Showing papers by "R. S. W. van de Wal published in 1999"

The importance of thermodynamics for modeling the volume of the Greenland Ice Sheet

Abstract: Two different kinds of ice flow models, one two-dimensional (2-D) and the other three-dimensional (3-D), have been used to test the importance of the thermodynamic response of the Greenland ice sheet. The basic difference between the two models is that the thermodynamics are neglected in the two-dimensional model, whereas these are incorporated in the three-dimensional model. The 2-D and 3-D models are compared in terms of volume response time and total volume of the ice sheet. Results indicate that the sawtooth character of the ice volume on glacial-interglacial timescales is, among other possible reasons, a result of the thermodynamic coupling. The changes in elevation over the last 130,000 years calculated with the three-dimensional version are 230 m for the Summit drill site and 190 m for the N-GRIP site. The standard deviation of the changes in elevation is 55 m for the Summit site and 43 m for the N-GRIP site. The present-day imbalance is merely a result of the rather constant climate over the last 10 kyr and is not determined by the thermodynamics. Consequently, Pleistocene temperatures do still exist in the ice sheet but are not important for volume calculations of the present-day ice sheet. For short-term perturbation experiments in the future, the pronounced sensitivity of the mass balance will determine the response of the Greenland ice sheet, whereas thermodynamics will play only a minor role.

Processes of buildup and retreat of the Greenland Ice Sheet

Abstract: Sensitivity experiments were conducted to study the influence of several physical processes on the evolution of the Greenland ice sheet over the last 250,000 years. The experiments were carried out by means of a three-dimensional thermodynamical model. The mass balance is modeled by a surface energy balance model., This means that variations in orbital parameters following from the astronomical (Milankovitch) theory can be incorporated in the ablation calculations. It is shown that with regard to the Greenland ice sheet, variations in shortwave radiation are only of minor importance for the changes in volume over time. Calculations of the various components of the energy balance under glacial and interglacial conditions show that the changes in absorbed shortwave radiation resulting from changes in orbital parameters are considerably smaller than the variations in the sensible heat flux caused by temperature variations. Thermodynamics are only of importance for the volume changes of the ice sheet if the climate, is on average, stable over a long period. In between 110 kyr B.P. and 20 kyr B.P. the ice volume increased slowly due to the cooling of the ice, which resulted in a higher effective viscosity and thus a thicker ice sheet. The long response time of the thermodynamics means that the thermodynamics cannot play a crucial role during transition from a glacial to a deglacial period which occur typically within several thousands of years. The mass balance height feedback has an important function in the buildup of the ice sheet after a warm period like the Eemian. Without isostatic bedrock compensation the ablation will remain very high in some marginal parts, yielding a considerably smaller ice sheet volume for the last glacial period.