Explain how the enhanced greenhouse effect leads to an increase in the average surface temperature of the Earth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Human activities have increased the concentration of greenhouse gases such as carbon dioxide and methane in the Earth's atmosphere since the Industrial Revolution.
Model answer (4 marks)
Greenhouse gases absorb infrared radiation emitted by the Earth’s surface, preventing it from escaping directly to space.
The absorbed energy is re‑emitted in all directions, so part of it is sent back towards the surface.
When the concentration of greenhouse gases increases, a larger fraction of the surface’s infrared radiation is absorbed and re‑emitted, returning more energy to the surface – this is the enhanced greenhouse effect.
The surface therefore receives more energy than it radiates away, raising its temperature until a new, higher equilibrium temperature is reached.
The absorbed energy is re‑emitted in all directions, so part of it is sent back towards the surface.
When the concentration of greenhouse gases increases, a larger fraction of the surface’s infrared radiation is absorbed and re‑emitted, returning more energy to the surface – this is the enhanced greenhouse effect.
The surface therefore receives more energy than it radiates away, raising its temperature until a new, higher equilibrium temperature is reached.
Examiner tips
- Use the exact terms ‘infrared radiation’, ‘re‑emitted’, ‘enhanced greenhouse effect’, and ‘new equilibrium temperature’.
- Show the causal chain: absorption → re‑emission → extra energy to surface → temperature rise.
- Keep the answer concise – each point should be a short sentence or two.
Common mistakes
- Confusing visible light with infrared radiation; only IR is relevant for greenhouse gases.
- Failing to mention that the extra energy is returned to the surface, not just trapped in the atmosphere.
- Using vague phrases like ‘more heat’ instead of ‘more infrared radiation is absorbed and re‑emitted’.
Mark scheme (4 marks)
- Greenhouse gases absorb infrared radiation (emitted by the Earth's surface) rather than allowing it to escape to space.
- The absorbed energy is re-emitted in all directions, including back towards the Earth's surface.
- A higher concentration of greenhouse gases means more infrared radiation is absorbed and re-emitted, so more energy is returned to the surface (enhanced greenhouse effect).
- The surface therefore receives more energy than it radiates away, so its temperature rises until a new equilibrium is reached at a higher temperature.
Key terms in this question
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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