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Warm summers shown to slow down glaciers

27 January 2011, by Tamera Jones

Hotter summers may not be such a disaster for the Greenland ice sheet after all, if the latest research is anything to go by. Scientists have found that Greenland glaciers flow slower in warmer summers than they do during cooler ones.

Russell Glacier

The Russell Glacier draining the Greenland icesheet inland from Kangerlussuaq on Greenland's west coast.

Although this seems counter-intuitive, the researchers explain that during cooler summers, a small amount of melted water from glaciers' surfaces seeps down to their bases, lubricating them so they move more easily.

But during warm summers, much more melted water makes its way underneath glaciers. This water bores out wider channels and so drains away faster, leading to less lubrication.

This is exactly what happens in mountain glaciers, 'where melt-induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient,' write the authors in their report, published in Nature today.

'Our findings are certainly at odds with what people might expect,' says Professor Andrew Shepherd from the University of Leeds, co-author of the study.

The team's findings address a key problem identified in the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC) published in 2007, and are likely to influence the next one which is due in 2012. The IPCC is made up of world-leading scientists who review and assess the latest scientific findings relevant to climate change.

In its fourth report, the IPCC didn't include the full effects of rapid changes in ice flow in its sea level projections, because scientists weren't sure exactly how such changes might evolve. While some studies had found that increased melt-water led to glaciers accelerating towards the sea, others had discovered a long-term decrease in Greenland's flow despite increased melting.

'Our findings are certainly at odds with what people might expect.'
Professor Andrew Shepherd from the University of Leeds

'By showing that this relationship appears to stop above a certain level of melting, we've taken away a large degree of uncertainty about how the climate could affect ice flow,' says Shepherd.

Understanding how the climate affects ice at the Earth's poles is crucial, because so much water is locked up as ice in the Greenland and Antarctic ice sheets. If all of Greenland's ice melted, sea levels would rise by about seven metres, threatening billions of people.

Rising temperatures in the Arctic in recent years have caused the Greenland ice sheet to shrink, prompting fears it may be close to a 'tipping point' beyond with melting would be irreversible. Some of this ice loss has been blamed on accelerating glaciers.

To find out exactly how the climate affects glacier flow, Shepherd and colleagues from the universities of Leeds, Edinburgh, Sheffield, and Vrije Universiteit Brussel in Belgium analysed satellite observations of ice velocity in Greenland over five years.

'Previous studies hadn't looked at how ice speeds vary throughout years of markedly different melting,and you need to do that to get the full picture,' says Shepherd.

Although they found that the initial ice speed-up was similar in all the years they looked at, glaciers in southwest Greenland flowed 62 per cent more slowly in the second half of warmer years than in cooler years.

'We were a little surprised to find that flow is that much slower in warmer years. We thought there wouldn't be an appreciable difference,' Shepherd says.

He is however keen to point out that, 'this doesn't mean the ice sheet is safe from change change - the impact of ocean-driven melting remains uncertain.'


Aud Venke Sundal, Andrew Shepherd, Peter Nienow, Edward Hanna, Steven Palmer & Philippe Huybrechts, Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage, Nature, published 27 January 2011, doi:10.1038/nature09740


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