Corect me if i'm wrong, but CO2 is not an inert gas either, an inert gas being one that does not interact with anything else. CO2 plainly reacts within plants in the process of photosynthesis.
Inert gases just don't take part in chemical reactions. They still undergo physical reactions: even if CO2 were chemically inert (like the real inert gases), it might still absorb energy radiated from the earth.