For the past 200 million years the
amounts of the different gases
in the air have been almost the
same as they are today.
The approximate composition of the air today is
| Gas | Proportion |
| Nitrogen (N2) | 78% |
| Oxygen (O2) | 21% |
| Argon (Ar) | 0.9% |
| Carbon Dioxide (CO2) | 0.04% |
| Other Noble Gases | small amounts |
| Water Vapour (H2O) | Variable |
The proportion of carbon
dioxide in the air
has risen from 0·03% to
0·04% in the last hundred years
due to the burning of fossil
fuels.
The amount of water vapour in the air varies
with temperature and weather - see the water cycle.