Hierarchy of the Models of the Earth Climate System

Models of the Earth climate system, along with the physical components of the climate (atmosphere, ocean, sea ice, active land layer), contain modules for describing (bio)geochemical processes in the Earth system, as well as the socio-economic processes in some cases. At the top of the hierarchy of such models one can see general-circulation models which are able to represent each of the considered components in detail, but are characterized by high computational cost. The simplest models of the Earth climate system are the energy-balance models and radiative-convective models characterized by low spatial resolution and allowance for only a small number of the most important climate-forming processes. Nevertheless, these models are characterized by a number of advantages, primarily, simple and understandable physics. Moreover, radiative-convective models are useful for studying a number of the processes allowed for in general-circulation models and tuning appropriate modules. In addition, there is a class of models of the Earth climate system of intermediate complexity, which take into account most of the processes presented in the general-circulation models (and sometimes the processes unaccounted for in the latters), but with a number of simplifications. The advantage of this class is related to an opportunity of integrating the model for the periods of tens of thousands years or even more. The review deals with all these classes of models with discussions of their features, including the conservation laws explicitly taken into account in them, as well as the classes of problems to which it is advisable to apply the models of the Earth climate system of different types. Additionally, the projects for comparing the models of the Earth climate system in which models of different classes are used simultaneously are discussed.