UPSC Prelims 2012 Q62: Environment & Ecology – Climate Change & Greenhouse Gases
The increasing amount of carbon dioxide in the air is slowly raising the temperature of the
atmosphere, because it absorbs
- (a) the water vapour of the air and retains its heat
- (b) the ultraviolet part of the solar radiation
- (c) all the solar radiations
- (d) the infrared part of the solar radiation
Answer: (d) the infrared part of the solar radiation
Explanation
Why carbon dioxide warms the atmosphere
Sunlight reaches the ground as short-wave radiation.
The warm Earth sends heat back as long-wave infrared radiation.
Carbon dioxide lets the short waves pass but absorbs the infrared, so heat stays in the lower atmosphere.
This is the greenhouse effect.
| Option | Verdict | Reason |
|---|---|---|
| (a) the water vapour of the air and retains its heat | Incorrect | Carbon dioxide does not absorb water vapour |
| (b) the ultraviolet part of the solar radiation | Incorrect | Ozone absorbs ultraviolet rays |
| (c) all the solar radiations | Incorrect | It is transparent to visible light |
| (d) the infrared part of the solar radiation | Correct | It absorbs infrared (heat) radiation |
Greenhouse gases:
| Gas | Main source |
|---|---|
| Carbon dioxide | Burning of fossil fuels |
| Methane | Paddy fields, cattle, landfills |
| Nitrous oxide | Fertilizers |
| Water vapour | Most abundant natural greenhouse gas |
Answer: (d) the infrared part of the solar radiation.
Why each option is right or wrong
- (a) Incorrect. Carbon dioxide does not absorb water vapour; both are greenhouse gases.
- (b) Incorrect. The ultraviolet part is absorbed by ozone in the stratosphere.
- (c) Incorrect. Carbon dioxide does not absorb all the solar radiations; visible light passes through.
- (d) CORRECT. Carbon dioxide absorbs the infrared part and so traps heat.
Key point
Greenhouse effect: short-wave sunlight comes in, long-wave infrared goes out and is absorbed by carbon dioxide, methane, nitrous oxide and water vapour. Ozone absorbs ultraviolet.
Remember
Carbon dioxide traps infrared heat.
Common mistakes
- Confusing the greenhouse effect (infrared) with ozone depletion (ultraviolet).
- Thinking carbon dioxide blocks incoming sunlight.
- Forgetting that the trapped heat is mostly the Earth's own outgoing radiation.
Exam tip
Ozone and ultraviolet go together; carbon dioxide and infrared go together.
Sources
- NCERT Class XI — Fundamentals of Physical Geography, Ch 9 (Solar Radiation, Heat Balance and Temperature)
- NCERT Class XII — Biology, Ch 16 (Environmental Issues: Greenhouse Effect and Global Warming)