The state of the Greenland Ice Sheet
Greenland is host to the second largest ice mass on the planet, accounting for ~11% of the global ice surface area. Recent estimations put the volume of ice contained in the ice sheet at 2.931 million gigatons - enough to raise global sea level by 7 metres (Bamber et al., 2001).
As the climate over Greenland is expected to warm over the 21st century, the prospect of rising sea levels will therefore become a real subject for concern. Since the 1990's, dramatic thinning rates have been observed at low elevations (Krabill et al., 2000; Luthcke et al., 2006, 2008; Pritchard et al., 2009); the result of ice flow changes and surface melting. Observations more recently from satellites have confirmed this accelerating trend of mass loss at the ice sheet margins (Velicogna, 2009).
Despite the erroneous claim by the Times Atlas that Greenland lost 15% of its ice cover since 1999, the ice sheet is in fact losing considerable mass, and at an accelerating rate. Between 2000-2010 the 39 largest outlet glaciers and ice shelves draining the Greenland Ice Sheet lost a cumulative area of 1535 squared km - an area just under the size of Greater London (Box and Decker, 2011).
van As, D., Hubbard, A., Hasholt, B., Mikkelsen, A.B., van den Broeke, M., Fausto, R.S. (2011) Surface mass budget and meltwater discharge from the Kangerlussuaq sector of the Greenland ice sheet during record-warm year 2010. The Cryosphere 5, 2319-2347. [.pdf]
Bamber, J. L., R. L. Layberry, S. P. Gogineni (2001) A new ice thickness and bedrock data set for the Greenland ice sheet, 1, Measurement, data reduction, and errors, J. Geophys. Res., 106(D24), 33773-33780
Bartholomew, I., Nienow, P., Mair, D., Hubbard, A., King, M.A., Sole, A. (2010) Seasonal evolution of subglacial drainage and acceleration in a Greenland outlet glacier. Nature Geoscience 3, 408-411 [DOI: 10.1038/NGEO863]. [link]
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Citterio, M. and Ahlstrøm, A. P. (2011) Greenland land ice extent and map, in preparation.
Irvine-Fynn, T.D.L., Hodson, A.J., Moorman, B.J., Vatne, G., Hubbard, A. (2011) Polythermal Glacier Hydrology: A review, Rev. Geophys., 49, RG4002, doi:10.1029/2010RG000350. [link]
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Joughin, I,. Smith, B.E., Howat, I.M., Scambos, T., Moon, T. (2010) Greenland flow variability from ice-sheet-wide velocity mapping. Journal of Glaciology 56, 415-430.
Krabill, W., W. Abdalati, E. Frederick, S. Manizade, C. Martin, J. Sonntag, R. Swift, R. Thomas, W. Wright, and J. Yungel (2000) Greenland ice sheet: High-elevation balance and peripheral thinning, Science, 289, 428 – 430.
Hubbard, A. (2011) The Times Atlas and actual Greenland ice loss, Geology Today 27, 212-215. [.pdf]
Luthcke, S.B., H.J. Zwally, W. Abdalati, D.D. Rowlands, R.D. Ray, R.S. Nerem, F.G. Lemoine, J.J. McCarthy and D.S. Chinn (2006) Recent Greenland Ice Mass Loss by Drainage System from Satellite Gravity Observations. Science, 314, 1286. [DOI: 10.1126/science.1130776]
Luthcke, S.B., A.A. Arendt, D.D. Rowlands, J.J. McCarthy and C.F. Larsen (2008) Recent glacier mass changes in the Gulf of Alaska region from GRACE mascon solutions. Journal of Glaciology, 54(188).
Pritchard, H.D., Arthern, R.J., Vaughan, D.G., Edwards, L.A. (2009) Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature 461, 971-975. [DOI: 10.1038/nature08471]. [link]
Shepherd, A., Hubbard, A., Nienow, P., King, M., McMillan, M. and Joughin, I. (2009) Greenland ice sheet motion coupled with daily melting in late summer, Geophysical Research Letters 36, L01501 [DOI: 10.1029/2008GL0357568]. [.pdf]
Sundal, A.V., Shepherd, A., Nienow, P., Hanna, E., Palmer, S., Huybrechts, P. (2011) Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage. Nature 469, 521-524. [DOI: 10.1038/nature09740]. [link]
Tedesco, M., Fettweis, X., van den Broeke, M.R., van de Wal, R.S.W., Smeets, C.J.P.P., van de Berg, W.J., Serreze, M.C., and Box, J.E. (2011) The role of albedo and accumulation in the 2010 melting record in Greenland. Environmental Research Letters, 6 (1) 014005. [.pdf]
Velicogna, I. (2009) Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE, Geophysical Research Letters 36, L19503, doi:10.1029/2009GL040222. [link]
van den Broeke, M. R., J. Bamber, J. Ettema, E. Rignot, E. Schrama, W. J. van de Berg, E. van Meijgaard, I. Velicogna and B. Wouters (2009) Partitioning recent Greenland mass loss, Science 326, 984-986. [link]
van de Wal, R.S.W., Boot, W., van den Broeke, M.R., Smeets, C.J.P.P., Reijmer, C.H., Donker, J.A.A. and Oerlemans, J. (2008) Large and Rapid Melt-Induced Velocity Changes in the Ablation Zone of the Greenland Ice Sheet, Science 321, pp.111-113. [DOI: 10.1126/science.1158540]. [link]
Despite the erroneous and much-publicised media statement by the Times Atlas that Greenland ice cover had reduced by 15% since 1999, data does indeed show the ice sheet to be losing considerable ice mass, and at an accelerating rate (see Hubbard, 2011 - Geology Today). This fact can be dramatically illustrated by one event - the breakup of Petermann Glacier in 2010 and the resulting 'ice island' that broke free into the North Atlantic. The Petermann story is not unique however. A survey of outlet glaciers between 2000-2010 found a general pattern of deglaciation, with 31/34 in retreat [LEFT]. Retreat is continuing at the 110 km wide Humboldt glacier and at the 23 km wide Zachariae ice stream. Humboldt, Zachariae, and Petermann (16 km wide) have bedrock trenches that lead inland below sea level to the thickest parts of the ice sheet. Sleeping giants are awakening…
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 and raising the 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.
Mass changes of the Greenland Ice Sheet are regularly computed remotely from satellite observations. One important set of instruments are the GRACE (Gravity Recovery And Climate Experiment) satellites. Since gravity is determined by mass, measurements of gravitational anomalies through time can show how ice sheets grow or melt. More details on how the satellites operate can be found on NASA's webpages.
The animation below shows mass change observed by the GRACE satellites between April 2003 - July 2009, computed at 10-day intervals and 200 km spatial resolution. Changes are referenced from April 5, 2003 (Luthcke et al., 2006, 2008). What the results show is that the periphery of the ice sheet has been experiencing strong melting, while accumulation in the interior has been gradually increasing since 2003.
Source: NASA/Goddard Space Flight Center Scientific Visualization Studio [4.7mb; 49s].
The graph (below) shows monthly GRACE results (black line; the orange line is a smoothed version) for the mass variability summed over the entire Greenland Ice Sheet between April 2002 and December 2009. The trend of the best fitting straight line is about 260 cubic km of ice volume lost per year. Also critical is the notable downward curvature, indicating that the mass loss rate has been accelerating during this time (Velicogna, 2009).
The map shows how this mass loss rate is distributed across Greenland, as determined from the GRACE solutions. By far, the most dramatic changes have occurred in the southeast and west of Greenland, where dramatic acceleration of outlet glaciers and accompanying ice thinning has been observed over the last few years. In contrast, the northern interior has been gaining mass, associated with increased accumulation rates there.
[LEFT] GRACE results for the mass variability summed over the entire Greenland Ice Sheet between April 2002 and December 2009. A gigaton of ice is equal to one cubic kilometre of water.
[RIGHT] Mass change distribution (2002-2009) across the ice sheet as determined by GRACE observations.
Figures source: John Wahr (University of Colorado at Boulder), Applications of Time-Variable Gravity Measurements from GRACE
- 1) through direct melt of the ice due to increased solar radiation;
- 2) through increased calving of icebergs into the ocean due to warm ocean currents; and
- 3) by dynamic thinning of the ice sheet.
Dynamic thinning is the thinning of the ice sheet due to acceleration of the ice sheet's outlet glaciers. The outlet glaciers of the Greenland Ice Sheet are natural conveyor belts transporting ice from the inland 'reservoir' to the margin. During summer meltwater collects on the surface of the ice sheet in lakes which overflow into rivers. Eventually this water plunges through the ice sheet via vertical pipes called moulins. The theory is that this water lubricates the bed of the ice sheet - causing the ice sheet to slide. As the ice sheet slides more ice is transported to lower, warmer elevations and therefore more ice is lost to the sea. As the ice sheet extends it becomes thinner, hence the term dynamic thinning.
The extent of this dynamic thinning in Greenland was captured by NASA's ICESat between 2003-2007, with showing strong thinning of the southeast and northwest ice margins. Worryingly deeply penetrating thinning has spread to high northern latitudes (Pritchard et al., 2009; Figure 1).
More detailed observations for these seasonal speed-ups have been made in western Greenland from repeat satellite imagery (Joughin et al. 2008). The figure far-left shows mean ice velocity between September 2004 - December 2006, while the figure right shows the relative speed-up in August 2006 - on average 36 m/year (48%) above the 76 m/year mean speed.
The data shows a relatively uniform speed-up extending over the bare-ice zone, however GPS data from North and South Lake also revealed additional short-term speed-ups (~100%) lasting just a matter of days. One such local speed-up was shown to coincide with the rapid drainage of a lake to the bed of the ice sheet.
The large-scale pattern of seasonal speed-up across the ice sheet is likely driven by processes that caused ice-front retreat and reduced back-stress, such as declining sea-ice extent near calving fronts. However the relative importance of melt- or calving-front-induced changes in controlling near-future ice-sheet mass balance is still ambiguous, and is something scientists are racing to find critical evidence for.