Carbon dioxide, Methane and Nitrous oxide are considered greenhouse gases.

Question

Consider the following statements:

Assertion (A): Carbon dioxide, Methane and Nitrous oxide are considered greenhouse gases.

Reason (R): Greenhouse gas absorbs short-wave radiation from the sun and is transparent to long-wave radiation from Earth. Choose the correct answer:

1. A. A and R are both correct and R is the correct explanation of A
2. B. A and R are both correct and R is NOT the correct explanation of A
3. C. A is correct, but R is NOT correct (Correct answer)
4. D. A is NOT correct, but R is correct

Correct Answer

Option C — A is correct, but R is NOT correct

Detailed Solution & Explanation

The correct answer is A is correct, but R is NOT correct.

Key Points

• The Assertion is correct. Carbon dioxide, methane and nitrous oxide are indeed greenhouse gases.
• The Reason is incorrect, and it states the mechanism backwards. Greenhouse gases are largely transparent to the incoming short-wave solar radiation, and absorb the outgoing long-wave (infrared) radiation emitted by the Earth's surface. The Reason claims the opposite.
• Since A is correct and R is not, the answer is C.

Additional Information

• How the greenhouse effect actually works: the Sun, being very hot, radiates mostly at short wavelengths, which pass through the atmosphere and warm the surface. The Earth, being far cooler, re-radiates that energy at long infrared wavelengths. Greenhouse gases absorb this outgoing infrared and re-emit it in all directions, including downward, warming the lower atmosphere and surface.
• Without the natural greenhouse effect the Earth's mean surface temperature would be about −18 °C instead of about 15 °C. The problem is not the greenhouse effect itself but its enhancement by rising concentrations of these gases.
• Why these gases and not others: absorption of infrared requires a change in dipole moment during molecular vibration. Nitrogen and oxygen, which make up 99% of the atmosphere, are symmetric diatomic molecules and are effectively transparent to infrared, which is why they are not greenhouse gases.
• Water vapour is the most abundant greenhouse gas by contribution, but it is treated as a feedback rather than a forcing, since its atmospheric concentration is governed by temperature rather than by direct emissions.
• Global Warming Potential (GWP) over 100 years, with CO₂ as the baseline of 1: methane about 28-30, nitrous oxide about 265-273, and the fluorinated gases — HFCs, PFCs, SF₆ and NF₃ — in the thousands to tens of thousands. SF₆ is the most potent, at over 23,000.
• The Kyoto Protocol basket of greenhouse gases comprises CO₂, CH₄, N₂O, HFCs, PFCs and SF₆, with NF₃ added in the Doha Amendment.