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is forced using fields of temperature, specific humidity,

zonal and meridional wind components, and surface pres-sure from either GCM or re-analysis output. Relaxation of

RACMO2 prognostic variables towards external forcings is

restricted to the boundary relaxation zone (Fig. 1). Externalforcings are updated every six hours and linearly interpo-

lated in time to yield accurate values in between. Sea

surface temperatures and sea-ice extent are also prescribedfrom the forcing model. The version of RACMO2 used for

this study includes a snow model that calculates tempera-ture, density and meltwater processes (percolation, reten-

tion, refreezing and runoff) in the snow (Ettema et al.

2009), and an improved albedo scheme, where the snowalbedo depends on snow grain size (Kuipers Munneke

et al. 2011). For this study, contributions from drifting

snow processes have not been included, because themodule of Lenaerts and Van den Broeke (2012) was not yet

fully implemented when we started the simulations.

For contemporary climate studies of the AIS (1–30 years),RACMO2 has been run on grids with 27 and 5.5 km hori-

zontal resolution (Lenaerts et al. 2012a, b). However, for the

number of simulation years considered here (660 years intotal), a horizontal resolution of 55 km is considered a good

trade-off between computational expense and spatial detail;

doubling the grid resolution would multiply the computa-tional time by a factor 10. Moreover, the annual integrated

SMB of the AIS at 55 km resolution (Van de Berg et al.

2006) is similar to that at 27 km resolution (Lenaerts et al.2012a). For the scenario runs, the largest uncertainty there-

fore derives not from the model resolution but from the

chosen forcing model and scenario. Given this information,and the fact that a 27 km resolution run is ten times as

expensive as a 27 km run, we chose 55 km as final resolu-tion. The model topography, grid resolution and lateral

relaxation boundary of the domain are shown in Fig. 1.

For the period 1980–1999, a RACMO2 reference sim-ulation, forced by ERA-40 re-analysis data from the

European Centre for Medium-Range Weather Forecasts

(Uppala et al. 2005), was performed in order to check thereliability of the GCM-forced RACMO2 simulations. In

this paper, ERA-40 has been used as forcing instead of its

successor ERA-Interim (Dee and et al. 2011), since thelatter only covered the period 1989–2009 at the time the

RACMO2 simulations were started. Other RACMO2

simulations forced by re-analysis data (ERA-40 or ERA-Interim) yielded realistic SMB results over Antarctica

Fig. 1 Map of Antarcticashowing the model domain, theboundary relaxation zone(dotted area) and modeltopography in meters above sealevel

Future SMB of Antarctica

123

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Modelled  surface  mass  balance  (kg  m-­‐2  yr-­‐1)  

kg  m-­‐2  y-­‐1  

E^ema  and  others,  2009  

Q1  and  Q2:  Greenland  cumula2ve  mass  loss  1990-­‐2010  

The  future  of  the  Greenland  ice  sheet:  an  average  warming  scenario  (RCP4.5)  

6.4 Results

Figure 6.10: Annual SMB for RACMO2-fHadGEM2 (grey bars), with 11-year runningaverage SMB for RACMO2-fERA (blue), RACMO2-fHadGEM2 (black) and RACMO2-fHadGEM2, assuming the refreezing capacity remains constant at 38% throughout the21st century (red). 104 Gt is added to the RACMO2-fHadGEM2 SMB to correct for theSMB bias between the two simulations for the present day (1992-2011) (Table 6.1).

uid water production increases strongly (rain and melt, +722 Gt yr�1), yet refreezingonly modestly increases in comparison (+133 Gt yr�1). In the RACMO2-fHadGEM2simulation the refreezing capacity is reduced from 38% to 29% at the end of the 21stcentury (Fig. 6.12c, blue line). This represents a 24% decrease in refreezing capacityin less than a century’s time. The loss of refreezing capacity is concentrated in thelower accumulation area, and marks the transformation of accumulation zone, withnet annual surface mass gain, to ablation zone, where surface mass is lost on an an-nual basis. To demonstrate the impact of the reduction in refreezing capacity, weadded to Fig. 6.10 the hypothetical situation in which the refreezing capacity of theGrIS were to remain constant throughout the 21st century. In that scenario, the SMBwould remain positive for several decades longer.

The reason for this loss of refreezing capacity is twofold. Upon refreezing in thecold firn sections of the ice sheet, the massive release of latent heat causes averagefirn temperature to increase by 4-5 K towards the end of this century. Locally thisfirn warming is projected to be as large as 18 K (Fig. 6.13) at locations where re-freezing and thus latent heat release increase most significantly (Fig. 6.11e). More

101

Van  Angelen  and  others,  2013  

Conclusions

Recent mass loss from Antarctica driven by glacier acceleration Recent mass loss from Greenland driven by glacier acceleration but mostly by increased surface meltwater runoff Total mass loss of both ice sheets accounts for ~1/3 of current sea level rise, and this contribution is increasing We have a good understanding of surface mass balance of the ice sheets, which reasonable confidence in its predictions Challenge: modelling ice dynamics and ice-ocean-atmosphere interactions in a coupled system