UPPSC 2020 Prelims GS Paper-I · Question 83 of 150

Correct — B, about –18°C. If Earth had an atmosphere with no greenhouse gases (or no atmosphere at all), the surface would be warmed only by absorbed sunlight and would radiate that energy straight back to space. Balancing incoming solar energy against outgoing long-wave radiation — for Earth's albedo (~0.3) and its distance from the Sun — gives an effective (equilibrium) temperature of roughly 255 K, which is about –18°C. Earth's actual global average surface temperature is about +15°C, so the natural greenhouse effect adds about 33°C of warming, which is what makes the planet habitable. The standard textbook figure for 'no greenhouse effect' is therefore –18°C.

• (a)0°C — 0°C is the freezing point of water, not Earth's no-greenhouse equilibrium temperature; the radiative-balance value is colder, about –18°C.
• (c)5°C — Too warm — 5°C is close to today's climate range, not the frozen equilibrium a greenhouse-free Earth would reach.
• (d)– 20°C — Close but not the standard figure; the accepted textbook value for Earth's effective temperature is about –18°C (255 K), not –20°C.

The greenhouse effect is the warming of a planet's surface because atmospheric gases (water vapour, CO2, methane, nitrous oxide, ozone) are largely transparent to incoming short-wave solar radiation but absorb and re-emit outgoing long-wave (infrared) radiation from the surface. This traps heat in the lower atmosphere. Earth's 'effective temperature' — the temperature it would have from radiative balance alone, without this trapping — is about 255 K (–18°C); the actual surface average is ~15°C.

This is a numerical-recall item resting on a single classic figure: no greenhouse effect ⇒ about –18°C. The trap options cluster around freezing (0°C, 5°C) or a near-miss (–20°C). Remember the two paired numbers — effective temperature –18°C and actual ~+15°C — and the ~33°C gap between them is the natural greenhouse contribution.

• Earth's effective (no-greenhouse) temperature ≈ 255 K ≈ –18°C
• Actual global average surface temperature ≈ 15°C
• Natural greenhouse effect adds ≈ 33°C of warming — essential for liquid water and life
• Main natural greenhouse gases: water vapour, CO2, methane (CH4), nitrous oxide (N2O), ozone (O3)

• Picking 0°C because it 'feels' like the freezing threshold — the radiative value is colder (–18°C)
• Confusing the effective temperature (–18°C) with the actual surface average (+15°C)

UPPSC/UPSC test the greenhouse mechanism both numerically (the –18°C vs +15°C figures) and conceptually (the atmosphere is warmed more by terrestrial long-wave radiation than by direct sunlight).

• practice — not a real PYQ

  The natural greenhouse effect raises Earth's average surface temperature by approximately:

  • (a)10°C
  • (b)18°C
  • (c)33°C
  • (d)50°C

  Answer(c) 33°C — the difference between the ~+15°C actual average and the ~–18°C effective temperature.

• practice — not a real PYQ

  The atmosphere is heated mainly by:

  • (a)Direct incoming short-wave solar radiation
  • (b)Outgoing long-wave terrestrial radiation
  • (c)Conduction from the Earth's core
  • (d)Latent heat of condensation only

  Answer(b) Outgoing long-wave terrestrial radiation — greenhouse gases absorb it, warming the lower atmosphere.