UPSC Prelims 2011 Q21: Environment & Ecology – Biogeochemical Cycles & Earth Systems
Human activities in the recent past have caused the increased concentration of carbon
dioxide in the atmosphere, but a lot of it does not remain in the lower atmosphere because of
1. Its escape into the outer stratosphere,
2. The photosynthesis by phytoplankton in the oceans,
3. The trapping of air in the polar ice caps.
Which of the statements given above is/are correct?
- (a) 1 and 2
- (b) 2 only
- (c) 2 and 3
- (d) 3 only
Answer: (b) 2 only
Explanation
Where does extra atmospheric CO₂ go?
Human activity keeps adding CO₂, but the lower atmosphere's concentration rises less than expected because natural sinks absorb much of it.
| Statement | Verdict | Reason |
|---|---|---|
| 1. Escape into the outer stratosphere | Incorrect | CO₂ does not escape the atmosphere into space; it stays within Earth's gravitational envelope |
| 2. Photosynthesis by phytoplankton in the oceans | Correct | Phytoplankton absorb CO₂ for photosynthesis, acting as a major carbon sink; this is part of the ocean's biological pump |
| 3. Trapping of air in polar ice caps | Incorrect | Air bubbles trapped in ice are a record of past atmosphere (useful for ice-core studies), not an active ongoing sink removing today's CO₂ |
Major natural CO₂ sinks:
| Sink | Mechanism |
|---|---|
| Oceans (phytoplankton + dissolution) | Absorb roughly a quarter of annual human CO₂ emissions |
| Forests and soil | Terrestrial photosynthesis and carbon storage |
Answer: (b) 2 only.
Why each option is right or wrong
- (a) Incorrect. Statement 1 is false; CO₂ does not escape into the outer stratosphere or space.
- (b) CORRECT. Only phytoplankton photosynthesis among the three is a genuine, correct mechanism.
- (c) Incorrect. Statement 3 is false; polar ice traps old air as a record, not as an active ongoing sink.
- (d) Incorrect. Statement 3 alone is wrong; it is statement 2 that correctly explains the missing CO₂.
Key point
Excess atmospheric CO₂ is absorbed mainly by oceanic phytoplankton photosynthesis (and dissolution), NOT by escape to space or by polar ice trapping (which only records past air, it doesn't remove present CO₂).
Remember
Missing CO₂ is mainly absorbed by ocean phytoplankton photosynthesis.
Common mistakes
- Thinking excess gas simply 'escapes' upward out of the atmosphere.
- Confusing ice-core air bubbles (a record of the past) with an active carbon sink today.
- Underrating the ocean's biological pump as a major CO₂ absorber.
Exam tip
Only living processes (like phytoplankton photosynthesis) actively remove today's CO₂. Physical trapping or escape are distractors.
Sources
- NASA Earth Observatory — The Carbon Cycle
- Intergovernmental Panel on Climate Change — Climate Change 2007: The Physical Science Basis, Chapter 7 (Couplings Between Changes in the Climate System and Biogeochemistry)