Vereisung der Antarktis im Känozoikum: Anwendung eines integrierten Klima-Eisschild-Modells.

Abstract

A prominent climate change of the Earth system was the onset of Antarctic glaciation near the Eocene-Oligocene transition (~34 million years ago). The causes of this change are not yet well understood. The most common hypothesis are that glaciation resulted from a cooling of Antarctica due to plate tectonic repositioning and associated changes in ocean circulation or by a response to declining atmospheric pCO2 supported by the Earth's orbital configuration relative to the Sun. In this thesis these hypotheses are tested through sensitivity experiments with a new climate-ice sheet modeling approach, which takes into account the global oceanic and atmospheric circulation and the Antarctic cryosphere. The numerical models chosen for this study are Huybrechts (1993) ice sheet model for the Antarctic ice sheet (AIS), and COSMOS, composed of the atmospheric general circulation model ECHAM5 and the ocean general circulation model MPI-OM. MPI-OM is initialized by runs of the Large Scale Geostrophic ocean model (LSG).The modelling procedure is validated for modern climate and the results compared to observational data. Furthermore the robustness of the method is assessed by analysing the climate and AIS response to a doubling of the global atmospheric carbon dioxide (pCO2). The AIS modelled with this methodology is comparable with observations. This method is also usable to investigate changes in the atmospheric pCO2.The response of the Antarctic continent to the opening of the Drake Passage and to the establishment of the Antarctic Circumpolar Current (ACC) is examined. Two different climate states have been reproduced with global tectonic configurations including open and closed Passage. A reduced southward heat flux and a decrease of both water and air temperature is found around and over Antarctica when the gateway is open. A more massive ice sheet develops on the continent in this case. The influence of a specific concentration of pCO2 in the atmosphere for the onset of a major AIS is investigated. The climate with a tectonic configuration similar to the Late Eocene and under different pCO2 are analyzed and the response of the AIS is examined. Lower atmospheric pCO2 levels result in lower surface atmospheric temperature over the Antarctic continent and in larger AIS. The effect of a favorable orbital configuration on the early formation of the AIS is analyzed. Four different experiments are conducted with the land-sea distributions similar to the Late Eocene and to the Late Oligocene by applying two distinct orbital setups, the modern and one yielding the coldest Antarctic summer. The effect of a favorable orbital position is to help the initial growth of the AIS under both tectonic configurations.The results of this study support the idea that the establishment of the ACC and low atmospheric pCO2 levels could have comparable significance in creating the conditions for a wide continental glaciation whereas orbital forcings do not seem to have a major impact.