An Introduction to Simple Climate Models used in the IPCC Second Assessment Report 

This Technical Paper is intended as a primer on the climate system and simple climate models (SCMs), and has two objectives:
(a) to explain how SCMs work, the processes that are included in them, what their strengths and weaknesses are in relation to more complex models, the purposes to which they are applied, and why they have been used extensively in the Working Group I volume of the IPCC Second Assessment Report (IPCC WGI, 19961);
(b) to fully document the procedures and assumptions used to generate the trace gas concentration, global mean temperature change, and global mean sea level rise projections presented in the SAR WGI (Section 6.3) and in the IPCC Technical Paper on Stabilization of Atmospheric Greenhouse Gases: Physical, Biological and Socio-economic Implications (IPCC TP STAB, 1997).

The major components of the climate system that are important for climatic change and its consequences, such as sea level rise, during the next century are: the atmosphere, oceans, terrestrial biosphere, glaciers and ice sheets and land surface. In order to project the impact of human perturbations on the climate system, it is necessary to calculate the effects of all the key processes operating in these climate system components and the interactions between them. These climate processes can be represented in mathematical terms based on physical laws such as the conservation of mass, momentum, and energy. However, the complexity of the system means that the calculations from these mathematical equations can be performed in practice only by using a computer. The mathematical formulation is therefore implemented in a computer program, which we refer to as a “model”. If the model includes enough of the components of the climate system to be useful for simulating the climate, it is commonly called a “climate model”. Climate system models are fundamentally different from statistical models used in some of the social sciences, which are based purely on empirical correlations and are unrelated to an underlying body of physic   law.