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UPSC Prelims 2025, Question 29

The concept being tested: Atmospheric insulation and greenhouse gas effect on Earth's temperature regulation.

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?

Options

  1. AI and III only
  2. BI and II only
  3. CI, II and IIICorrect answer
  4. DII and III only

The answer: I, II and III

The greenhouse effect explains why Earth's surface is habitable rather than frozen. Incoming solar radiation is mostly short-wave and passes through the atmosphere with little absorption, warming the surface; the surface then re-radiates long-wave infrared radiation outward. Greenhouse gases, chiefly carbon dioxide, water vapour and methane, are structurally suited to absorbing this long-wave radiation and re-emitting it back toward the surface, trapping heat that would otherwise escape to space. Calculations of Earth's radiative balance without an atmosphere put the average surface temperature near minus 18 degrees Celsius, well below freezing point everywhere, against the actual average of about plus 15 degrees Celsius. Since this trapped heat sets that average temperature, and carbon dioxide is specifically an efficient absorber of outgoing radiation, all three statements describe accurate, complementary parts of the same mechanism, so the correct combination is I, II and III.

What UPSC was testing

The trap in this question is treating a three-statement item as needing at least one false statement to eliminate; here all three are correct because they describe the same greenhouse mechanism from three angles, the no-atmosphere counterfactual, the resulting average temperature, and the specific gas responsible, so the safer strategy was to verify each statement independently rather than assume UPSC always plants a false one.

How often this comes up

This concept has appeared 11 times in UPSC Prelims General Studies Paper I between 2015 and 2025 — in 2018, 2019, 2022, 2024 and 2025.

The option-by-option autopsy for this question — why each of the other three is wrong, and which neighbouring concept each one was built from — is in the app.

More questions on this

Other past-year questions touching Albedo & Earth's Radiation Balance.

UPSC 2025 · Q26

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