A naturally occurring gas It is also a by-product of burning fossil fuels and biomass, as well as land-use changes and other industrial processes. It is the principal anthropogenic greenhouse gas that affects the earth’s temperature (i.e. resulting from human influence). It is the reference gas against which other GHGs are indexed (a ‘common currency’, for ease of measurement and explanation).. Carbon dioxide constitutes approximately 0.036 per cent of the atmosphere.