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

1. Radiative forcing is a measure of the balance between incoming solar radiation and outgoing long-wave radiation.
2. A positive radiative forcing indicates that Earth’s system is gaining more energy than it loses.
3. GHGs and some aerosols increase radiative forcing, contributing to warming.
4. Other aerosols, such as sulphates, may increase cloud reflectivity and contribute to cooling, sometimes producing regional effects such as global dimming.

Greenhouse effect

1. The greenhouse effect is a natural and essential process that keeps the Earth warm enough to sustain life.
2. The Sun emits radiation across a broad spectrum, including visible light, ultraviolet radiation, and infrared radiation.
3. Most of the Sun’s short-wave radiation passes through the atmosphere and warms the Earth’s surface.
4. The warmed surface then emits long-wave infrared radiation back toward the atmosphere.
5. Greenhouse gases absorb this long-wave radiation and re-emit it, keeping much of the heat within the Earth’s system.

The Enhanced Greenhouse Effect

1. Human activities have increased the concentration of greenhouse gases in the atmosphere.
2. This enhances the natural greenhouse effect, causing additional trapping of heat.
3. Major contributors include:
   1. Fossil fuel combustion.
   2. Deforestation.
   3. Agriculture and livestock production.
   4. Industrial processes.
4. The enhanced greenhouse effect leads to global warming, which is one component of broader climate change.

How the Greenhouse Effect Works

1. The Sun emits solar radiation, consisting mainly of visible and ultraviolet light.
2. This radiation passes through the atmosphere and warms the Earth’s surface.
3. The Earth re-emits the absorbed energy as infrared radiation.
4. Some of this infrared radiation escapes directly into space.
5. A significant portion is absorbed by greenhouse gases.
6. These gases re-emit infrared radiation in all directions.
7. Part of this energy returns to the Earth’s surface, warming the lower atmosphere.

From Global Warming to Climate Change

1. Global warming describes the rise in the average global temperature.
2. Climate change refers to a much broader range of long-term changes, including:
   1. shifting weather patterns,
   2. rising sea levels,
   3. melting ice sheets and glaciers,
   4. ecosystem disruption,
   5. and changes in global precipitation patterns.