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Saturday, August 21, 2010

Jason Box: Petermann not the only major ‘loser’ in Greenland (see also: Zachariae, Humboldt, and 79N)

Petermann not the only major ‘loser’ in Greenland

MODIS, August 14th, 2010

The recent ice island detachment at Petermann glacier is part of a larger pattern of deglaciation observed at 31/34 glaciers (91%) in our survey.

We just updated our survey to include year 2010. Retreat continues at the 110 km (68 mi) wide Humboldt glacier and at the 23 km (14 mi) wide Zachariae ice stream. Humboldt, Zachariae, and Petermann (16 km or 10 mi wide) have bedrock trenches that lead inland below sea level to the thickest parts of the ice sheet.

Sleeping giants are awakening…
Cumulative area change at Greenland’s glacier top 5 “losers.” 2010 areas are measured ~1 month prior to the end of summer melt when the survey usually is made . We do not expect 2010 area changes to be much different using the future data. If anything, we expect the losses to be larger. Click here for a full resolution graphic.

The front areas at Jakobshavn glacier, the world’s overall fastest glacier, and at 79 N glacier, are not losing area in 2010. Jakobshavn area changes are probably less indicative of its stability because the ice is moving so fast it just jams into its ice-choked fjord resulting in growth of the front area (see Amundson, Fahnestock, Truffer, Brown, Lüthi and Motyka.  2010. Ice me´lange dynamics and implications for terminus stability, Jakobshavn Isbræ, Greenland. J. Geophys. Res., 115 (F1), 1–12. F01005.). Jakobshavn remains flowing ~2x faster than it was prior to the loss of its ice shelf 1997-2003. Ian Howat has likened this glacier to a fire hose spewing about as fast as it can.

The 79 N and Zacharaiae glaciers are outlets to the Northeast Greenland Ice Stream (see: Joughin, Fahnestock, MacAyeal, Bamber, and Gogineni. 2001, Observation and analysis of ice flow in the largest Greenland ice stream, J. Geophys. Res., 106, 34,021–34,034). The northeast ice stream has not accelerated much. If surface climate is any indicator (J. Box is convinced it is), the lesser warming rates in northeast Greenland may partly explain the relative stability.

The Bottom Line Importance
Losses at the front of glaciers translate to less ice flow-resistance and in turn accelerated flow. Flow acceleration leads to further thinning by stretching. In turn the “grounding line,” where the glacier begins to float migrates inland. For the largest glaciers that have bedrock trenches leading inland to the thickest parts of the ice sheet, there is no expected mechanism to prevent retreat from continuing, hastening ice sheet volume losses. Ice movement from land to sea rises global sea level. As climate warming continues, we expect some acceleration of global sea level rise; by how much remains the subject of intense scientific inquiry that’s making gradual progress.

This blog entry was composed by Jason Box with assistance from David Decker.


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