Its a logarithmic response curve. We simply wont get much more direct warming from increasing CO2 levels now. What they count on is mythical positive feed back via an increase in water vapor due to the increased warmth from the CO2.
yep, and from all the evidence, feedback is turning negative. More storminess (e.g. increased rainfall) is a negative feedback. Storm tracks further south is negative and blocking patterns such as the ones that cause extreme rainfall in one area and heat waves / drought in another.