UPSC Prelims 2025 Q31: Environment & Ecology – Climate Change & Greenhouse Gases
- (a) Both Statement II and Statement III are correct and both of them explain Statement I
- (b) Both Statement II and Statement III are correct but only one of them explains Statement I
- (c) Only one of the Statements II and III is correct and that explains Statement I
- (d) Neither Statement II nor Statement III is correct
Answer: (b) Both Statement II and Statement III are correct but only one of them explains Statement I
Explanation
Statement-by-Statement Breakdown:
| Statement | Claim | Verdict |
|---|---|---|
| Statement I | CO₂ emissions from cement industry account for a significant share of global CO₂ | TRUE - cement produces ~8% of global CO₂ emissions (frequently discussed figure) |
| Statement II | CO₂ has strong infrared radiation absorption properties | TRUE - CO₂ absorbs IR in specific bands (particularly ~15 micrometres), making it an effective greenhouse gas |
| Statement III | Water vapour is the most abundant greenhouse gas in the atmosphere | TRUE - water vapour contributes ~50% of the natural greenhouse effect |
Why only Statement II explains Statement I:
CO₂'s IR absorption (II) is the direct physical mechanism that causes it to trap heat and explains why its emissions are significant (Statement I).
Water vapour (III) is a feedback amplifier — it responds to warming caused by CO₂, it doesn't explain CO₂'s direct role as a forcing agent.
Answer: (b) Both Statement II and Statement III are correct but only one of them explains Statement I.
Why each option is right or wrong
- (a) Incorrect. Both II and III cannot explain Statement I together. Statement III (water vapour) does not explain why CO₂ emissions from cement are significant — it's a separate phenomenon.
- (b) CORRECT. Both Statements II and III are correct, but only Statement II (CO₂'s IR absorption) directly explains Statement I. Water vapour (III) is a feedback amplifier, not the reason CO₂ emissions cause warming.
- (c) Incorrect. Both II and III are correct — you cannot say only one of them is correct.
- (d) Incorrect. Both II and III are factually correct statements about greenhouse gases.
Key point
CO₂ = direct forcing agent (absorbs IR, explains Statement I). Water vapour = most abundant GHG but a feedback (amplifies warming, does not explain CO₂'s role).
Remember
CO₂ drives greenhouse forcing. Water vapor is the most abundant GHG but a feedback amplifier, not the initial driver.
Common mistakes
- Thinking water vapour explains Statement I. Water vapour is a feedback — it increases BECAUSE temperatures rise (due to CO₂), it doesn't independently explain why CO₂ emissions are problematic.
- Not knowing the distinction between forcing agents (CO₂, CH₄ — cause initial warming) and feedback amplifiers (H₂O vapour — respond to and amplify warming).
- Forgetting that cement production generates CO₂ through calcination (CaCO₃ → CaO + CO₂) — a chemical process, not just energy combustion.
Exam tip
GHG hierarchy: CO₂ = forcing agent (causes warming directly). H₂O vapour = most abundant but a FEEDBACK (amplifies, doesn't initiate). Cement CO₂ = calcination + combustion. ~8% of global CO₂.
Sources
- NCERT Class XI/XII - Environment & Ecology (Climate Change chapter)
- IPCC AR6 WGI Chapter 6: Radiative Forcing
- UPSC CSE Syllabus - Climate Change & Greenhouse Gases