Additional increments of CO2 will cause relatively less direct warming because we already have so much CO2 in the atmosphere that it has blocked most of the infrared radiation that it can. (climate-skeptic.com)
BTW: There is no physical evidence that the assumption of a net positive feedback is correct, and much physical evidence to suggest that the real world feedbacks are net negative and will reduce the proposed direct warming effect of CO2 towards (closer too) 0. (joannenova.com.au)
As if that were not bad enough, the official story is that feedbacks triggered by direct warming roughly triple it, causing not 1 but 3 degrees» warming per CO2 doubling. (breitbart.com)