APES 9.3 The Greenhouse Effect

Solar Radiation Overview

• Not all incoming solar radiation reaches Earth's surface.

• Diagram demonstrating possibilities of solar radiation:

  • About 25% is reflected back into space by atmosphere and clouds.

  • Around 20% is absorbed by atmosphere/clouds, momentarily stored, then re-radiated.

  • Only the remaining energy reaches the Earth's surface.

• On reaching Earth’s surface:

  • Can either be absorbed or reflected.

  • Absorbed radiation is emitted as infrared radiation.

• Importance of Albedo:

  • Low albedo surfaces (e.g., deep ocean, blacktop) absorb sunlight and emit infrared radiation.

  • High albedo surfaces (e.g., snow, ice) reflect sunlight back into space.

The Greenhouse Effect

• Definition: Greenhouse gases trap heat and radiate it back to Earth, keeping the planet warm enough to support life.

• Key Process:

  • Solar rays (UV and visible light) strike Earth, converted to infrared radiation.

  • Infrared radiation is absorbed by greenhouse gases (e.g., CO2, methane, etc.) and re-radiated towards Earth.

  • Without the greenhouse effect, Earth would be too cold for life.

Greenhouse Gases and Their Sources

• Carbon Dioxide (CO2):

  • Most critical greenhouse gas.

  • Major sources:

    • Fossil fuel combustion.

    • Decomposition of organic matter.

    • Deforestation reduces carbon sequestration.

• Methane (CH4):

  • Sources include:

    • Natural gas extraction and combustion.

    • Anaerobic decomposition in wetlands and permafrost melting.

• Nitrous Oxide (N2O):

  • Main source: Management of agricultural soils, especially from fertilization.

  • Nitrifying bacteria convert excess nitrates into nitrous oxide.

• CFCs (Chlorofluorocarbons):

  • A class of compounds including greenhouse gases, with various sources.

• Water Vapor:

  • Though a greenhouse gas, it is not a major concern; its presence in the atmosphere is temperature dependent.

  • Unlike carbon dioxide and others, water vapor condenses when temperatures drop, thus leaving the atmosphere.

Potency of Greenhouse Gases (Global Warming Potential)

• Global Warming Potential (GWP):

  • Measure of a gas's contribution to warming the climate over a 100-year period, relative to CO2 (GWP = 1).

• Methane (CH4):

  • GWP ranges from 23 to 84 times that of CO2.

  • Residence time: Approximately 12 years.

• Nitrous Oxide (N2O):

  • GWP of 300.

  • Long residence time: Approximately 115 years.

• CFCs:

  • GWP from 1,600 to 13,000.

  • Moiety-dependent with residence times from 50 to 500 years.