A Simple Model of Multiple Climate Regimes

Among the most intriguing enigmas of the climate system is that on the one hand, the earth’s climate appears to be exquisitely sensitive to relatively minor variations in the distribution of insolation owing to orbital variations, but on the other hand it is in a grosser sense stable, in that its oceans have not frozen over (at least not in the last 500 million years) or boiled away. To this enigma may be added the evidence that climate may undergo extraordinarily abrupt transitions. An attractive idea to help explain these characteristics is the notion that the earth possesses a limited number of stable climate regimes that may overlap to produce multiple equilibrium states for the same solar forcing. Here we present a simple model that produces such overlapping stable equilibria, based on a few key feedback processes. These include control of atmospheric clouds and water vapor by the large-scale circulation of the atmosphere, control of the depth and intensity of the ocean’s thermohaline circulation by tropical cyclones, and the dependence of atmospheric CO2 content on ocean temperature and the strength of the thermohaline circulation. We will show that these key feedback processes produce a climate with two or three stable, overlapping climate regimes. Subjecting this system to variations in climate forcing can account for several observed features of the climate system, including abrupt transitions, sensitivity to orbital variations, arctic warmth and high bottom water temperature during the Eocene and late Cretaceous, tempestites, and possible episodes of deep ocean anoxia during the Cretaceous.