Explain why increasing the concentration of greenhouse gases in the atmosphere causes the effective radiating level of the Earth to rise to a higher altitude, and explain how this leads to an increase in the Earth's average surface temperature.

IB DP Physics Standard Level (2023 syllabus) — B.2 Greenhouse effect · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The effective radiating level is the average altitude from which infrared radiation escapes to space. As greenhouse gas concentrations increase, this level rises to a higher, cooler altitude.

Model answer (4 marks)

Increasing the concentration of greenhouse gases increases the opacity of the atmosphere to infrared radiation. The atmosphere therefore becomes transparent to outgoing IR only at a higher altitude, where the optical depth is unity. This higher altitude is cooler, so the effective radiating level is at a lower temperature. According to the Stefan–Boltzmann law the power radiated to space is proportional to T⁴; a cooler radiating level means less energy is lost to space. The Earth then receives more energy from the Sun than it radiates, creating a net energy gain. The atmosphere–surface system warms until the temperature at the new, higher radiating level rises enough to restore the balance between incoming solar and outgoing IR radiation, giving a higher average surface temperature.

Examiner tips

  • Use the term ‘effective radiating level’ and explain its altitude change; link opacity to optical depth; mention Stefan–Boltzmann law; show the energy imbalance and feedback to surface temperature.
  • Show the causal chain: higher GHG → higher opacity → higher radiating level → cooler level → less IR to space → energy surplus → warming.
  • Include the word ‘optical depth’ or ‘optical thickness’ to demonstrate understanding of absorption over a path length.

Common mistakes

  • Confusing the radiating level with the surface temperature; writing that the radiating level becomes hotter rather than cooler.
  • Failing to mention the Stefan–Boltzmann law or the energy imbalance; using vague terms like ‘more radiation’ without explaining the mechanism.

Mark scheme (4 marks)

  1. A greater concentration of greenhouse gas molecules means the atmosphere absorbs infrared radiation more strongly / over a shorter path length, so the altitude at which the atmosphere becomes transparent to outgoing infrared radiation increases.
  2. At a higher altitude the temperature of the atmosphere is lower, so the effective radiating level is at a lower temperature.
  3. Because the radiating level is cooler, the power radiated to space (Stefan–Boltzmann law / P ∝ T⁴) is reduced, so the Earth radiates less energy to space than it receives from the Sun — creating an energy imbalance / net energy gain.
  4. This net energy surplus causes the whole atmosphere–surface system to warm until the radiating level's temperature is restored to a value that once again balances incoming and outgoing radiation, resulting in a higher average surface temperature.

Key terms in this question

effective radiating level · greenhouse gas

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