Topic 9.3: The Greenhouse Effect

Objective

Not all solar radiation reaches Earth's surface:
- Reflection: 25% of incoming solar radiation is reflected back into space by the atmosphere and clouds.
- Absorption: 20% is absorbed by the atmosphere and clouds, then radiated back to space or Earth.
The remainder reaches Earth's surface and can either:
- Be absorbed: Converts to infrared radiation.
- Be reflected: Influenced by albedo.
- Low Albedo Surfaces: Absorb sunlight (e.g., oceans, soil, urban areas) and emit infrared radiation.
- High Albedo Surfaces: Reflect sunlight (e.g., snow, ice caps).

Gases in Earth's atmosphere trap heat and radiate it back to Earth.
Importance: Without the greenhouse effect, Earth would be too cold for life.
Mechanism:
- Solar radiation (UV and visible light) strikes Earth, converting to infrared radiation (IR).
- Earth emits IR, which greenhouse gases absorb and radiate back to Earth.
- Greenhouse gases include CO2, water vapor, methane, etc.

Carbon Dioxide (CO2):
- Major sources: Fossil fuel combustion, decomposition, deforestation.
- Serves as a benchmark for measuring other gases.
Methane (CH4):
- Sources: Natural gas extraction, anaerobic decomposition (bogs, permafrost).
Nitrous Oxide (N2O):
- Main source: Agricultural soil management (fertilizers).
CFCs: Also greenhouse gases.
Water Vapor:
- Absorbs IR but condenses and leaves the atmosphere with temperature changes, unlike other gases.

Global Warming Potential: Measure of a gas's ability to warm the climate over 100 years relative to CO2.
Factors:
- Residence Time: Duration a gas remains in the atmosphere.
- Infrared Absorption: Efficiency of absorbing and emitting IR.
GWP Comparisons:
- CO2: GWP = 1.
- Methane: GWP = 23-84 times CO2; stays in atmosphere ~12 years.
- Nitrous Oxide: GWP = 300; residence time ~115 years.
- CFCs: GWP = 1600-13,000; residence time 50-500 years.