Greenland's vast ice sheets are proving surprisingly resilient. That means the glaciers will melt only slowly as the climate warms, provided it does so steadily. However, wild changes in the weather could make them melt much faster.
Greenland's ice sheets are shrinking already as the climate warms, and some glaciologists fear that they could accelerate their own destruction. If they all melted, they would raise global sea levels by 6.5 metres – though even in a world 8 °C warmer than now this might take 1000 years.
The feared self-destruct device is water. As a glacier melts, water runs down to its base. In theory, this lubrication should accelerate the glacier's slide downhill and melt it sooner.
But this positive feedback is "limited", says Christian Schoof of the University of British Columbia in Vancouver, Canada, who has built a new model of glaciers that simulates how they respond to meltwater. He says glaciers can get rid of excess water because of the way their internal structure changes.
Channels and cavities
Glaciers have two drainage systems: meltwater carves out conduits called Röthlisberger channels in the base of the glacier, and cavities form whenever the ice passes over a bump in the ground. The cavities are the real threat because water stays in them, lubricating the glacier for longer.
However, Schoof's model shows that once the amount of meltwater exceeds a critical value, it burrows larger Röthlisberger channels for itself. As a result, extra water above that critical amount simply drains away and the glacier slows down. "It's a very efficient route for water to be evacuated," Schoof says.
Schoof's model could help explain what is happening to Greenland's glaciers, says glaciologist Roderik van de Wal of Utrecht University in the Netherlands, who has found no sign of the feared feedback effect. "The melt has been increasing but glacier velocities have not," he says. "This is a physical explanation."
Ice and storms
But there is one fly in this otherwise comforting ointment. Changeable weather can cause short-term spikes in water supply, adding more water than the glacier's drainage can cope with and temporarily speeding it up. "The variability of the weather is key," Schoof says.
That could be a problem, as Greenland is expecting more frequent rainstorms this century thanks to climate change. These storms might drive the glaciers to destruction.
Fortunately, this effect may also be quite small. Rainstorms have been observed accelerating glaciers, but each one's effect lasts only a few days, says Hilmar Gudmundsson of the British Antarctic Survey in Cambridge, UK. "You need a lot of storms to make a big difference," he says.
Journal reference: Nature, DOI: 10.1038/nature09618
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