UPSC Prelims 2025 Q29: Greenhouse Effect and Role of Atmosphere
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?
- I and III only
- I and II only
- I, II and III
- II and III only
Answer: (c) I, II and III
Statement-wise explanation
- I: correct — Without an atmosphere there is no greenhouse effect. Earth's average surface temperature would then fall to about −18°C instead of today's 14–15°C (National Geographic, UCAR), well below 0°C, the freezing point of water. The wording is loose: the UCAR source says 'on average', and an airless sunlit surface can still get hot by day (the Moon's equator reaches about 120°C). The examiner meant the planet-wide average, and the key accepts the statement.
- II: correct — The atmosphere absorbs the long-wave (infrared) heat the surface gives off and sends part of it back down. UCAR states that the heat absorbed and trapped by the atmosphere keeps the average surface temperature at about 15°C. This natural greenhouse effect accounts for the gap of roughly 33°C between −18°C and 15°C.
- III: correct — Nitrogen and oxygen make up about 99% of dry air, and they barely absorb infrared radiation. Trace gases such as carbon dioxide, water vapour and methane absorb it strongly. Carbon dioxide absorbs strongly near 15 micrometres, a wavelength at which Earth gives off a lot of heat. UCAR uses almost the same words: some gases 'like carbon dioxide, are particularly good at absorbing and trapping radiation'.
How to eliminate
All three statements describe the basic greenhouse effect from NCERT Class XI geography, so knowing that concept solves the question. Statements II and III are textbook definitions: the atmosphere traps outgoing heat, and CO2 is a greenhouse gas. Once you accept both, only two options remain: the one with II and III only, and the one with all three. The question then turns on Statement I. The word 'everywhere' is absolute, and absolute words usually point to a false statement. Here, though, the statement restates the standard figure: without the atmosphere, Earth's mean temperature would be about −18°C instead of about 15°C. The fact to remember is −18°C versus +15°C, a natural greenhouse warming of about 33°C. Lesson: when a statement paraphrases a well-known textbook figure, judge the figure, not just the absolute word. This question's wording follows a UCAR explainer almost line for line. That explainer says 'on average' where UPSC wrote 'everywhere'.
Know more
How the atmosphere warms the Earth
The Sun sends energy as short waves (visible light). Clear air lets most of it through. The warmed ground then gives off heat as long waves (infrared): this is terrestrial radiation. Certain trace gases absorb these long waves and send part of the heat back down. This is the greenhouse effect, and it is the reason the atmosphere is heated mainly from below, by the ground, and not directly by the Sun.
NASA's average energy budget (top of atmosphere ≈ 340 W/m²):
| Fate of incoming sunlight | Share |
|---|---|
| Reflected back to space (clouds, particles, ice, snow): the planet's albedo | ~29% |
| Absorbed in the atmosphere (water vapour, dust, ozone) | ~23% |
| Absorbed by the surface | ~48% |
Over the long run, outgoing long-wave energy balances what is absorbed. This balance is the heat budget.
Which gases trap heat?
| Gas | Share of dry air | Absorbs infrared? |
|---|---|---|
| Nitrogen (N₂) | ~78% | Barely |
| Oxygen (O₂) | ~21% | Barely |
| Argon (Ar) | <1% | No |
| CO₂, CH₄, N₂O, water vapour | Traces | Strongly |
The main gases absorb almost no heat. A tiny fraction of the air does the warming. NASA notes that water vapour causes most of the natural effect, and CO₂, methane and other gases add to it.
Key numbers: without greenhouse warming, Earth's mean temperature would be about −18°C. With it, the mean is about 14–15°C. Bodies with little or no air show the contrast: the Moon swings from about 120°C by day to −130°C at night, because nothing holds or spreads the heat.
Natural vs enhanced effect: the natural effect keeps Earth habitable. Adding more greenhouse gases strengthens it and causes global warming.
NCERT Concepts to Revise
- Greenhouse effect: All three statements describe parts of this one process, from the −18°C baseline to heat trapping.
- Greenhouse gases: Statement III asks which atmospheric gases are good at absorbing and trapping radiation.
- Terrestrial radiation: The radiation trapped in Statements II and III is the Earth's own long-wave heat, not incoming sunlight.
- Heat budget of the Earth: Statement II's 'maintains average temperature' rests on the balance between energy received and energy lost.
- Albedo: Explains how much incoming sunlight is reflected away before any warming happens, which shapes the no-atmosphere temperature.