Understanding the Holocene climate record is an exceptional challenge because of the countless possible interactions between components characterized by hugely different spatial and temporal scales. A spectrum of climate models is therefore required. Conceptual models made of a few equations are useful to formulate general hypotheses. Earth models of intermediate complexity and comprehensive models are needed to study mechanisms of climate change in more detail.
The Holocene is a time during which no major global change in climate regime occurred. In fact, the astronomical theory of paleoclimates predicts no glacial inception before the next 50 000 years. We do not exclude the possibility, however, that climate chose the trajectory of a long interglacial by chance, or perhaps was helped by a small kick from anthropogenic activities as early as 6000 years ago. A short Holocene with a currently ongoing glacial inception was another option that is now definitively excluded given the present atmospheric carbon dioxide concentrations.
Although the Holocene is globally stable, regional changes in regime occurred. Detection of local climate instability is naturally highly relevant in the context of global warming. The Holocene records provide the opportunity to do so. Claussen (this volume) describes abrupt transitions associated with land-surface feedbacks. Here, we discussed ocean circulation instabilities. Sudden coolings in northern Europe and Greenland may be ascribed to stops or drastic reduction of deep-ocean convection in the North Atlantic. Analysis of the dynamics of the convective feedback and interactions with sea-ice and atmosphere dynamics suggest that climate may be "trapped" in this cold state for several decades to several centuries, the exact duration being ruled by the stochastic character of the climate system. This relatively new theory certainly deserves further consideration.
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