Carbon Dioxide is a greenhouse gas, and %CO2 in the earth's atmosphere is increasing from the burning of fossil fuel. So increased CO2 content will tend to increase the average temperature of the Earth's atmosphere. The question under debate is, how much? "Conventional wisdom" says around 2.5ºC, of which CO2 itself accounts for about 1ºC , the rest being poorly-understood amplification from, e.g., increased water vapor. SM has been looking for a good study on CO2 sensitivity for years, with no joy so far (12-07). Climate sensitivity is defined as the change in global mean surface temperature (in °C) from a doubling of the atmospheric CO2 concentration.
Carbon Dioxide has a linear structure O=C=O where the '=' is a double bond. Since it's symmetrical, it doesn't absorb much in the visible or short-wave IR but since vibrations or bends will create a dipole, it will absorb long-wave (i.e. low energy) IR. Note that this is different than O2 or N2 which cannot produce a dipole by vibrations and which have nothing to bend. But CO2 is also different than water vapor which has a naturally bent structure and thus a natural dipole and is therefore a much stronger absorber of IR in the bands where it absorbs. However there are some bands where CO2 absorbs while H2O doesn't, and in these bands, increases in the concentration of CO2 in the atmosphere can increase the Greenhouse effect.
Carbon dioxide is not a main topic at Climate Audit, and Steve Mcintyre has repeatedly requested that discussions of CO2 remain narrowly focussed to specific questions: "The main features of CO2 infrared behavior are proven beyond any caviling. If you want to discuss a published article and criticize or endorse things in that article, fine, but please no personal theories. I don’t have time to engage such theories and, all too often, despite my explicit statements that I’m not responsible for posts here, I end up being held responsible elsewhere for all opinions and theories expressed here or for not commenting on them." — Steve McIntyre, comment #430.
Consequently there are not a large number of posts on the subject at CA, but below are a few.