Usually Co2 is not considered as pollutant, but its higher concentration forms a thick layer above the earth’s surface, Checks the radiation of the heat from the earth surface, Because of this, temperature of the earth’s surface increases, this is called “green house effect” or global warming.
Main greenhouse gases in Earth’s atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone.

In this Phenomenon cover of co2 layer around the earth, allow the short wavelength incoming solar radiation to come in but does not allow the long wavelength of out going heat radiation from warm surface of earth and surface keep the earth warm. The consequent increase in the global mean temperature is referred to as global warming.