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The Greenland Ice Sheet
The Greenland ice sheet represents the largest continuous mass of ice in the northern hemisphere, covering a land surface area of about 22 million km^2, storing enough freshwater to raise sea levels by 7m and representing a huge reflective surface crucial for regulating earth’s temperature (SPRI). In recent years it has become increasingly apparent that the Greenland ice sheet is changing for the warmer, smaller and less stable.
With alarming regularity, the Greenland ice sheet has undergone unprecedented summer ablation (mass loss, primarily through melting and calving of ice into the oceans). The mass balance of the Greenland ice sheet has been a key area of interest for many scientists, including (amongst others) Jason Box, Alun Hubbard, Julienne Stroeve, Marco Tedesco, Richard Alley and Edward Hanna. These researchers have closely examined the mass balance of the Greenland ice sheet using both historical and modern measurements; they are becoming progressively less surprised by record melt years and have even described the ice sheet as existing “on borrowed time” (e.g. Hanna, 2012).
Jason Box even predicted in 2012 that the ablation (melting) zone could cover the entire Greenland ice sheet – with no accumulation zone at all during high summer – by 2022 (Box et al, 2012). He was almost prematurely proven correct that very same year with 97% ablation zone coverage in July. Between May and July 2012 the Greenland ice sheet experienced record warming, with the southern-most weather station reading 24.8 degrees on 29th May (Hanna, 2012). These temperature extremes are superimposed upon a general temperature increase of about 2.3 degrees (summer average) over the past two decades, far in excess of the northern hemisphere average of 0.5 degrees (dmi.dk, 2012) and illustrating the ice sheet’s sensitivity to climatic change. In the same year, huge discharge from outflow rivers in Kangerlussuaq (SW Greenland) resulted in the total destruction of the Kangerlussuaq bridge and inundation of some areas of the town. The video below shows the damage to the bridge due to extreme meltwater discharge in the Watson River (Kangelussuaq) (video credit: M. Tedesco). 2010 was also a record melt year (Tedesco et al, 2011; van As et al, 2011), and 2011, although not a record year, was far above the 1979-2010 average and very nearly showed unprecedented rates of ablation .
Why are we seeing such extreme melt conditions on the Greenland ice sheet, out of proportion to the northern hemisphere temperature changes?
Like all ice masses, the Greenland ice sheet responds in complex, often non-linear ways to changes in temperature. As Hanna (2012) explains, every 1 degree rise in temperature increases melt by approximately 30%, meaning that in the extreme summer of 2012, about double the normal volume of melt water was generated by the ice sheet. This has a number of consequences, and initiates numerous positive feedbacks (mechanisms which amplify climate signals). Firstly, and possibly most importantly, melt water accumulates on the ice sheet surface. The effect of this is a general darkening of the ice surface. This is clearly visible from satellite and aerial photographs, such as Tyler Jones’s photo below.
The dark blue colouration of the ice surface, although beautiful, is also bad news for melt rates. The darker colour absorbs more incoming solar radiation and warms up, enhancing local melt rates and generating further surface melt water. This was compounded by lower then average rates of high albedo snow precipitation in 2012. The albedo anomaly in 2012 is illustrated in the image below. Albedo lowering is more pronounced near the ice sheet margins than in the deep interior areas. Nevertheless, the lower graph confirms that the area-averaged albedo for the entire ice sheet has consistently declined since 2000 (Box et al, 2012). Not only is the melting of the Greenland ice sheet accelerating, the albedo is progressively decreasing. This feeds back and causes further melt, which in turn further darkens the ice surface.
Aside from albedo feedbacks, Greenland’s glaciers might be flowing faster towards the sea as a result of enhanced melt water generation. The ‘like butter’ study described the drainage of melt water from ice surfaces into the englacial zone warming the ice from the inside and accelerating its movement, crucially implicating latent, as well as sensible heat fluxes in ablation control (NASA.gov, 2013). As a result of these feedbacks, the mass of the Greenland ice sheet has been in constant, accelerating decline. GRACE (Gravity Recovery and Climate Experiment) data is derived from satellite measurements of the mass of the entire Greenland ice sheet, and illustrates the decline in ice mass, which is also mirrored in Canadian ice caps and mountain glaciers (Box et al, 2013).
Over the past decade we have seen successive summers of unprecedented mass loss from the Greenland ice sheet, demonstrated not only by satellite data, but high temperatures, melt rates and melt runoff recorded empirically. Reliable predictions of future responses to continued warming are not yet available, although the general outlook is bleak: we are sure that more warming means more melt (recent work undertaken by Box et al (2012) suggests a secondary feedback involving enhanced deposition of snow in the ice sheet interior; however this is not thought to outweigh melt-enhancing albedo feedbacks closer to the ice margins).
Hanna (2012) also suggests that anthropogenic activity may have contributed to shifting the polar jet stream in the northern hemisphere, drawing warm air up over the ice sheet and adding to regional melting. Clearly, the fate of the ice sheet is uncertain. Current estimates suggest that Greenland will not be ice free for several thousand years, although there could be enough mass loss to raise sea levels by tens of centimetres by 2100, endangering coastal regions. This is a fascinating, fast-moving and crucial area of research.
Box, J. E., Fettweis, X., Stroeve, J. C., Tedesco, M., Hall, D. K., and Steffen, K.: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere, 6, 821-839, doi:10.5194/tc-6-821-2012, 2012
Box, J.E., Cappelen, J., Chen, C., Decker, D., Fettweis, X., Mote, T., Tedesco, M., van de Wal, R.S.W., Wahr, J. 2013. Greenland ice sheet. Arctic Report Card. http://www.arctic.noaa.gov/reportcard/greenland_ice_sheet.html
DMI (Danish Meteorological institute): http://www.dmi.dk/en/groenland/maalinger/greenland-ice-sheet-surface-mass-budget/
Hanna, E. Greenland’s ice sheet is melting fast – I’m not surprised. Guardian, 2012: http://www.theguardian.com/commentisfree/2012/jul/26/greenland-ice-sheet-borrowed-time#_
SPRI (Scott Polar Research Institute): http://www.spri.cam.ac.uk/research/projects/greenlandicesheet/
Tedesco M., X. Fettweis, M. R. van den Broeke, R. S. W. van de Wal, C. J. P. P. Smeets, W. J. van de Berg, M. C. Serreze, and J. E. Box. 2011. Record summer melt in Greenland in 2010. EOS AGU, Volume 92, Number 15
van As, D., Hubbard, A., Hasholt, B., Mikkelsen, A. B., van den Broeke, M., and Fausto, R. S.: Surface mass budget and meltwater discharge from the Kangerlussuaq sector of the Greenland ice sheet during record-warm year 2010, The Cryosphere Discuss., 5, 2319–2347