Carbon dioxide is a greenhouse gas released by human activities such as burning fossil fuels. Explain how an increase in atmospheric carbon dioxide causes global temperatures to rise, and describe TWO ways in which this temperature rise could affect living organisms.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 3.3 Climate change · Explain · 5 marks · View as Markdown

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

Scientists have recorded a steady rise in atmospheric carbon dioxide concentrations since the Industrial Revolution. Global average temperatures have increased by approximately 1.2 °C since pre-industrial times, and this rise is predicted to continue.

Model answer (5 marks)

Carbon dioxide is a greenhouse gas that absorbs infrared radiation emitted by the Earth’s surface. The absorbed energy is re‑radiated in all directions, including back towards the surface, which increases the net energy retained by the planet and raises global temperatures.

Higher temperatures can:
1. Melt polar ice caps, raising sea levels and destroying the habitat of cold‑adapted species such as polar bears and penguins.
2. Shift species’ geographical ranges; organisms move to higher latitudes or altitudes to stay within their optimum temperature range, altering community composition and food webs.

Examiner tips

  • Show the greenhouse‑effect mechanism in two clear steps; use the terms ‘absorb’, ‘re‑radiate’ and ‘trapped energy’.
  • For the effects, give two distinct examples that link temperature rise to a biological consequence, citing habitat loss or range shift.
  • Use concise, exam‑style language and avoid unnecessary explanation.

Mark scheme (5 marks)

  1. Carbon dioxide (and other greenhouse gases) absorb/trap infrared radiation (heat) that would otherwise escape from Earth
  2. This trapped energy is re-radiated back towards Earth's surface, causing temperatures to rise
  3. Rising temperatures cause melting of ice caps / rising sea levels / loss of habitat for organisms that live in cold environments (e.g. polar bears / penguins)
  4. Rising temperatures cause a shift in the geographical range/distribution of species — organisms may migrate to higher latitudes or altitudes to remain within their optimum temperature range
  5. Rising temperatures affect the timing of seasonal behaviours (e.g. flowering, breeding, migration) which can disrupt food webs / predator–prey relationships

Key terms in this question

greenhouse gas

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