I would call it a pollutant. It certainly meets the Encylopdia Brittanica definition:
"the addition of any substance (solid, liquid, or gas) or any form of energy (such as heat, sound, or radioactivity) to the environment at a rate faster than it can be dispersed, diluted, decomposed, recycled, or stored in some harmless form."
But I don't see the value in a semantic debate.
Is anthropogenic CO2 increasing rapidly? is it a greenhouse gas ? will that change the climate?
Those questions have been answered decisively in the affirmative.