Climate Sensitivity: The Significance of the Altitude-mass-balance Feedback on Glaciers and Ice Sheets

Abstract

An ice mass increasing its extent will generally also increase its mean surface elevation (anything else being unchanged). Since temperature drops rapidly with height this often implies a more positive mass balance and thus further growth. This powerful feedback loop is at least as important as the temperature-albedo feedback. In this work, we present a general study of the nature of the altitude-mass-balance feedback, with special attention for the role of differences in the bedrock geometry. One- and two-dimensional experiments on various bedrock topographies are run, using a numerical ice-sheet model based on plane shear flow of ice. Different climates are simulated by lowering or raising uniformly the mass-balance distribution over the ice sheet.These studies were first done on steady state ice sheets. They show that some geometries can largely enhance this altitude mass-balance feedback. On an island, for instance, it is possible to have two different stable steady states under the same climatic environment: no ice sheet, or a large one over the whole island. When the geometry of the island becomes more complex, for instance with two hills of unequal heights, four different equilibria can be found for the same climatic conditions: no ice sheet, one small one, two small ones or a very large one. Which will develop depends entirely on the history of the mass-balance fluctuations.