8.24 - The greenhouse effect
Follow energy from the Sun to Earth's surface and back through the atmosphere. Explain how carbon dioxide, methane and water vapour keep the surface warmer by absorbing and emitting infrared radiation.
Energy arrives and leaves
Energy from the Sun travels through space as electromagnetic radiation. Some passes through the atmosphere and is absorbed by Earth's surface, warming it. The surface also emits radiation, mainly infrared radiation at Earth's temperatures.
Incoming sunlight and outgoing infrared radiation are different parts of the energy story. Earth does not just reflect all the energy it receives: its warmed surface emits its own infrared radiation.
Greenhouse gases absorb some of this outgoing infrared radiation. Important examples are carbon dioxide (CO₂), methane (CH₄) and water vapour (H₂O). Nitrogen and oxygen, the main gases in air, do not have the same strong infrared-absorbing effect.
Absorption and emission keep Earth warmer
When greenhouse gas molecules absorb infrared radiation, they gain energy. Energy can be transferred between molecules, and greenhouse gases also emit infrared radiation in all directions. Some travels towards the surface, where it is absorbed; some travels towards space.
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