Explain why the Earth's surface temperature remains approximately constant despite the Earth continuously absorbing radiation from the Sun.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Earth absorbs solar radiation primarily in the visible and ultraviolet range. The atmosphere contains greenhouse gases such as carbon dioxide and water vapour.
Model answer (4 marks)
The Earth absorbs solar radiation in the visible/UV range.
1. It re‑emits the absorbed energy as infrared (thermal) radiation.
2. Greenhouse gases absorb part of this outgoing IR and re‑emit it in all directions, including back to the surface.
3. The surface receives this back‑radiation, so the net energy absorbed equals the energy radiated to space.
4. When this balance is reached, the surface temperature remains essentially constant.
1. It re‑emits the absorbed energy as infrared (thermal) radiation.
2. Greenhouse gases absorb part of this outgoing IR and re‑emit it in all directions, including back to the surface.
3. The surface receives this back‑radiation, so the net energy absorbed equals the energy radiated to space.
4. When this balance is reached, the surface temperature remains essentially constant.
Examiner tips
- Use the word ‘re‑emits’ and specify infrared; mention greenhouse gases re‑emit in all directions; state the energy balance; note the wavelength shift due to temperature difference.
Common mistakes
- Saying the surface emits visible light; confusing absorption with reflection; ignoring the role of greenhouse gases; not linking the balance to constant temperature.
Mark scheme (4 marks)
- The Earth re-emits radiation (as infrared / thermal radiation) back towards space.
- Greenhouse gases in the atmosphere absorb some of this outgoing infrared radiation and re-emit it in all directions, including back towards the surface.
- A balance (equilibrium) is reached where the rate of energy absorbed by the Earth equals the rate of energy emitted/radiated to space.
- The Earth's surface emits radiation at a longer wavelength (infrared) than the incoming solar radiation (visible/UV) because the surface is at a lower temperature than the Sun.
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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