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The Ups and Downs of Greenland's Ice Sheet Wastage
Murray, T., Scharrer, K., James, T.D., Dye, S.R., Hanna, E., Booth, A.D., Selmes, N., Luckman, A., Hughes, A.L.C., Cook, S. and Huybrechts, P. 2010. Ocean regulation hypothesis for glacier dynamics in southeast Greenland and implications for ice sheet mass changes. Journal of Geophysical Research 115: 10.1029/2009JF001522.

A few years back, during the early 2000s, many of the world's climate alarmists thought that one of Al Gore's and James Hansen's nefarious "tipping points" had been reached, when the authors report that "the Greenland Ice Sheet's annual ice discharge doubled," and its outlet glaciers were dramatically "thinning, accelerating, and retreating." But as they go on to show, the horror of it all was rather short-lived.

What was done
After first reporting some of what happened both before and after reaching the pseudo tipping point that drove concerns, if not sea levels, to unparalleled heights, Murray et al. went on to "describe the oceanographic setting of the southeast Greenland region and then undertake two analyses to explore the relationship between oceanic processes and glacier dynamics," all of which helped to put things in proper perspective.

What was learned
The eleven researchers report that in 2006, after the initial acceleration of ice loss, "two of the largest outlet glaciers in the sector, Helheim and Kangerdlugssuaq, were reported to have slowed down simultaneously (Howat et al., 2007), ceased thinning (Stearns and Hamilton, 2007; Howat et al., 2007), and readvanced (Joughin et al., 2008), and there was some indication that other glaciers in the region followed suit (Howat et al., 2008; Moon and Joughin, 2008)." In addition, their new work revealed that "the slowdown from 2006 was widespread and synchronized throughout southeast Greenland," and except for a minor reactivation at Helheim during 2007, "continued until at least 2008."

As for the mechanics of the oscillatory phenomenon, Murray et al. present evidence that suggests that the original ice wastage speedup "was the result of warm ocean waters coming into contact with the glaciers," and that this speedup "was probably terminated in part by increased discharge from the glaciers themselves, which increased ice sheet runoff and iceberg calving" that in turn "introduced additional cold water strengthening the East Greenland Coastal Current," which slowed glacier melting until warmer water again began to dominate the Current's waters.

What it means
Murray et al. write that their findings are suggestive of "a negative feedback that currently mitigates against continued very fast loss of ice from the ice sheet in a warming climate," and they thus conclude that "we should expect similar speedup and slowdown events of these glaciers in the future, which will make it difficult to elucidate any underlying trend in mass loss resulting from changes in this sector of the ice sheet."

Howat, I.M., Joughin, I.R. and Scambos, T.A. 2007. Rapid changes in ice discharge from Greenland outlet glaciers. Science 315: 1559-1561.

Howat, I.M., Joughin, I., Fahnestock, M., Smith, B.E. and Scambos, T.A. 2008. Synchronous retreat and acceleration of southeast Greenland outlet glaciers 2000-2006: Ice dynamics and coupling to climate. Journal of Glaciology 54: 646-660.

Joughin, I., Howat, I., Alley, R.B., Ekstrom, G., Fahnestock, M., Moon, T., Nettles, M., Truffer, M. and Tsai, V.C. 2008. Ice-front variation and tidewater behavior on Helheim and Kangerdlugssuaq glaciers, Greenland. Journal of Geophysical Research 113: 10.1029/2007JF000837.

Moon, T. and Joughin, I. 2008. Changes in ice front position on Greenland's outlet glaciers from 1992 to 2007. Journal of Geophysical Research 113: 10.1029/2007JF000927.

Stearns, L.A. and Hamilton, G.S. 2007. Rapid volume loss from two East Greenland outlet glaciers quantified using repeat stereo satellite imagery. Geophysical Research Letters 34: 10.1029/2006GL028982.

Reviewed 22 December 2010