What is CO2's Relationship to Temperature?

I just had a reader ask me this via email:Hi Coby,I'd like to know what models assume with regard to the relationship between radiation transmission through CO2 and its concentration in the atmosphere. Is it a linear decrease with ppm, exponential?A sceptic website makes a big issue of this, on the grounds that a 10% (say) increase in CO2 will not produce a 10% decrease in