C13 - The Earth's atmosphere

• Scientists think that the Earth was formed about 4.6 billion years ago
• To begin with, the Earth was a molten ball of rock and minerals
• For its first billion years the Earth was a very hot, turbulent place with volcanoes belching fire and gases into the atmosphere
• Gases scientists speculate were found in the Earth's early atmosphere

  • Carbon dioxide (CO2)
  • Water vapour (H2O)
  • Nitrogen (N2)
  • Methane (CH4)
  • Ammonia (NH3)
• How the Earth's early atmosphere was formed

  Volcanoes released carbon dioxide, water vapour, and nitrogen, which formed the early atmosphere
• Water vapour in the atmosphere condensed as the Earth cooled down, and fell as rain, collecting in hollows to form the first oceans
• Comets could also have brought water to the Earth as icy comets rained down and melted
• The early atmosphere was probably mainly carbon dioxide, with some water vapour, nitrogen gas, and traces of methane and ammonia
• There would have been very little or no oxygen at that time, resembling the atmospheres of Mars and Venus today
• Life first appeared on Earth around 3.4 billion years ago, with simple organisms similar to bacteria
• Around 2.7 billion years ago, bacteria and algae evolved that could use photosynthesis to produce oxygen as a waste product
• Photosynthesis


  Carbon dioxide + water (energy from sunlight) -> oxygen + glucose
• As plants evolved and colonised the Earth, the atmosphere became richer in oxygen, making it possible for the first animal forms to evolve
• Many of the earliest living microorganisms could not tolerate high oxygen concentrations and largely died out
• The Earth's early atmosphere was formed by volcanic activity
• Much of the carbon from the old carbon dioxide-rich atmosphere became locked up within the Earth's crust in rocks and fossil fuels
• Carbon dioxide gas was also removed from the early atmosphere by dissolving in the oceans and reacting with metal oxides to form insoluble carbonate compounds
• Over the past 200 million years, the level of carbon dioxide in the atmosphere has not changed much due to the natural cycle of carbon
• Volcanoes also produced nitrogen gas, which gradually built up in the early atmosphere
• Reactions of methane and ammonia with oxygen

  Methane (CH4) + 2 oxygen -> Carbon dioxide + 2 water

  Ammonia (NH3) + 3 oxygen -> Nitrogen + 3 water
• Nitrogen gas is very unreactive, allowing its levels to build up in the atmosphere
• Percentage composition of gases in the Earth's atmosphere today

  • Nitrogen - 78%

  Oxygen - 21%

  Argon - 0.9%

  Carbon dioxide - 0.04%

  Trace amounts of other gases
• The noble gases are all found in air, with argon being the most abundant at about 0.9%
• Greenhouse gases


  • carbon dioxide
  • methane
  • water vapour
• How greenhouse gases warm up the Earth

  1. Greenhouse gases let short-wavelength radiation pass through

  2. Earth's surface emits longer wavelength infrared radiation

  3. Greenhouse gases absorb infrared radiation

  4. Radiation stimulates bonds in molecules to vibrate, bend, and stretch more vigorously, raising their temperature

  5. Energy radiated from Earth's surface gets trapped in atmosphere, increasing temperature
• Without carbon dioxide in Earth's atmosphere, average temperature would be about -19°C and life as we know it could never have evolved
• Over the past century, amount of carbon dioxide released into atmosphere has greatly increased due to increased use of fossil fuels
• Methane gets into atmosphere from swamps, rice fields, grazing cattle, and decomposing waste in landfill sites
• There is no doubt amongst scientists that levels of greenhouse gases, especially carbon dioxide, in the atmosphere are increasing
• As more trees are cut down (deforestation), carbon dioxide removed from air by photosynthesis is reduced
• As temperature rises, carbon dioxide becomes less soluble in water, making oceans less effective as 'CO2 sinks'
• Most scientists agree that a trend in global warming has started, based on evidence presented in scientific journals
• Analysis of gases trapped in ice cores suggests current levels of carbon dioxide are higher than at any time in the last 440,000 years
• Despite advances in science, effects of increasing greenhouse gases cannot be predicted with certainty, even with powerful computers
• Boiling an electric kettle

  May increase amount of carbon dioxide in Earth's atmosphere
• Reasons for increased carbon dioxide in atmosphere in recent past

  • Increased use of fossil fuels
  • Deforestation
  • Oceans less effective as CO2 sinks
• Annual variations in atmospheric carbon dioxide levels show peaks and troughs
• Overall trend in atmospheric carbon dioxide levels has been ever upwards
• Media reports on global warming should be treated with caution as they can be biased or oversimplified
• Conclusions drawn from data in Figure 3 have uncertainty due to limited historical data