Interactions and feedbacks between ice sheets, climate, sea level and the solid Earth

Abstract

Anthropogenic climate change is one of the largest challenges facing society today. Although mitigating the magnitude of climate change by reducing CO2 emissions is of vital importance, even the most optimistic of future scenarios will still result in significant warming. One of the inevitable consequences of the warming is sea-level rise due to the melting of land ice and the thermal expansion of the. In order to determine an adaptation and mitigation strategy, accurate projections of sea-level rise are required. The vast size of the large ice sheets on Greenland and Antarctica lead to a slow response to a change in climate: it takes years to decades before a change in climate even begins to significantly affect ice sheet growth or retreat, and hundreds to thousands of years to reach a new equilibrium. This implies that observational alone is insufficient to predict future ice-sheet evolution. To overcome this problem, palaeoglaciology, the study of the past behaviour of glaciers and ice sheets, is helpful. In the geological past, Earth’s climate has undergone major changes, some even larger than the ongoing present anthropogenic change, though probably not quite as fast. Often these climate changes, in particular the glacial cycles of the Pleistocene, were accompanied by large changes in ice-sheet size, and therefore sea level. By studying the relation between the climate and the ice sheets in the past, and trying to reproduce their evolution with models, we contribute to the understanding of the Earth system, and the response of ice sheets in the future. A complication that is implicit in palaeoglaciology more than in future projections, is that the interaction between ice sheets and climate works in two directions. Changes in temperature and precipitation patterns will affect how much snow accumulates and melts on an ice sheet. However, changes in ice extent and geometry also affect local temperatures through changes in surface albedo and altitude, and in addition affect precipitation through orographic forcing and changes in large-scale atmospheric circulation. Changes in ocean temperatures affect the melting of floating ice, but the addition of large amounts of fresh melt water to the ocean can affect ocean currents. As a consequence, a change in any part of the Earth system will directly or indirectly affect all the other parts. Throughout Earth’s geological past, all the different components changed in concert, continuously affecting each other through coupling effects and the associated feedback processes. Any model that aims to successfully simulate the behaviour of one or more components will therefore need to include an accurate representation of the interactions and feedback processes in the Earth system. This thesis aims to incorporate several of these feedback processes into an existing ice- sheet model, and use that model configuration to reproduce the behaviour of ice-sheets during different episodes during the past several million years.