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Thursday, January 22, 2009

Greenland glacier's speed doubles two hours after daily peak surface melting

Glacier's speed doubles two hours after daily peak surface melting

21 January 2009

The surface temperature of a so-called land-terminating glacier appears to controlling how fast it moves on a daily and annual basis, according to new research.

Russell Glacier

The Russell glacier in south-west Greenland. Resarchers have found that it speeds up just after midday and slows at night.

Dr Andrew Shepherd and colleagues have shown how the Russell Glacier in south-west Greenland speeds up after peak melting during the day and after late summer melting -- when the glacier's surface reaches its warmest.

It seems meltwater from the surface slips through the glacier acting as a lubricant, making it easier for the ice at the bottom to slide over bedrock. The study, published in Geophysical Research Letters, helps explain why the Greenland ice sheet is shrinking - a problem which could lead to sea levels rising sooner than expected. Greenland contains enough ice to raise sea levels by around seven metres.

In the last 15 years the air temperature over Greenland has been rising faster than anywhere on Earth. But at the moment best estimates suggest that the ice sheets of Greenland and Antarctica only make a small contribution to sea level rise -- around 0.1-0.8 mm/year of the 3-mm/year average.

'The most recent IPCC predictions for sea level rise are wrong, because they don't include ice dynamics.' Andrew Shepherd, University of Edinburgh

However, in its Fourth Assessment Report in 2007, the Intergovernmental Panel on Climate Change (IPCC) stated that the mechanisms that link climate and ice dynamics are poorly understood and are one of the great uncertainties in forecasting future sea level rise. Because so little is known about ice dynamics, the IPCC didn't include the effects of ice flow in its models.

'The most recent IPCC predictions for sea level rise are wrong, because they don't include ice dynamics,' says Shepherd.

Since the IPCC's 2007 report, researchers have been trying to plug the gap in their knowledge of exactly how the Earth's ice sheets will respond to a warming climate.

Although earlier research has shown that meltwater can find its way to the base of alpine glaciers, nobody has been able to link warmer temperatures to the speed at which land-terminating glaciers in an ice sheet flow.

Shepherd and his team placed three GPS sensors on the surface of the Russell Glacier along the 'line of flow' to monitor the movement of ice during late summer. They also used satellite images to monitor lake formation on the surface of the ice to find out if the glacier moved when water disappeared from these lakes. Shepherd guessed that water from the lakes could be streaming to the base of the glacier and helping it move faster.

After peak melting time -- usually around midday -- the team found that the speed of ice doubled two hours later, but slowed down 12 hours afterwards once air temperatures had cooled. During late summer, the ice accelerated as the temperature increased. The speed of the glacier was double the winter average.

Accelerating ice

Using a model to estimate the amount of meltwater produced by the glacier, the team found that the amount of daily meltwater matched the acceleration of the ice. The glacier moved the most after water in the lakes had drained. At one site, the ice moved by as much as 250 m in one year. Land-terminating glaciers move much more slowly than glaciers that reach the sea -- so-called outlet glaciers, which travel several kilometres a year.

'Nobody knew that ice sheet glaciers could respond so quickly. Up until now, people thought glaciers didn't react to their environment very quickly at all,' says Shepherd, a glaciologist from the University of Edinburgh's School of Geosciences and leader of the research team.

In a different study, published in Nature Geoscience last week, Andreas Vieli and Faezeh Nick from Durham University found the reasons for rapid ice loss in outlet glaciers.

Whereas the loss of ice in the land-terminating Russell Glacier in Shepherd's study is dominated by surface melt, Vieli unearthed a different mechanism for ice loss in outlet glaciers.

Vieli and his team found that the Hellheim Glacier, in east Greenland, speeds up in response to changes at its calving end -- the part of the glacier that juts into the sea.

They wanted to see how different effects on the glacier compared with the ice loss researchers have witnessed. So they used computer models to test how various scenarios affected the glacier.

When they increased meltwater lubrication at the glacier's base, unlike Shepherd's study, the glacier didn't speed up towards the ocean. But when they decreased the stability at the front end of the glacier -- such as would happen in a warming climate -- ice thinning, acceleration and retreat quickly spread far up the glacier. This exactly mimics what researchers have seen in Greenland.

'Essentially, higher air temperatures and warmer seas mean you're going to get more calving in a glacier like the Hellheim Glacier. But as these glaciers retreat inland more, maybe surface melt will become the dominant factor making them lose ice,' says Vieli.

Vieli is keen to point out that you have to be careful about extrapolating ice loss from just a few years' observations into the future. Researchers are still a long way off predicting how fast sea levels will rise as Greenland's ice sheet disappears. Even so, both of these studies are a step forward in refining the ice aspect of current climate models.

Greenland ice sheet motion coupled with daily melting in late summer
Andrew Shepherd, Alun Hubbard, Peter Nienow, Matt King, Malcolm McMillan, and Ian Joughin. Geophysical Research Letters, Vol 36, L01501, doi:10.1029/2008GL035785, 2009.

Large-scale changes in Greenland outlet glacier dynamics triggered at the terminus
Faezeh M Nick, Andreas Vieli, Ian M Howat and Ian Joughin. Nature Geoscience, doi:10.1038/NGEO394.

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