Greenhouse Gases (SL IB Physics)

• The main greenhouse gases have both natural and human-generated origins
• In order of decreasing contributions, these are:
  • Water vapour (H₂O) - evaporation from the oceans / seas and plants
  • Carbon dioxide (CO₂) - volcanic eruptions, wildfires and respiration
  • Methane (CH₄) - emission from oceans and soils as part of decomposition, termites also emit methane 
  • Nitrous oxide (N₂O) - soils and oceans

• 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

• There are many greenhouse gases, and those that contribute most to the greenhouse effect are:
  • Carbon dioxide (CO₂)
  • Water vapour (H₂O)

• These have the most significant impact on the greenhouse effect
• There are other greenhouse gases which have a lesser effect, such as:
  • Ozone (O₂ and O₃)
  • Methane
  • Nitrous oxides

• While only around 25% of the (mostly 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
  • For example, incoming UV radiation is absorbed by ozone
  • Re-emitted infrared radiation is absorbed by the main greenhouse gases

• This absorbed radiation keeps Earth at a habitable temperature
  • However, if there is an imbalance in the chemical composition of the atmosphere, this can lead to fluctuations in the Earth’s mean surface temperature

• The relative significance of a greenhouse gas depends on its concentration in the Earth's atmosphere and how much the gas can absorb specific wavelengths of radiation

The degree of absorption for a particular molecule varies depending on the wavelength of the radiation. The dark parts show the percentage of radiation that is absorbed by each type of greenhouse gas.

Ozone (O₃)

• Ozone absorbs close to 100% of the Sun's incoming ultraviolet rays
• It also strongly absorbs the wavelengths of the outgoing infrared radiation leaving the Earth's atmosphere, between 9 μm and 10 μm
• However, it is not a significant contributor to the greenhouse effect as it is found in much smaller concentrations in the atmosphere 

Carbon dioxide (CO₂)

• Carbon dioxide is a good absorber of infrared radiation with wavelengths between 1.5 - 30 µm
• In particular, it strongly absorbs radiation with a wavelength of 15 μm
• The increasing concentration of carbon dioxide in the atmosphere makes it one of the most significant contributors to the greenhouse effect

Water vapour (H₂O)

• Water vapour is the best absorber of infrared radiation with wavelengths between 0.8 - 35 µm
• The concentration of water vapour in the atmosphere increases as the air becomes warmer

Total atmosphere

• Overall, most of the ultraviolet, infrared and microwave radiation is absorbed by the atmosphere
• The atmosphere is mostly transparent to incoming visible radiation, which means that the gases in the atmosphere do not absorb or emit much visible radiation