Greenhouse Effect

• Albedo


  The proportion of light that is reflected by a given surface
• Emissivity


  The power radiated by a surface divided by the power radiated from a black body of the same surface area and temperature
• Stars are good approximations to a black body, whereas planets are not
• Black body


  A perfect absorber and emitter of radiation
• For a perfect black body, emissivity is equal to 1
• The solar constant is the amount of solar radiation across all wavelengths that is incident in one second on one square metre at the mean distance of the Earth from the Sun
• The solar constant varies year-round because the Earth is in an elliptical orbit around the Sun and the Sun's output varies by about 0.1% during its 11-year sunspot cycle
• Main greenhouse gases


  • Water vapour (H2O)
  • Carbon dioxide (CO2)
  • Methane (CH4)
  • Nitrous oxide (N2O)
• Greenhouse gas


  A gas that absorbs re-radiated radiation from the Earth's surface, trapping it in the Earth's atmosphere
• Carbon dioxide (CO2) and water vapour (H2O) have the most significant impact on the greenhouse effect
• 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
• Ozone (O3)


  • Absorbs close to 100% of the Sun's incoming ultraviolet rays
  • Strongly absorbs the wavelengths of the outgoing infrared radiation leaving the Earth's atmosphere, between 9 μm and 10 μm
  • Not a significant contributor to the greenhouse effect as it is found in much smaller concentrations in the atmosphere
• Carbon dioxide (CO2)


  • Good absorber of infrared radiation with wavelengths between 1.5 - 30 µm
  • Strongly absorbs radiation with a wavelength of 15 μm
  • Increasing concentration makes it one of the most significant contributors to the greenhouse effect
• Water vapour (H2O)


  • Best absorber of infrared radiation with wavelengths between 0.8 - 35 µm
  • Concentration increases as the air becomes warmer
• Overall, most of the ultraviolet, infrared and microwave radiation is absorbed by the atmosphere, while the atmosphere is mostly transparent to incoming visible radiation
• Infrared radiation


  Wavelength range of 9 μm to 10 μm
• Infrared radiation leaving the Earth's atmosphere is not a significant contributor to the greenhouse effect as it is found in much smaller concentrations in the atmosphere
• Carbon dioxide (CO2)


  Good absorber of infrared radiation with wavelengths between 1.5 - 30 μm, particularly at 15 μm
• Increasing concentration of carbon dioxide in the atmosphere makes it one of the most significant contributors to the greenhouse effect
• Water vapour (H2O)


  Best absorber of infrared radiation with wavelengths between 0.8 - 35 μm
• Concentration of water vapour in the atmosphere increases as the air becomes warmer
• Most of the ultraviolet, infrared and microwave radiation is absorbed by the atmosphere
• The atmosphere is mostly transparent to incoming visible radiation
• Greenhouse effect


  Absorbed radiation keeps Earth at a habitable temperature
• If there is an imbalance in the chemical composition of the atmosphere, this can lead to fluctuations in the Earth's mean surface temperature
• Resonance model of global warming

  1. Incoming solar radiation is absorbed by Earth's surface

  2. Earth re-radiates this as infrared

  3. Greenhouse gases absorb infrared and reflect it back to Earth's surface

  4. Higher greenhouse gas concentration traps more heat energy in atmosphere
• Greenhouse gases absorb the long-wave radiation emitted by Earth, warming the atmosphere
• Molecular energy level model

  Greenhouse gases have a natural frequency in the infrared region, so they absorb and re-emit infrared radiation
• Solar radiation is primarily short-wave, while the radiation that is re-emitted by Earth is long-wave radiation
• Human activity is increasing the number of greenhouse gases in the atmosphere
• Carbon dioxide (CO2) levels in the atmosphere have increased by more than 100 parts per million (ppm) to 420ppm in 2020
• Increased amounts of greenhouse gases have led to the enhanced greenhouse effect
• Less long-wave radiation (heat) can escape the atmosphere
• Average global temperatures have increased over 1°C since pre-industrial times
• The average global temperature has risen remarkably in the past 100 years
• The greenhouse effect arises due to natural causes, but the enhanced greenhouse effect arises due to human activity
• Human sources of greenhouse gases

  • Burning of fossil fuels - power stations, vehicles
  • Burning of wood
  • Deforestation
  • Decay of organic matter - manure, waste in landfill, crops
  • Artificial fertilisers
• The enhanced greenhouse effect causes the average global temperature to increase and is the result of human activity
• Energy balance problems


  Considers how much incoming energy from the Sun is used and how much is returned to space
• If incoming and outgoing energy are in balance, the Earth's temperature will remain constant