CLIMBER-2: a climate system model of intermediate complexity. Part II: model sensitivity

A set of sensitivity experiments with the climate system model of intermedi ate complexity CLIMBER-2 was performed to compare its sensitivity to change s in different types of forcings and boundary conditions with the results o f comprehensive models (GCMs). We investigated the climate system response to changes in freshwater flux into the Northern Atlantic, CO2 concentration , solar insolation, and vegetation cover in the boreal zone and in the trop ics. All these experiments were compared with the results of corresponding experiments performed with different GCMs. Qualitative, and in many respect s, quantitative agreement between the results of CLIMBER-2 and GCMs demonst rate the ability of our climate system model of intermediate complexity to address diverse aspects of the climate change problem. In addition, we used our model for a series of experiments to assess the impact of some climate feedbacks and uncertainties in model parameters on the model sensitivity t o different forcings. We studied the role of freshwater feedback and vertic al ocean diffusivity for the stability properties of the thermohaline ocean circulation. We show that freshwater feedback plays a minor role, while ch anges of vertical diffusivity in the ocean considerably affect the circulat ion stability. In global warming experiments we analysed the impact of hydr ological sensitivity and vertical diffusivity on the long-term evolution of the thermohaline circulation. In the boreal and tropical deforestation exp eriments we assessed the role of an interactive ocean and showed that for b oth types of deforestation scenarios, an interactive ocean leads to an addi tional cooling due to albedo and water vapour feedbacks.