Carbon dioxide (CO2) is a colourless and odourless gas with a slightly sour taste. CO2 is the molecular formula of the chemical compound of carbon (C) and oxygen (O).
With the help of sunlight and the green leaf pigment, chlorophyll, vegetation converts CO2 back into O2 (oxygen). Oxygen is in turn converted into CO2 by humans and animals breathing. This would create a balance, if the process was not disrupted, for example by combustion gases.