The direct warming effect of doubling the CO2 in the air is generally agreed to be little more than 1 C. However climate models produce exaggerated warming forecasts by assuming that strongly positive or amplifying temperature feedbacks, such as the increased capacity of warmer air to carry water vapour, a greenhouse gas. They assume that these positive feedbacks will turn 1 C of direct warming into 3 C of imagined catastrophe.
Fallacy: slothful induction