It is claimed that a doubling of carbon dioxide in the atmosphere will cause a temperature increase of 1.2°C and the only question is how much increase will be due to forcing effects, mostly by water vapor. Computer models are used to evaluate the forcing.
There is an equation for calculating the 1.2°C temperature increase; and it is such a simple equation that it seems to never be questioned. The problem is, it is too simple. It is a three component equation.
The equation is this: Heat increase = 5.35 ln C/C0. Temperature increase = 0.75 times heat increase. C sub zero is the starting concentration of CO2 and C is the final amount. Since the question is doubling, this ratio is two. The natural log of two is multiplied times the constant 5.35. Then the heat increase is multiplied times 0.75 to get the temperature increase. If only seventh grade general science were so simple.
Determining where this equation came from is no easy task. Steve McIntyre tried to trace down the citations for it in the IPCC documents and failed (1). All of the references led to no real explanation. The citation problem wasn't just glaciergate, Amazongate ad infinitum; it was also in the claimed, unquestionable physics.