B.2.7—Greenhouse gases

Main gases

The syllabus identifies water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) as the main greenhouse gases.

Natural and human origins

Each of these gases has natural sources and sources affected by human activity. For example, water vapour participates in the natural water cycle, while combustion, agriculture and land-use changes can alter atmospheric concentrations of several greenhouse gases.

Abundance is not the only factor

A gas’s contribution depends on both its atmospheric abundance and how strongly it absorbs infrared radiation. Do not rank gases using concentration alone.

Common trap

O₂, N₂ and Ar are not treated as the main greenhouse gases in this syllabus objective. The question may test recognition of the listed gases, not whether a molecule is simply present in the atmosphere.

How it is assessed

The evidence repeats a multiple-choice identification question asking which listed gas is not considered a greenhouse gas.

Command terms

State / Identify

What earns marks

Recognize H₂O, CO₂, CH₄ and N₂O as the main greenhouse gases in this syllabus. For a recognition question, reject O₂; do not infer greenhouse effect from atmospheric abundance alone.

Watch for

Selecting a listed greenhouse gas as the non-greenhouse gas, or treating natural origin as evidence that a gas cannot contribute to the greenhouse effect.

Representative question

[Maximum number: 1]

Which of the following is not considered to be a greenhouse gas?

$\mathrm{N}_{2} \mathrm{O}$

$\mathrm{H}_{2} \mathrm{O}$

$\mathrm{O}_{2}$

$\mathrm{CH}_{4}$

B.2.7—Greenhouse gases

Identify the Main Greenhouse Gases

B.2.7 Exam Analysis

Assessment in practice

Question 1