ice shelf, the water flowing into the cracks would have split the floating ice.
“Warmer temperatures allow meltwater to pool on the ice sheet, where it can flow down the plumbing”
The models suggest that something similar could happen in Antarctica as it warms. Over time, as “plumbed” ice forms on land and flows down to the sea, the ice shelf could regrow. The plumbing would channel the water to the sea without forcing apart cracks in the ice.
However, Alley cautions that the regrowth would be temporary, as witnessed in Greenland, where the ice shelf in front of the Jakobshavn glacier finally fell apart in the mid-1990s, as seawater eroded it from underneath (Journal of Geophysical Research, ).
The idea could be put to the test by studying the glaciers behind the now-disintegrated Larsen A and B ice shelves. If these ice sheets develop plumbing and push this ice towards the sea, then we may see the ice shelves regrow. “Whether the regrowth could slow down the ice loss, that’s a question we have to go after next,” says Alley.
