Methane and carbon dioxide are both greenhouse gases. Explain how an increase in the concentration of greenhouse gases in the atmosphere leads to climate change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists have recorded a steady rise in atmospheric methane levels over the past century, largely due to livestock farming, landfill sites and the extraction of fossil fuels. Methane is considered to be a more potent greenhouse gas than carbon dioxide.
Model answer (5 marks)
Greenhouse gases allow short‑wave solar radiation to reach the surface. The surface absorbs this energy and re‑emits it as long‑wave infrared radiation. Greenhouse gases absorb the outgoing infrared radiation instead of letting it escape into space. The absorbed energy is then re‑emitted in all directions, including back towards the Earth, which warms the surface and the lower atmosphere. When the concentration of greenhouse gases increases, more infrared radiation is absorbed and retained, raising global temperatures and driving climate change.
Examiner tips
- Use the exact sequence: solar → surface absorption → infrared emission → absorption by GHGs → back‑radiation → warming
- Mention that higher concentrations mean more infrared is trapped, leading to temperature rise
Common mistakes
- Confusing short‑wave with long‑wave radiation
- Saying GHGs allow radiation to escape instead of trapping it
- Omitting the step that absorbed energy is re‑emitted back towards Earth
Mark scheme (5 marks)
- Greenhouse gases allow short-wave solar radiation / sunlight to pass through the atmosphere and reach Earth's surface
- Earth's surface absorbs this radiation and re-emits it as long-wave infrared / heat radiation
- Greenhouse gases absorb this outgoing infrared / heat radiation (rather than allowing it to escape into space)
- The absorbed energy is re-emitted in all directions, including back towards Earth, warming the surface / atmosphere
- Higher concentrations of greenhouse gases mean more infrared radiation is absorbed / retained, so global temperatures rise further, causing climate change
Key terms in this question
greenhouse gases · climate change · concentration
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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