Topic 1 - How the presence of life has brought about environmental change

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Atmospheric oxygen

How the presence of life on Earth has brought about environmental change

Atmospheric oxygen

  • Oxygen is a reactive element and its concentration in the atmosphere declined as it racted with other elements.
  • For example, by oxidation weathering with elements, such as iron, on the surface of exposed rocks.
  • The continued presence of oxygen in the atmosphere relies on processes that continually replace it as other processes remove it. 
  • A small amount of oxygen was released by the photolysis of water. 
  • Photosynthetic bacteria, then, much later, photosynthetic plants gradually released much larger amounts of oxygen.
  • This oxygen absorbed UV light from the Sun, causing the oxygen molecules to split.
  • This produced monatomic oxygen that reacted with diatomic ocygen to produce ozone.
  • These reactions allowed the ozone layer to develop, which provided protection from UV light to living organisms on Earth. 
  • Before the ozone layer developed, abundant life was not possible on land so early organisms lived in the oceans where water protected them from the UV light.
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Atmospheric carbon dioxide

How the presence of life on Earth has brought about environmental change

Atmospheric carbon dioxide

  • Carbon dioxide is naturally released into the atmosphere by volcanoes.
  • It is an essential greenhouse gas, helping to retain heat in the atmosphere.
  • Without it, the Earth would be too cold to support life.
  • However, excessive carbon dioxide levels would cause temperatures to rise too high for lifw to survive.
  • The light output of the Sun inceases by about 10% every billion years, so it is about 30% brighter now than when life first developed.
  • If the carbon dioxide content had remained constant, then the Earth would have heated up and life would have died out, but living organisms have helped to maintain suitable atmospheric temperatures by removing carbon dioxide from the atmosphere and storing the carbon in fossil fuels and carbonate rocks such as limestone and chalk.
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The water cycle

How the presence of life on Earth has brought about environmental change

The water cycle

  • Heat energy from absorbed sunlight causes water in the sea to evaporate.
  • This may be carried over land where it falls as rain then flows back to the sea.
  • Most rain falls relatively close to the coast, but transpiration by plants returns water vapour to the atmosphere so it can be blown further inland.
  • Transpiration from leaves is unavoidable as the moist stomata must be open to allow exchange of gases such as carbon dioxide.
  • It is important in cooling plants and in the upward transport of dissolved minerals.
  • The individual features of the Earth and their inter-connections can be very complex.
  • Although they are interdependent, it is often easiest to focus on one aspect at a time, such as the biosphere, hydrosphere, atmosphere and lithosphere.
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