A stochastic model of global atmospheric response to enhanced greenhouse warming with cloud feedback

An atmosphere-ocean climate box model is used to examine the influence of cloud feedback on the change in the climate system's variability in response to enhanced greenhouse warming. The model consists of three nonlinear stochastic differential equations that are simplified forms of the first law of thermodynamics for the atmosphere and ocean and the continuity equation for the atmospheric component of the hydrological cycle. The model is driven by random fluctuations in the mean evaporative flux, which is routed and distributed among the components of the system through the fluxes of energy and moisture. The model suggests that cloud feedback can lead to the occurrence of two climatic regimes into which the present climate may evolve as a result of an enhanced greenhouse warming. In the first regime, the mean values of the model parameters, such as temperature, precipitation and cloudiness, as well as the amplitude and timescale of their fluctuations all increase moderately. In the second regime these mean values increase substantially, and the amplitude and timescale of their fluctuations rise sharply. The model also predicts the existence of climatic hysteresis; that is, for the climate system to return from either of these regimes back to the present regime, a substantial decrease in the long-wave forcing is required. (C) 1998 Elsevier Science B.V. All rights reserved.