By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide and small amounts of other gases. Air also contains a variable amount of water vapour, on average around 1% at sea level and 0.4% over the entire atmosphere. Air composition, temperature and atmospheric pressure vary with altitude and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth’s troposphere (the lowest part of the atmosphere). At high altitudes the air can be described as becoming thinner which will cause any combustion process to be incomplete.

It wasn’t always like this, the atmosphere has changed quite a bit over time. The issue for CO2 is that because it is such a small part of the atmosphere even a single percentage point change is very significant.

Greenhouse gases, which are mainly trace gases, include water vapour, carbon dioxide, methane, nitrous oxide and ozone. Various industrial pollutants also may be present as gases or aerosols, such as chlorine (elemental or part of compounds), fluorine compounds and elemental mercury vapour. Sulphur compounds such as hydrogen sulphide and sulphur dioxide (SO2) can be present, which may be derived from natural sources or from industrial sources.