UPSC Prelims 2025 Q29: Environment & Ecology – Climate Change & Greenhouse Gases
Consider the following statements:
I. Without the atmosphere, temperature would be well below freezing point everywhere on the
Earth's surface.
II. Heat absorbed and trapped by the atmosphere maintains our planet's average temperature.
III. Atmosphere's gases, like carbon dioxide, are particularly good at absorbing and trapping
radiation.
Which of the statements given above are correct?
- (a) I and III only
- (b) I and II only
- (c) I, II and III
- (d) II and III only
Answer: (c) I, II and III
Explanation
The atmosphere and Earth's temperature — the greenhouse effect
| Statement | Verdict | Explanation |
|---|---|---|
| I. Without atmosphere, well below freezing | Correct | Without greenhouse gases the average would be about −18°C instead of about +15°C |
| II. Trapped heat keeps the average temperature | Correct | The atmosphere absorbs outgoing terrestrial (long-wave) radiation and re-radiates part of it back |
| III. Gases like CO₂ absorb and trap radiation | Correct | CO₂, water vapour, methane, N₂O absorb infrared strongly |
How it works:
| Radiation | Wavelength | Atmosphere's response |
|---|---|---|
| Incoming solar | Short-wave | Mostly passes through |
| Outgoing terrestrial | Long-wave (infrared) | Largely absorbed by greenhouse gases |
The natural greenhouse effect makes Earth habitable; extra greenhouse gases cause global warming.
Answer: (c) I, II and III.
Why each option is right or wrong
- (a) Incorrect. It leaves out II, yet trapped heat is precisely what maintains Earth's average of about 15°C.
- (b) Incorrect. It leaves out III; CO₂ and other greenhouse gases are strong absorbers of infrared radiation.
- (c) CORRECT. All three describe the natural greenhouse effect: without it Earth would be about −18°C.
- (d) Incorrect. It leaves out I, but without the atmosphere Earth's surface would be well below freezing.
Key point
Greenhouse effect raises Earth's average from about −18°C to about +15°C; CO₂ and water vapour trap long-wave radiation.
Remember
Without the greenhouse effect, Earth would average about −18°C.
Common mistakes
- Treating the greenhouse effect as purely harmful; the natural effect is what keeps Earth habitable.
- Thinking greenhouse gases block incoming sunlight; they mainly trap outgoing long-wave radiation.
- Forgetting water vapour is the most abundant greenhouse gas.
Exam tip
Short-wave in, long-wave out. Greenhouse gases are transparent to the first and absorb the second.
Sources
- NCERT Class XI — Fundamentals of Physical Geography, Ch 9 (Heat Budget)
- IPCC AR6 — Working Group I, Summary for Policymakers