Yes, carbon dioxide truly is a greenhouse gas.

With regard to the greenhouse effect, 99.96% of the dry atmosphere might as well be vacuum. Nitrogen (N<sub>2</sub>), oxygen (O<sub>2</sub>), and argon are extremely weak greenhouse gases. The 99.96% of the atmosphere that doesn't directly participate in the greenhouse effect is however present. At sea level, a molecule of carbon dioxide will travel freely for about a tenth of nanosecond before colliding with another gas molecule. This mean time between collisions increases to a few tens of nanoseconds at the top of the troposphere. A greenhouse gas molecule such as CO<sub>2</sub> that has just absorbed a thermal infrared photon will very quickly transfer that absorbed energy to the non-greenhouse gases that comprise the vast majority of the atmosphere.