The Earth's Atmosphere

• How long ago did the Earth form?

  4.6 billion years ago
• What gases were present during the Earth's formation?

  Carbon dioxide, water vapour, methane, ammonia
• What was released by intense volcanic activity during Earth's formation?

  Mainly carbon dioxide and water vapour
• What happened to water vapour during Earth's formation?

  It condensed to form the oceans
• What happened to some carbon dioxide during Earth's early history?

  It dissolved into the oceans
• What changes occurred in the atmosphere 3-4 billion years ago?

  Decrease in carbon dioxide, increase in oxygen and nitrogen
• How did green plants and algae affect the atmosphere?

  They reduced carbon dioxide and increased oxygen
• What happened to microorganisms that couldn't tolerate oxygen?

  They were killed off by the increasing oxygen
• What happens to carbon from carbon dioxide over time?

  It becomes locked up in sedimentary rocks
• How is nitrogen produced in the atmosphere?

  By bacteria removing nitrates from decaying plant material
• What was the atmospheric composition 1-2 billion years ago?

  Decrease in carbon dioxide, increase in nitrogen and oxygen
• What is the current percentage of oxygen in the atmosphere?

  About 20%
• What is the current percentage of nitrogen in the atmosphere?

  About 80%
• How has the amount of carbon dioxide changed in the atmosphere now?

  It has decreased significantly
• The Atmosphere Today

  . The proportions of gases in the atmosphere have been more or less the same for about 200 million years

  . Water vapour may also be present in varying quantities (0-3%)
• Increase of Oxygen Levels

  . Algae and plants photosynthesise

  . During photosynthesis, carbon dioxide and water react to produce glucose and oxygen:

  carbon dioxide + water --> glucose + oxygen

  6CO2 + 6H2O --> C6H12O6 +6O2

  . Algae first started producing oxygen about 2.7 billion years ago

  . Over the next billion years, plants evolved and the amount of oxygen in the atmosphere increased

  . Eventually the level of oxygen in the atmosphere increased enough to allow animals to evolve
• Key Point

  The Earth's early atmosphere was mainly carbon dioxide. Over time the levels of oxygen have increased and the levels of carbon dioxide have decreased
• Decrease of Carbon Dioxide Levels

  . As plants and algae have evolved, the level of carbon dioxide in the atmosphere has decreased. This is because plants use carbon dioxide during photosynthesis

  . Carbon dioxide has also decreased as carbon becomes locked up in sedimentary rocks (e.g. limestone) and fossil fuels (e.g. coal, crude oil and natural gas)

  . Limestone contains calcium carbonate and can be formed from the shells and skeletons of sea creatures

  . Coal is a sedimentary rock formed from plant deposits that were buried and compressed over millions of years

  . The level of carbon dioxide in the atmosphere has also been reduced by the reaction between carbon dioxide and sea water. This reaction produces:

  - insoluble carbonates that are deposited as sediment

  - soluble hydrogen carbonates

  . However, too much carbon dioxide dissolving in the oceans can harm marine life, such as coral reefs