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Rapid retreat of Greenland’s outlet glaciers may be temporary

The loss of ice from Greenland ranks as one of the most troubling, and poorly understood, aspects of climate change. Melting of the colossal ice sheet, which is already undoubtedly underway, has the capacity to raise global sea levels by an astounding 7 meters.

Not only is Greenland losing mass from direct surface melting, its outlet glaciers (those that terminate in the sea) are spewing large icebergs directly into the ocean at an increasingly alarming speed as they retreat. Many have worried that these changes are a sign of what’s in store. But a new study published this week in Nature Geoscience [subscription] suggests that the recent rapid retreat of many of Greenland’s outlet glaciers will be short-lived.

A team led by Andreas Vieli at Durham University, UK, used a computer model to reconstruct the recent behaviour of the Helheim Glacier, one of Greenland’s largest outlet glaciers. Helheim retreated some 7 kilometers between 2002 and 2005, during which time it discharged considerable volumes of ice into the ocean.

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Vieli’s team looked at whether the recent changes observed in the Helheim Glacier could be explained by one of two hypotheses; the first was that increased surface meltwater reaching the base of the glacier was speeding its slide toward the sea. The second potential explanation was that changing conditions in the area where the glacier meets the sea would trigger a domino effect on the glacier itself, leading to even faster ice flow and thinning upstream.

Their model showed that only the second hypothesis could explain past changes in the Helheim Glacier. Though they only analysed changes in this one outlet glacier, the authors say it is representative of many outlet glaciers south of 70°N that have recently thinned and rapidly released ice to the ocean.

Over the next 50 years, the discharge of ice from Helheim will slow, to at most 10% of previous short-term projections, say the authors. Their model showed that, in most cases, retreat of the glacier eventually stops due to shallowing of the fjord, which slows its release of ice. The exception to this could be glaciers that flow into the sea through a deep channel, such as Jakobshavn Isbrae, which may continue to lose ice rapidly in the future.

The new research suggests that most of the future mass loss from Greenland will result directly from surface warming. The study should help inform estimates of how glacial dynamics will contribute to sea level rise. These processes have previously been poorly understood , leading the IPCC to controversially omit them from estimates of sea level rise in their 2007 report.

Olive Heffernan

Image credit: Retreat of the Helheim Glacier, Greenland. NASA images created by Jesse Allen, Earth Observatory, using data provided courtesy of NASA/GSFC/METI/ERSDAC/JAROS, and the U.S./Japan ASTER Science Team.


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