Explain why increasing the concentration of greenhouse gases in the atmosphere leads to an increase in the equilibrium surface temperature of the Earth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The concentration of carbon dioxide in the Earth's atmosphere has increased significantly since the Industrial Revolution, and global mean surface temperatures have risen correspondingly.
Model answer (4 marks)
Greenhouse gases absorb the long‑wavelength infrared radiation emitted by the Earth’s surface.
The absorbed energy is re‑emitted isotropically, so a part of the re‑emitted radiation goes back towards the surface, adding to the energy received by the surface.
When the concentration of greenhouse gases rises, a larger fraction of the surface’s infrared radiation is absorbed and subsequently re‑emitted towards the surface, so the surface receives more power per unit area.
At the new equilibrium the surface must radiate this extra energy away. By the Stefan–Boltzmann law (P∝T⁴) the surface temperature must increase to radiate the higher power, giving a higher equilibrium surface temperature.
The absorbed energy is re‑emitted isotropically, so a part of the re‑emitted radiation goes back towards the surface, adding to the energy received by the surface.
When the concentration of greenhouse gases rises, a larger fraction of the surface’s infrared radiation is absorbed and subsequently re‑emitted towards the surface, so the surface receives more power per unit area.
At the new equilibrium the surface must radiate this extra energy away. By the Stefan–Boltzmann law (P∝T⁴) the surface temperature must increase to radiate the higher power, giving a higher equilibrium surface temperature.
Examiner tips
- Use the term ‘infrared (long‑wavelength) radiation’ and ‘re‑emitted in all directions’ to show understanding of the mechanism.
- Show the link between increased absorption → more energy to surface → higher radiated power → higher T⁴.
- Mention the Stefan–Boltzmann law explicitly to justify the temperature rise.
Common mistakes
- Confusing short‑wave solar radiation with long‑wave infrared radiation.
- Failing to state that the re‑emitted radiation is isotropic and includes a component back to the surface.
- Not linking the increased absorbed energy to the need for a higher surface temperature via the Stefan–Boltzmann law.
Mark scheme (4 marks)
- Greenhouse gases absorb infrared (long-wavelength / thermal) radiation emitted by the Earth's surface.
- The absorbed energy is re-emitted in all directions, including back towards the Earth's surface, increasing the energy input to the surface.
- A higher concentration of greenhouse gases means more infrared radiation is absorbed (and re-emitted towards the surface), so the surface receives more power per unit area.
- At the new equilibrium, the surface must radiate at a higher power (Stefan–Boltzmann law: power ∝ T⁴), so the equilibrium surface temperature is higher.
Key terms in this question
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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