Atmospheric dynamics: the composition is not constant

The proportions of the gases are not constant: they change continuously through natural processes in which living organisms play a pivotal role.

• Respiration — living organisms consume oxygen from the atmosphere and release carbon dioxide into it.
• Photosynthesis — plants (and algae) absorb carbon dioxide from the air and release oxygen.
• Geological phenomena, such as volcanic eruptions, add large amounts of gases (water vapour, CO₂, SO₂, N₂, H₂S, and dust particles) to the atmosphere.
• Decomposition of dead plants and animals releases nitrogen compounds that bacteria convert to N₂ (denitrification), and the ocean exchanges CO₂ and O₂ with the air (Module 7).
• Human activities — burning fuels, industry, agriculture, deforestation — change the proportions of CO₂, CH₄, NOx, SO₂ and others (Modules 10 and 12).

The cycles of these processes are normally balanced over long times: the atmosphere is a dynamic system.