9.1.4 How Carbon Dioxide Decreased

Formation of Oceans and Sediments:

• Condensation of Water Vapour:
  • The formation of oceans as water vapour condensed played a significant role in reducing atmospheric carbon dioxide.
  • Carbon dioxide dissolved in the newly formed oceans, leading to the formation of carbonate sediments like limestone $(CaCO₃)$, which removed CO₂ from the atmosphere.

• Continued Reduction by Photosynthesis:
  • The advent of photosynthesising organisms further reduced atmospheric CO₂ levels.
  • As plants and algae absorbed carbon dioxide for photosynthesis, the amount of CO₂ in the air continued to decline.

Modern Atmospheric Composition:

• Over the course of billions of years, these processes reduced carbon dioxide levels and increased oxygen levels, leading to the modern atmosphere we have today.
• By about 200 million years ago, the atmosphere had stabilised into its current composition, with 78% nitrogen, 21% oxygen, and trace amounts of other gases, including carbon dioxide.