The mass balance of the Greenland ice sheet: sensitivity to climate change as revealed by energy-balance modelling

The sensitivity of the mass balance of the Greenland ice sheet to climate change is studied with an energy-balance model of the ice/snow surface, applied at 200 m elevation intervals for four characteristic regions of the ice sheet. Solar radiation, longwave radiation, turbulent heat fluxes and refreezing of melt water in the snow pack are treated separately. The daily cycle is fully resolved. For the climatology chosen as input (mainly from work by A. Ohmura), the mean specific balance produced by the model is 0.079 m/yr (water equivalent). Comparing this with the total accumulation, 0.313 m/yr, it is obvious that the ablation is quite large. However, a 1 K decrease of the imposed annual mean temperature leads to a specific balance of 0.147 m/yr, a 1 K increase to 0.003 m/yr. Because of this large sensitivity, it appears that the present state of balance cannot be determined from climatological data. The calculations show that changes in the earth's orbit during the Holocene must have had a significant effect on the mean specific balance of the Greenland ice sheet. The balance was smaller than today during most of the last 10 000 years, probably by as much as 0.05 m/yr. Further experimentation showed that the changes in the specific balance can be related to changes in summer insolation. The difference between a typical high latitude minimum in insolation (e.g., 25 000 BP) and a high latitude maximum in insolation (e.g., 10 000 BP) is equivalent to the effect of a 2 K difference in annual mean temperature. So when considering mass balance changes for a particular glacier during the Holocene, it is important to consider the orbital and climatic effects separately, because they may work in the same or in the opposite direction, depending on the location. In a warmer world, ablation will increase, but this will be compensated to some extent by increased snowfall. For a uniform warming of 1 K, and including a precipitation rate proportional to maximum possible atmospheric water content, the model predicts a contribution to sea level rise of 0.27 mm/yr.