The atmosphere of Venus is 90 times thicker than earth, the dense atmosphere creates tremendous pressure and thats what causes the extreme heat. Mars with 95% of its atmospere composed of CO2 must also be a searing hot oven. Yet its mean tempreature is -46 degrees C.
CO2 does have some warming effect, but extra CO2 does not have the same effect. The US Air Force came up with a model, called Modtran, which proves that every new molecule of CO2 put in the atmosphere has less effect than the previous on an exponential scale. Put differently the first 20ppm of CO2 has more effect than the next 367ppm combined and so on.
