Global climate models: Past, present and future

gerrit.lohmann [ at ]


One of the main features of climate spectra is their redness which originates from stochastic mechanisms (see e.g. the time scale arguments of Hasselmann, 1976). The variance increases toward the longer time scales and is limited by the negative feedback mechanisms in the climate system. Apart from this there is climate variability at distinct time scales due to external forcing (e.g. Milankowitch cycles), or internal oscillations (e.g. ENSO, decadal oscillations). The understanding of long-term climate variability is essential for the detection of anthropogenic climate changes.Geological data reveal that there have been abrupt changes in the past climate. Examples are the terminations of Pleistocene glaciations, the sudden cooling after the last deglaciation (Younger Dryas), and the formation of ice sheets. Model studies suggest the importance of multiple equilibria of the climate system. Especially the role of the oceanic thermohaline circulation has been discussed recently.The oceanic thermohaline circulation occupies a central position in the understanding of climate variability and predictability because of its link to long-term variability and climate changes. In addition to paleoclimatic shifts, interdecadal climate variability may originate from changes of North Atlantic Deep Water formation. The changes in distribution of winter convective activity in the North Atlantic are highly correlated with the dominant pattern of atmospheric variability, the North Atlantic Oscillation.Models with different levels of complexity (conceptual models, box models, coupled atmosphere-ocean circulation models) have been used to examine stability, variability, and predictability of the climate in the North Atlantic region. In this sensitive part of the global thermohaline circulation complicated feedback mechanisms act on many different timescales. The interactions involve different climatic components and processes such as the atmosphere, oceanic deep water formation, gyre circulation, and sea ice.

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Stute, M. , Clement, A. and Lohmann, G. (2001): Global climate models: Past, present and future , Proceedings National Academy of Sciences, USA, 98 (19), pp. 10529-10530 .

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