The present thesis examines the ocean and atmospheric dynamics of present-day climate and Last Glacial Maximum (LGM) through Ocean and Atmosphere General Circulation models. Simulating the glacial climate different LGM reconstructions of sea surface temperatures (SST) and sea-ice margins are used as forcing fields for the models: CLIMAP (1981), a modification of CLIMAP (1981), with additional cooling in the tropics, and reconstructions as produced from Weinelt et al. (1996) and GLAMAP 2000, which show seasonally ice free conditions in the Nordic seas. The stability of the thermohaline circulations (THC) under different reconstructions is investigated together with the corresponding atmospheric dynamics. The stability analysis, by means of freshwater flux hysteresis maps reveals mono-stability for each glacial background state, which appears to be a robust feature of the glacial ocean. The impact of the changed orography in North America together with the ice-albedo feedback due to the largely expanded Laurentide Ice Sheet and the reduction of the CO2 concentration are assessed. The results show a strong dependence of the glacial Northern Hemisphere circulation pattern to the changed orography. The Laurentide Ice Sheet forces a deflection of the westerlies, their enhancement and a southward displacement. The oceanic heating contributes only 20-40% to the North Atlantic cooling. Motivated by the extreme climates in the Earth's history, namely the full earth glaciation in the Neoproterozoic era (600-800 million years ago), known as "snowball" Earth, the atmospheric model is forced with extreme boundary and initial conditions. The impact of land albedo, oceanic heat transport, CO2 concentration, initial temperature conditions on the extreme climates are examined. Changing only one boundary or initial condition, the model produces open ice free tropical oceans. Using a proper combination of the varied forcing parameters a full 'Earth glaciation' results. Oceanic heat transport and orography have only a minor influence on the climate instability