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UPSC Prelims 2025 · Environment · Nature’s Machinery
Consider the following statements: Which of the statements given above are correct?
- Without the atmosphere, temperature would be well below freezing point everywhere on the Earth’s surface.
- Heat absorbed and trapped by the atmosphere maintains our planet’s average temperature.
- Atmosphere’s gases, like carbon dioxide, are particularly good at absorbing and trapping radiation.
- (a) I and III only
- (b) I and II only
- (c) I, II and III
- (d) II and III only
Show the answer and explanation
Answer: (c) I, II and III
All three describe the natural greenhouse effect.
Statement by statement
1. ✓ CORRECT Without the greenhouse effect Earth’s average would be about −18 °C instead of +15 °C.
2. ✓ CORRECT The atmosphere absorbs outgoing infrared and re-radiates part of it down — the natural greenhouse effect.
3. ✓ CORRECT CO₂, water vapour and methane absorb infrared radiation strongly.
The idea
The atmosphere works like a blanket. Sunlight warms the ground; the ground gives off heat; gases in the air absorb part of that heat and send it back down. Without this natural greenhouse effect the Earth would average about −18°C instead of about 15°C.
Sunlight reaches the ground → Ground gives off heat (infrared) → Carbon dioxide and water vapour absorb it → Part is sent back down → Average temperature stays near 15°C
The trap
“Everywhere” in statement I sounds absolute, but without air even the tropics would freeze at night and average below zero.
Also know
The Moon shows what happens without air: at the same distance from the Sun, its surface is about 120°C by day and far below −100°C at night. Venus shows the opposite extreme: a thick carbon dioxide atmosphere and a surface hot enough to melt lead.
🔒 Natural greenhouse effect: −18 °C → +15 °C (≈ +33 °C).
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