Climate Change: Greenhouse Effect (DP IB Biology: HL)

• When radiation from the sun hits the earth, it is radiated back from the earth's surface as long-wave radiation
• A greenhouse gas is a gas that absorbs this re-radiated radiation, trapping it in the earth's atmosphere so that it is not lost to space
  • Greenhouse gases in the atmosphere have a similar effect to the glass in a greenhouse, hence the term greenhouse gas, and their effect being known as the greenhouse effect

• The greenhouse effect is important to ensure that Earth is warm enough for life; if it were not for the insulating effects of greenhouse gases, Earth would see similar dramatic temperature fluctuations to its neighbouring planets
  • Temperatures on Mars range between 20°C and −153°C

• There are many greenhouse gases, and those that contribute most to the greenhouse effect are:
  • Water vapour
  • Carbon dioxide

• Water vapour enters the atmosphere when it evaporates from the surface of the oceans, and when it is released by transpiration
• Carbon dioxide enters the atmosphere when living organisms respire, or when organic matter or fossil fuels are burned

• While water vapour and carbon dioxide are the greenhouse gases with the most significant impact on the greenhouse effect, there are various other greenhouse gases which have a lesser effect, e.g.
  • Methane
  • Nitrous oxides

• Methane is released by:
  • Methanogenic bacteria when they break down organic waste
    • Methanogenic bacteria carry out methanogenesis in anaerobic environments such as waterlogged ground and landfill sites, and in the stomachs of some types of livestock

  • Coal mining when deposits of methane are released from rock

• Sources of nitrous oxides include:
  • Fossil fuel combustion e.g. vehicle exhausts
  • Agriculture e.g. the production and use of fertilisers
  • The activity of some types of bacteria

Factors affecting the impact of a Greenhouse Gas

• The significance of the impact of any particular greenhouse gas depends on two factors:
  • Its ability to absorb long-wave radiation
  • Its concentration in the atmosphere

• A gas may have a strong ability to absorb radiation, but if its atmospheric concentration is relatively low, then it will be a low-impact greenhouse gas
  • This is the case for methane, which can absorb more long-wave radiation per molecule than carbon dioxide, but which is present at a much lower atmospheric concentration

• The atmospheric concentration of a greenhouse gas depends on:
  • The balance between release and removal of that gas from the atmosphere
    • A gas that is released into the atmosphere at a faster rate than it is removed will increase in concentration e.g. carbon dioxide is released by the combustion of fossil fuels at a faster rate than photosynthesis can remove it

  • The length of time that gas is present in the atmosphere
    • Methane only remains in the atmosphere for 10-12 years before it is oxidised, reducing the overall impact of methane on the greenhouse effect
    • Water vapour is the most abundant greenhouse gas in the atmosphere, but because water vapour usually only spends an average of 9 days in the atmosphere before it returns to the ground, scientists are less concerned about its impact on the greenhouse effect than they are about carbon dioxide, which accumulates in the atmosphere over hundreds of years

The Earth emits longer-wave radiation

• The solar radiation that reaches Earth from the sun contains a range of wavelengths; primarily shorter wavelengths in the UV and visible parts of the light spectrum, with some longer wavelength infra-red radiation
• After Earth has absorbed this radiation, it re-emits radiation back into the atmosphere, but this re-emitted radiation is entirely longer wavelength, in the infra-red part of the spectrum

Solar radiation is primarily short-wave, while the radiation that is re-emitted by earth is long-wave radiation

Greenhouse Gases absorb longer-wave radiation

• While only around 25% of the (primarily short wavelength) solar radiation is absorbed by the atmosphere on its way to Earth, around 80% of the (long wavelength) re-emitted radiation from Earth is absorbed on its way back into the atmosphere
  • E.g. incoming UV radiation is absorbed by ozone
  • Re-emitted radiation is absorbed by greenhouse gases

• This absorbed radiation keeps Earth at a habitable temperature

Greenhouse gases absorb the long-wave radiation emitted by Earth, warming the atmosphere