Quantifying ice sheet response to variations in initial bedrock topography

Quantifying ice sheet response to variations in initial bedrock topography (IceBed) The majority of the world's population lives along the coastline. In the present-day world of melting ice sheets and consequent global sea level rise, ice sheet research is therefore extremely relevant. The effect of the topography on which the ice sheet grows on the resulting ice volume and stability is not well known. In this project we aimed at quantifying this effect under various warm climate conditions. We first assessed the climate during the last interglacial period (roughly 130-115 thousand years ago) using a range of time slice simulations performed with the Norwegian Earth System Model (NorESM). We showed that a reduction in greenhouse gas forcing (e.g. atmospheric CO2) causes a global cooling distributed similarly in all seasons. In contrast, insolation forcing (caused by variations in the orbital cycles of the Earths rotation around the Sun) induces regional and seasonal variations in temperature. The model simulations confirm the timing of maximum summer temperatures in the North Atlantic to be around 125.000 years ago. We used our simulated last interglacial climates to assess the evolution of Greenland ice sheet retreat during this period. Our results indicate a moderate Greenland ice sheet reduction, equivalent to a global mean sea-level rise of ~1-2 m compared to today. However, the exact retreat strongly depends on the model set-up. Unfortunately, verification using the data available is complicated. Further analyses of the Greenland ice sheet evolution during this past warm period are continued in an on-going doctoral project. In order to put the last interglacial and present-day ice sheet changes in a geological perspective, we reviewed the long-term evolution of the Greenland ice sheet, with a focus on the last glacial cycle. We show, among others, that the projected future melt rates are likely higher than those observed during the last deglaciation. A special focus of IceBed is on the much-debated early onset of Northern Hemisphere glaciation during the Eocene-Oligocene climate transition, approximately 34 million years ago. Across this transition, a decrease in atmospheric CO2 and high-latitude temperatures caused a large-scale ice sheet expansion on Antarctica. At the same time, in the Northern Hemisphere, the inception of ice on Greenland is controversial. Indirectly, ice formation is suggested by the occurrence of ice-rafted debris off the coast of Greenland, and by changes in erosion regime in east Greenland. However, despite the temperature decrease of approximately 5°C, the climate just after the transition is still much (maybe even 10°C) warmer than today. This would inhibit the formation of any ice on Greenland. We show that acknowledging changes in the elevation of the topography on which the ice forms (bedrock topography) can solve this inconsistency. During the late Eocene / early Oligocene the bedrock topography was likely higher than today due to tectonic processes related to the break-up of the North Atlantic (i.e. Greenland being pushed into Canada) and the position of the Icelandic mantle plume. When allowing for higher bedrock topography, we do simulate an ice sheet on Greenland under the relatively warm climate of the early Oligocene. Concluding, in the IceBed project we showed that applying appropriate initial bedrock topography is essential for quantifying the size and extent of the ice sheet that grows on this topography.

IceBed aims at quantifying the effect of initial bedrock topography on ice sheet volume, extent and stability, and related sea level variations (objective 1). The project also intends to improve our understanding of atmospheric CO2-ice sheet interactions by assessing CO2 thresholds required for glacial inception on various bedrock topographies (objective 2). This knowledge will be applied to the much-debated early onset of Northern Hemisphere glaciation under high Eocene/Oligocene CO2 levels. IceBed will analyse the probability of such an early onset by elevating the initial bedrock topography (objective 3). These objectives will be achieved by using the three-dimensional ice sheet model SICOPOLIS, for which a climate-forcing module converting CO2 and i nsolation to surface energy and mass balances will be developed. A set of hypothetical, but realistic initial bedrock topographies will be designed and used as model boundary condition. The elevated initial bedrock topography required for the Northern Hem isphere glaciation simulations will be derived from geodynamic model results. IceBed combines the disciplines of ice sheet and climate modelling with geology and geodynamics. It will advance these fields and additionally produces information essential f or future global sea level projections and hydrocarbon exploration offshore Norway. IceBed also gives the applicant the opportunity to engage in popular dissemination activities, increase her publication record and enlarge her scientific and industrial network, thereby promoting the gender balance in these male dominated research fields. The transfer of scientific knowledge and skills, expansion of international cooperation (with the University of Bristol, United Kingdom) and development of new interd isciplinary methods associated with this project is highly beneficial for the ongoing excellence of Norwegian research.