Recently, significant advances have occurred in the development of Earth System Models of Intermediate Complexity (EMIC), which are designed to bridge the gap between the three-dimensional comprehensive models and simple models. The main characteristic of EMICs is that they describe most of the processes implicit in comprehensive models, albeit in a more reduced (i.e., more parametrized) form. They also explicitly simulate the interactions among several components of the climate system including biogeochemical cycles. On the other hand, EMICs are computationally efficient enough to allow for long-term climate simulations over several tens of thousands of years or a broad range of sensitivity experiments over several millennia. As for AOGCMs, but in contrast to simple models, the number of degrees of freedom of an EMIC exceeds the number of adjustable parameters by several orders of magnitude. Currently, there are several EMICs in operation such as: two-dimensional, zonally averaged ocean models coupled to a simple atmospheric module (e.g., Stocker et al., 1992; Marchal et al., 1998) or geostrophic two-dimensional (e.g., Gallee et al., 1991) or statistical-dynamical (e.g., Petoukhov et al., 2000) atmospheric modules; three-dimensional models with a statistical-dynamical atmospheric and oceanic modules (Petoukhov et al., 1998; Handorf et al., 1999); reduced-form comprehensive models (e.g., Opsteegh et al., 1998) and those that involve an energy-moisture balance model coupled to an OGCM and a sea-ice model (e.g., Fanning and Weaver, 1996). Some EMICs have been used to investigate both the climate of the last glacial maximum (see Section 8.5) as well as to investigate the cause of the collapse of the conveyor in global warming experiments (Stocker and Schmittner, 1997; Rahmstorf and Ganopolski, 1999) while others have been used to undertake a number of sensitivity studies including the role of sub-grid scale ocean mixing in global warming experiments (Wiebe and Weaver, 1999).

EMIC development involves the same evaluation procedure as AOGCMs use, albeit restricted due to the reduced complexity of some, or all, of the constituent sub-components. While EMIC evaluation is in its early stages, the nature of these models allows for a detailed comparison with both historical and proxy observational data. Initial analyses (referenced above) suggest that EMICs hold promise as exploratory tools to understand important processes, and their interactions and feedbacks within the climate system. However, they are not useful for assessing regional aspects of climate change.

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