ON THE CLIMATE REGIME SHIFT 0.9 M.Y. AGO

The Mid-Pleistocene Climate Shift near 0.92 Ma represents a major change in the nature of ice age fluctuations, from a modest-amplitude quasi-linear response to Milankovitch forcing to a high-amplitude non-linear response which at the same time favors longer periods. The nature of the response can be defined in terms of changes in systemic properties, including the introduction of threshold switches and positive feedback. Perhaps the most important of these is the introduction of a threshold condition contingent on building up instability within the ice masses. A number of long-term trends have been invoked to rationalize the arrival of a new climate regime at the time, including changes in topography allowing access of seawater at the bottom of major ice sheets, a drop in carbon dioxide in the atmosphere, and changes in deep-sea circulation. New evidence is presented supporting the "Nansen hypothesis" (modification of shelf topography where the ice meets the ocean). Also, there is evidence that the pCO2 has dropped somewhat over the last 400,000 years, which supports a long-term trend of falling CO2. Other possible positive feedback mechanisms are explored, including deep-sea circulation. We hypothesize that the Nordic Heat Pump is part of this feedback, and that it increases its efficiency as positive feedback at the time of the regime shift.