Forming the modern atmosphere

Compared to the Earth’s early atmosphere, the modern atmosphere is oxygen-rich. It contains:

  • less water vapour
  • less carbon dioxide
  • more oxygen
Air is composed of 78% nitrogen, 21% oxygen and 1% other gases, including 0.9% argon and 0.04% carbon dioxide.The composition of the modern atmosphere

Name the most abundant gas in the modern atmosphere.


Water vapour

The Earth cooled after it formed. When it became cool enough, water vapour in the atmosphere condensed. It fell as rain, creating the oceans.

Carbon dioxide

Carbon dioxide is a very soluble gas. It dissolves readily in water. As the oceans formed, carbon dioxide dissolved and the amount of it in the atmosphere decreased.


Plants make their own food by photosynthesis. In this process, carbon dioxide from the atmosphere reacts with water. Glucose is produced, with oxygen as a by-product.

Photosynthesis by primitive plants and algae caused the removal of carbon dioxide from the air, and the release of oxygen. This had two effects:

  • the amount of carbon dioxide decreased further
  • the amount of oxygen in the atmosphere gradually increased
Blue-green algae cells, viewed through using electron microscope
Green algae cells, viewed using an electron microscope

Describe two reasons why the amount of carbon dioxide in the atmosphere decreased over time.

Carbon dioxide dissolved in the oceans, and primitive plants used it for photosynthesis.