The composition and evolution of the Earth's atmosphere

• Proportions of different gases in the atmosphere

  • ~80% nitrogen
  • ~20% oxygen
  • Small proportions of various other gases, i.e. CO2, H2O(g) and noble gases
• For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today
• The Earth's early atmosphere

  • Evidence is limited because of the time scale of 4.6 billion years and theories have changed/developed over time
  • One theory suggests that during the first billion years of the Earth's existence, there was intense volcanic activity that released gases that formed the early atmosphere
  • At the start of this period, the atmosphere may have been like the atmospheres of Mars and Venus today: mainly CO2 with little or no O2(g)
  • Volcanoes also produced nitrogen (N2) which gradually built up in the atmosphere & there may have been small proportions of methane (CH4) and ammonia (NH3)
  • Water vapour condensed to form the oceans, and CO2 dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of CO2 in the atmosphere
• How oxygen increased


  1. Algae & plants produced the O2 that is now in the atmosphere by photosynthesis

  2. Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere

  3. Over the next billion years plants evolved and the % oxygen gradually increased to a level that enabled animals to evolve
• How carbon dioxide decreased

  1. Algae and plants decreased the % CO2 in the atmosphere by photosynthesis

  2. CO2 was also decreased by the formation of sedimentary rocks that contain carbon (e.g. limestone and coal) and by the production of fossil fuels from the remains of dead plants and animals when they decayed