Edexcel IGCSE Biology 复习笔记 4.4.2 Greenhouse Gases

• A greenhouse gas is a gas that absorbs infrared radiation from the Sun so it remains trapped in the Earth’s atmosphere
• This is important to ensure Earth is warm enough for life, however if levels of these gases in the atmosphere increase it leads to an increase in the greenhouse effect which causes the Earth’s average temperature to rise
• There are many greenhouse gases, the most important are:
  • Water vapour
  • Carbon dioxide
  • Methane
  • Nitrous oxides
  • CFCs

   

• Human activities have led to increasing levels of carbon dioxide and methane in the atmosphere:
  • Carbon dioxide is produced during the combustion of fossil fuel
  • Methane is produced by cattle as they digest grass and released by rice paddy fields

   

The greenhouse effect

• The Sun emits rays that enter the Earth’s atmosphere
• The heat bounces back from the Earth’s surface
• Some heat is reflected back out into space
• Some heat is absorbed by greenhouse gases and is trapped within the Earth’s atmosphere – this is normal
• However, as the levels of greenhouse gases in the atmosphere rise due to human activities the Earth’s average temperature rises beyond normal (an enhanced greenhouse effect), causing global warming

How the greenhouse effect works

The consequences of global warming

• The consequences of global warming due to an enhanced greenhouse effect include:
  • Ocean temperatures increasing, causing melting of polar ice caps / rising sea levels / flooding / coral bleaching
  • Increasing temperatures causing extreme weather like super storms, flooding, droughts
  • Changes in or loss of habitats due to these extreme weather events
  • Decreases in biodiversity as food chains are disrupted and extinction rates increase
  • Increases in migration of species to new places, including increased spread of pests and disease

   

The evidence for global warming

• There is scientific consensus (almost all scientists agree) that global warming is happening and that human activities are largely responsible for the most recent warming
• This scientific consensus is based on systematic reviews of thousands of scientific research papers that have been ‘peer reviewed’ by other scientists (the method used by scientists to check each other’s work in order to ensure that research findings are valid)
• Although they can make good predictions, it is difficult for scientists to say for certain what the consequences of global warming will be