There is no doubt that orography plays a crucial role in the initiation of ice sheets. Mass balance observations on glaciers in highmountain and polar climates show that the specific balance varies first of all with altitude. The height–mass balance feedback (HMB-feedback) is the most effective process to turn local ice caps into big ice sheets. Here, a study is presented on the processes that determine the strength of the HMB-feedback. This is done by carrying out a set of integrations with a numerical ice-sheet model for a bounded domain and a sinusoidally varying bed shape. Results are analysed for different wavenumbers and amplitudes. It turns out that the relative importance of the geometric effect (higher mountains allow\ud earlier glacierisation) and the ice-mechanical effect (dependence of ice flow on the bed slope) varies significantly with wavenumber and amplitude. With regard to full glaciation of the domain, this implies that for a given wavenumber there is a preferred amplitude (that is, the drop in the equilibrium-line altitude needed to initiate full glaciation has a minimum)
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