The stability of the Greenland ice sheet has global implications through sea level. Even smaller reductions in ice-sheet size can raise sea level enough to lead to complications for societies worldwide. In this work, the stability and evolution of the Greenland ice sheet on long timescales is investigated as well as its early history. The studies are performed using an ice-sheet model in combination with relevant forcing from observed and modeled climate.
Changes in ice-sheet geometry influences atmospheric flow (and vice versa) hereby changing the forcing patterns. Changes in the overall climate also alter the patterns. On this basis, output from a climate model is used to construct adaptive forcing patterns that are computationally fast and takes into account that the patterns respond to changes in a non-uniform way both spatially and temporally. The adaptive patterns were applied to study the steady-state response of the Greenland ice sheet to a warmer climate.
The threshold of irreversible decay was found to lie between a temperature increase of 4-5 K relative to present day when basal sliding was neglected in the ice-sheet model. Introducing basal sliding into the ice-sheet model shifted this threshold towards colder temperatures in line with a recent study, but the new threshold value depends on the choice of method.
It was found using the adaptive patterns that the Greenland ice sheet can reform under present-day conditions. A further study where additional coupling between the ice-sheet model and climate model is included shows, however, that a Föhn effect is activated and hereby increasing temperatures inland and inhibiting further ice-sheet expansion into the interior. This indicates that colder than present temperatures are needed in order for the ice sheet to regrow to the current geometry, and it is thus bistable under present day conditions.
The Greenland ice sheet in the Pliocene is also studied, and the great variability in ice-sheet size during the period is demonstrated. The effects of a new hypothesis abot mountain building in Greenland on ice-sheet initiation are also investigated. Accordingto this hypothesis, two stages of uplift since the Late Miocene lead to the present-day topography. The results of the ice-sheet simulations show geometries in line with geologicobservations through the period, and it is found that the uplift events enhance the effect of the climatic deterioration for the build-up of the Greenland ice sheet that lead to the intensification of the Northern Hemisphere glaciations at the end of the Pliocene.
A study of output from the climate model, EC-EARTH, reveals some of the challenges faced when using this to force ice-sheet evolution or when full coupling of ice-sheet models and climate models is desired. Precipitation is generally difficult for models to simulate.
A method was tested where precipitation in Greenland is inferred from the large-scale circulation-patterns using principal component analysis. The method has limitations, but is able to predict the precipitation variability in South Greenland fairly well.