Explain how methane and carbon dioxide act as greenhouse gases to cause global warming.

Pearson Edexcel International GCSE Biology (4BI1) — 4.4 Human influences on the environment · Explain · 4 marks · View as Markdown

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

Cattle farming and the burning of fossil fuels release large quantities of methane and carbon dioxide into the atmosphere. Scientists have recorded a steady rise in average global temperatures over the past century.

Model answer (4 marks)

Methane and carbon dioxide are greenhouse gases that absorb thermal (infra‑red) radiation emitted by the Earth’s surface and re‑radiate it back towards the ground.

When their atmospheric concentrations rise, a larger proportion of the outgoing heat is trapped, so less energy escapes to space.

The increased heat retention causes the mean surface temperature of the planet to rise – this is global warming.

A consequence of this warming is the melting of polar ice caps, which in turn raises sea levels and can lead to more extreme weather events.

Examiner tips

Use the term ‘greenhouse gases’ and mention ‘thermal/infra‑red radiation’ to gain full marks.
Show the causal chain: gas → heat trapping → temperature rise → consequence.
Include a specific consequence such as ‘melting polar ice caps’ to demonstrate understanding.

Common mistakes

Confusing greenhouse gases with other pollutants (e.g. sulphur dioxide).
Failing to mention that the gases re‑radiate heat back to Earth rather than just trapping it.
Giving a consequence that is not directly linked to temperature rise, such as ‘air pollution’.

Mark scheme (4 marks)

Methane and carbon dioxide are greenhouse gases that trap heat (thermal/infrared radiation) in the Earth's atmosphere
As levels of carbon dioxide and methane in the atmosphere increase, more heat is trapped
This leads to a gradual rise in the average temperature of the Earth (global warming)
Named consequence of global warming — e.g. polar ice caps melt / sea levels rise / more extreme weather events / changes in species distribution

Key terms in this question

greenhouse gases · global warming · methane · carbon dioxide