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); and (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).