Explain how the enhanced greenhouse effect leads to an increase in global average temperatures.

IB DP Biology Standard Level (2023 syllabus) — D4.3 Climate change · Explain · 4 marks · View as Markdown

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

Atmospheric concentrations of greenhouse gases such as carbon dioxide and methane have increased significantly since industrialisation, altering Earth's energy balance.

Model answer (4 marks)

Greenhouse gases absorb the long‑wave radiation emitted by the Earth’s surface.

The absorbed energy is then re‑emitted in all directions, part of which is directed back toward the surface.

Human activities such as burning fossil fuels and deforestation increase the concentrations of these gases, strengthening the absorption and back‑radiation.

Consequently, more energy is retained in the atmosphere and on the surface, raising the global average temperature.

Examiner tips

  • Use the term "long‑wave (infrared) radiation" and "re‑emitted" to show understanding of the process. Mention the human sources (fossil fuels, deforestation) to link to the cause. Show the causal chain: absorption → back‑radiation → energy retention → temperature rise.
  • common_mistakes
  • :
  • Confusing short‑wave solar radiation with long‑wave infrared. Failing to mention the human activities that increase greenhouse gas levels. Using vague phrases like "more heat" without explaining the mechanism.

Mark scheme (4 marks)

  1. Greenhouse gases absorb outgoing long-wave (infrared) radiation from Earth's surface
  2. This absorbed energy is re-emitted in all directions, including back towards Earth's surface
  3. Increased concentrations of greenhouse gases due to human activities (e.g. combustion of fossil fuels, deforestation) enhance this effect
  4. The net result is that more energy is retained in the Earth's atmosphere/surface, raising global average temperatures

Key terms in this question

enhanced greenhouse effect · global average temperature

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