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Science

Extreme Ice Sheet Melting Does Not Need Extreme Heat

New research has provided evidence to suggest that massive melting of ice sheets like the Greenland ice sheet does not need corresponding record temperature highs, rather, just persistent warm weather over several years.

Marco Tedesco standing on the edge of one of four moulins (drainage holes) he and his team found at the bottom of a supraglacial lake during the expedition to Greenland in the summer, 2011.

Such results suggest that glaciers and ice sheets could undergo a self-amplifying cycle of melting and warming simply if there is persistently warm temperatures, and that such a cycle could be very difficult to stop.

The findings come courtesy of a new analysis conducted by Dr. Marco Tedesco, assistant professor in the Department of Earth and Atmospheric Sciences at The City College of New York, and graduate student Patrick Alexander.

“We are finding that even if you don’t have record-breaking highs, as long as warm temperatures persist you can get record-breaking melting because of positive feedback mechanisms,” said Professor Tedesco, who directs CCNY’s Cryospheric Processes Laboratory and also serves on CUNY Graduate Center doctoral faculty.

Combining data gathered during a four-week expedition to the Jakobshavn Isbræ glacier in western Greenland together with microwave satellite recordings and the output from a model of the ice sheet, Tedesco and Alexander found a loss of snow and ice for 2011 that is close to record breaking, and a loss that continued without the help of the record highs that scoured the ice sheet in 2010.

The model showed that melting that took place between June and August of this year was well above the 1979 to 2010 average, and that in fact the 2011 melting was the third most extensive since 1979; 2010 and 2007 sitting above in first and second. The amount of snow gained minus the snow and ice that melted away – known as the “mass balance” – equalled last year’s record values.

Temperatures and an albedo feedback mechanism accounted for the record losses, Professor Tedesco explained.

This is where the ‘self-amplifying cycle’ comes into place. Albedo is the amount of solar energy absorbed or reflected by a surface area. A regular thick covering of white snow reflects a lot of energy, whereas bare ice does not, being darker. Absorbing more energy from the sun naturally means that the ice warms up faster, thus melts faster.

Professor Tedesco likens the melting process to a speeding steam locomotive. Higher temperatures act like coal shoveled into the boiler, increasing the pace of melting. In this scenario, “lower albedo is a downhill slope,” he says. The darker surfaces collect more heat. In this situation, even without more coal shoveled into the boiler, as a train heads downhill, it gains speed. In other words, melting accelerates.

Only new falling snow puts the brakes on the process, covering the darker ice in a reflective blanket, Professor Tedesco says. The model showed that this year’s snowfall couldn’t compensate for melting in previous years. “The process never slowed down as much as it had in the past,” he explained. “The brakes engaged only every now and again.”

The team’s observations indicate that the process was not limited to the glacier they visited; it is a large-scale effect. “It’s a sign that not only do albedo and other variables play a role in acceleration of melting, but that this acceleration is happening in many places all over Greenland,” he cautioned. “We are currently trying to understand if this is a trend or will become one. This will help us to improve models projecting future melting scenarios and predict how they might evolve.”

Source: City College of New York
Image Source: Patrick Alexander




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