Before CO2, you had pollutants like NOx, which were toxic pollutants, and CFCs, which were pollutants but not toxic. CFCs were considered pollutants because they damaged the Earth in a way that would have a negative impact on human health, but non-toxic because their harm was not directly effected on the human body. CO2 would be the kind of pollutant that CFCs are. (CFCs are toxic in high concentrations, but the mechanism of their harm when released into the environment is not toxicity.)
The topic is about controlling proportions of different molecules in the environment to maintain a preferable quality of life for citizens of the world.
