Decay of the Greenland Ice Sheet due to surface-meltwater-induced
of basal sliding
Simulations of the Greenland Ice Sheet are carried out with a high-resolution version of the ice-sheet model SICOPOLIS for several global-warming scenarios for the period 1990–2350. In particular, the impact of surface-meltwater-induced acceleration of basal sliding on the stability of the ice sheet is investigated. A parameterization for the acceleration effect is developed for which modelled and measured mass losses of the ice sheet in the early 21st century agree well. The main findings of the simulations are: (i) the ice sheet is generally very susceptible to global warming on time-scales of centuries, (ii) surface-meltwater-induced acceleration of basal sliding leads to a pronounced speed-up of ice streams and outlet glaciers, and (iii) this ice-dynamical effect accelerates the decay of the Greenland Ice Sheet as a whole significantly, but not catastrophically, in the 21st century and beyond.
In Chapter 10 (“Global Climate Projections”) of the Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change (IPCC), an increase of the mean global sea level by 18–59 cm for the 21st century (more precisely: 2090–2099 relative to 1980–1999) is projected for the six SRES marker scenarios B1, B2, A1B, A1T, A2 and A1FI (Meehl et al. 2007). The main causes for this sea level rise are thermal expansion of sea water and melting of glaciers and small ice caps, and to a lesser extent changes of the surface mass balance of the Greenland and Antarctic Ice Sheets. However, recent observations suggest that ice flow dynamics could lead to additional sea level rise, and this problem is explicitly stated in the AR4:
“Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.” (IPCC 2007).
These conjectured dynamical processes are (i) basal sliding accelerated by surface meltwater, (ii) reduced buttressing due to the loss of ice shelves, and (iii) penetration of ocean water under the ice. The first process is more relevant for the Greenland Ice Sheet (the focus of this study), whereas the latter two may affect the stability of the West Antarctic Ice Sheet. On the observational side, recent results from satellite gravity measurements for the period 2002-2005 indicate surprisingly large mass losses of ( sea level equivalent) for the Greenland Ice Sheet (Chen et al. 2006). Furthermore, major outlet glaciers (Jacobshavn ice stream, Kangerdlugssuaq and Helheim glaciers) have sped up drastically since the 1990’s (Rignot and Kanagaratnam 2006).
2 Ice-sheet model SICOPOLIS
For this study, we use the ice-sheet model SICOPOLIS (“SImulation COde for POLythermal Ice Sheets”), which simulates the large-scale dynamics and thermodynamics (ice extent, thickness, velocity, temperature, water content and age) of ice sheets three-dimensionally and as a function of time (Greve 1997; http://sicopolis.greveweb.net/). It is based on the shallow-ice approximation (e.g. Hutter 1983) and the rheology of an incompressible, heat-conducting power-law fluid [Glen’s flow law, see Paterson (1994)]. External forcing is specified by (i) the mean annual air temperature at the ice surface, (ii) the surface mass balance (precipitation minus runoff), (iii) the sea level surrounding the ice sheet and (iv) the geothermal heat flux prescribed at the bottom of a lithospheric thermal boundary layer of 5 km thickness. For all simulations of this study, the horizontal resolution is 10 km, the vertical resolution is 81 grid points for the cold-ice column, 11 grid points for the basal layer of temperate ice (if existing) and 11 grid points for the lithosphere layer, the time-step is 0.25 a, and the geothermal heat-flux distribution and model parameters are those used by Greve (2005).
3 WRE1000 scenario
Future global warming shall be prescribed exemplarily by the WRE1000 scenario, which assumes stabilisation of the atmospheric CO concentration at 1000 ppm (Cubasch et al. 2001). The corresponding temperature change from 1990 (the “present”) until 2350 is shown in Fig. 1 (along with similar scenarios with lower stabilisation concentrations).
In order to obtain temperature and precipitation forcings for the Greenland Ice Sheet, the argumentation by Greve (2004) is followed. The surface temperatures shown in Fig. 1 are amplified by a factor 2 and imposed as uniform increases over the ice sheet, and the precipitations are assumed to increase by per degree of ice-sheet-surface-temperature change. Surface melting is parameterized by the degree-day method in the version by Greve (2005). This approach is a critical simplification, and it should rather be replaced by an energy-balance model for more accurate results. However, since the objective of this study is to assess the impact of ice-dynamical processes on the decay of the Greenland Ice Sheet rather than making precise predictions of the decay itself, the use of the degree-day method is a reasonable compromise.
4 Basal sliding
where is the basal-sliding velocity, the sliding coefficient, the basal shear traction in the bed plane, the ice density, the gravity acceleration, the ice thickness and the overburden pressure. The term represents the exponentially diminishing sub-melt sliding, where is the temperature relative to pressure melting (in ) and the sub-melt-sliding coefficient.
Acceleration of basal sliding by surface meltwater is parameterized by an extension of the approach by Greve and Otsu (2007). The sliding coefficient is expressed as
where , is the surface melt rate (runoff), is the surface meltwater coefficient, and and are adjustable exponents. The idea behind this parameterization is to relate the sliding speed-up to the local surface melt rate, and account for the less efficient percolation of meltwater to the base in regions where the ice is thick by the dependency on the inverse ice thickness.
The parameterization employed by Greve and Otsu (2007) corresponds to . For this case, the authors show in their Appendix A that data reported by Zwally et al. (2002) from the Swiss Camp in central west Greenland give rise to the estimate . We will also consider the cases and , for which the same arguments lead to estimates of and , respectively.
Five simulations with different settings for the acceleration of basal sliding by surface meltwater will be discussed in order to investigate to what extent this process can increase the vulnerability of the Greenland Ice Sheet to future warming. In run #1, acceleration of basal sliding by surface meltwater is not considered (). Runs #2-4 have been conducted with , and , respectively, and values of chosen according to the estimates given at the end of Sect. 4 (designated in Table 1 as “100%”). Run #5 corresponds to the most extreme scenario considered by Greve and Otsu (2007), with the settings and (50 times the above estimate, therefore designated in Table 1 as “5000%”). All simulations start with the present-day ice sheet as initial condition, and the model time is from 1990 until 2350.
An overview of the main results is given in Table 1. The average loss of ice volume between 2002 and 2005 can be compared with the measured value by Chen et al. (2006) of (see introduction), thus providing an observational constraint for the simulations. Evidently, the ice-volume loss is far too small for run #1 (no acceleration of basal sliding by surface meltwater), and it is too small by about a factor 2 for run #2 . By contrast, the agreement is quite good for run #3  and very good for run #4 . On the other hand, the extreme case of run #5 [, very large ] produces more than 6 times more ice-volume loss than observed. Therefore, runs #3 and 4 seem to be most realistic.
From a theoretical point of view, the set-up of run #4 is preferable to that of run #3, because it is clear that the percolation of surface meltwater to the base will be the less efficient the thicker the ice is. This is accounted for in run #4, for which the acceleration of basal sliding decreases with increasing ice thickness (), whereas this is not the case in run #3 (). Consequently, run #4 shall be considered as the “best” simulation.
Comparison of the results of run #4 and run #1 (no acceleration of basal sliding by surface meltwater) shows that the contribution to sea-level rise by 2100 is larger for run #4 (0.18 vs. 0.12 m). The impact of the acceleration effect on ice flow becomes evident by inspection of Fig. 2 which shows the simulated surface velocities in 2100 for the two runs. Therefore, the acceleration of basal sliding by surface meltwater, which is most likely the major ice-dynamical process relevant for the Greenland Ice Sheet in the context of global warming, has a significant, but not catastrophic effect on the decay of the ice sheet in the 21st century.
The absolute difference between the two runs becomes larger in the more distant future; however, the relative difference becomes smaller: by 2200 the contribution to sea-level rise is 0.12 m () larger for run #4, and by 2300 it is 0.21 m () larger. Figure 3 shows the simulated surface topographies in 2350 (at the end of the simulations). It is nicely illustrated that for both runs #1 and #4 the ice sheet shows a strong response on the imposed warming scenario and retreats all around the margin (most pronounced in the south-west), while the surface-meltwater-induced acceleration of basal sliding accounted for in run #4 speeds up the decay.
Two additional simulations with larger exponents, namely and , and values of chosen in analogy to the “100%” runs #2-4, have also been conducted. For these cases, maximum surface velocities of more than occur close to the ice margin, which is unrealistic. Apparently, the speed-up effect is too pronounced for these settings, and so they have been discarded.
The simulations discussed in this study suggest that ice-dynamical processes can speed up the decay of the Greenland Ice Sheet significantly in the 21st century and beyond. However, a catastrophically accelerated decay can only be obtained with unrealistic parameter settings and thus seems to be unlikely.
This study was supported by a Grant-in-Aid for Scientific Research (Category B, No. 18340135) from the Japan Society for the Promotion of Science.
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