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 ✓ UPSC's answer
- (d) II and III only
Why the answer is (c)
• The natural greenhouse effect keeps the Earth's mean surface temperature at about 15 °C; without an atmosphere it would be roughly −18 °C, well below freezing — statement I is correct.
• The atmosphere lets in short-wave solar radiation, absorbs the long-wave (infrared) radiation re-emitted by the surface and radiates part of it back down; this trapped heat maintains the planet's average temperature — statement II is correct.
• Greenhouse gases such as carbon dioxide, water vapour, methane and nitrous oxide are especially effective at absorbing infrared radiation because of their molecular structure — statement III is correct.
• Human additions of these gases enhance the effect and drive global warming.
• Hence I, II and III, option (c).
Why the other options are wrong
- (a) I and III only
- Statement II on trapped heat maintaining average temperature is also correct.
- (b) I and II only
- Statement III on CO2 absorbing radiation is also correct.
- (d) II and III only
- Statement I is also correct: without an atmosphere the Earth would be below freezing.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2025, held on 25 May 2025. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.