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How the greenhouse effect warms the Earth

Chemistry of the atmosphere • Carbon dioxide and methane

Key concepts

What you'll likely be quizzed about

  • Shortwave radiation consists of visible and near-visible light from the Sun, characterized by high energy per photon.
  • The atmosphere is largely transparent to these wavelengths, allowing much solar energy to reach and warm the Earth's surface.
  • In contrast, longwave radiation is the infrared energy emitted by the warmed surface, which possesses lower energy per photon and interacts strongly with specific gas molecules.
  • This difference in behavior leads to surface warming as shortwave transparency allows energy to penetrate, while longwave absorption by gases helps trap heat in the atmosphere.

Flashcards

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What role does water vapour play in the greenhouse effect?

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Water vapour acts as a feedback mechanism; warmer air can hold more water vapour, which absorbs infrared radiation and further enhances warming.

Key notes

Important points to keep in mind

Greenhouse effect: transparency to shortwave + absorption of longwave by gases causes warming.

Shortwave = solar (visible); longwave = terrestrial (infrared).

CO2 and CH4 absorb infrared because of molecular vibrational/rotational modes.

Absorbed infrared is re-emitted in random directions or converted to heat by collisions.

Increased greenhouse gases reduce outgoing longwave radiation and raise equilibrium temperature.

Water vapour amplifies warming as a feedback mechanism, not the primary driver of warming.

Clouds can reflect shortwave (cooling) and absorb longwave (warming); net impact varies with type.

Gas radiative impact depends on absorption bands, concentration, and atmospheric lifetime.

Enhanced greenhouse effect refers to warming from increased human emissions.

Restoration of radiative balance requires warming until outgoing longwave equals incoming shortwave.

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