6.1C Greenhouse effect Notes

1. The Earth’s atmosphere contains a mixture of gases and particles that influence how much heat escapes into space.
2. Greenhouse gases (GHGs) and aerosols absorb and re-emit some of the infrared (long-wave) radiation emitted by the Earth’s warmed surface.
3. This prevents part of the outgoing radiation from leaving the planet and contributes to the warming of the atmosphere.
4. Important GHGs include carbon dioxide (CO₂), water vapour (H₂O), methane (CH₄) and nitrous oxides (N₂O).
5. A major warming aerosol is black carbon, which absorbs sunlight and enhances atmospheric warming.

Key Greenhouse Gases

1. Water Vapour (H₂O)

1. It is the most abundant greenhouse gas in the atmosphere.
2. It comes from evaporation, transpiration, and sublimation.
3. Its concentration increases as temperatures rise, forming a positive feedback loop:
   more warming leads to more evaporation, which leads to more water vapour, causing additional warming.
4. Despite its importance, water vapour is usually excluded from climate models because:
   1. its atmospheric lifetime is short,
   2. its concentration changes rapidly,
   3. and it cannot realistically be mitigated.

2. Carbon Dioxide (CO₂)

1. Carbon dioxide comes mainly from the combustion of fossil fuels, deforestation, and industrial processes such as cement production.
2. It is the most influential human-generated greenhouse gas because it remains in the atmosphere for centuries and exists in high concentrations.
3. Its current concentration (about 0.04 percent) is higher than at any point in at least 800,000 years.

3. Methane (CH₄)

1. Key sources include livestock digestion, rice paddies, landfills, and natural gas extraction.
2. Methane is over twenty times more effective at trapping heat than carbon dioxide over a 100-year period.
3. Its concentrations are lower than carbon dioxide, but it is highly potent and a significant contributor to warming.

4. Nitrous Oxides (N₂O)

1. Main sources include agricultural fertilisers, industrial processes, and fossil fuel combustion.
2. Molecule for molecule, nitrous oxide has a warming potential nearly 300 times stronger than carbon dioxide.

Aerosols: Tiny Particles with Big Impacts

Black Carbon (Soot)

1. Black carbon is produced from the incomplete combustion of:
   1. Fossil fuels
   2. Wood
   3. Biomass
2. It is a major component of PM2.5 and remains in the atmosphere for only days to weeks, but has substantial warming effects.

How Black Carbon Warms the Atmosphere

1. It absorbs sunlight, heating the surrounding air.
2. When deposited on snow or ice, it reduces albedo, causing surfaces to absorb more heat and melt faster.
3. It contributes to positive feedback by accelerating ice loss and further reducing albedo.

Radiative Forcing: Measuring the Impact