UPSC CSE Prelims
Greenhouse Effect Previous Year Questions (PYQs)

Detailed Explanation:

Answer: Option 4 — 1, 2 and 3

All three statements about methane hydrate deposits are correct. Methane hydrates are ice-like structures that trap methane and are vulnerable to release under warming conditions, creating a potential climate feedback loop.

✅ Statement 1 – Correct: Global warming can destabilize methane hydrate deposits in permafrost and ocean sediments, triggering methane release—a potent greenhouse gas that amplifies warming.
✅ Statement 2 – Correct: Large methane hydrate reserves are found in Arctic tundra permafrost and in deep-sea sediments along continental margins.
✅ Statement 3 – Correct: Methane in the atmosphere oxidizes to carbon dioxide through hydroxyl radical reactions after approximately 12 years, though CO₂ persists much longer in the atmosphere.

📝 Short Notes: Methane Hydrates

• Structure: Ice-like crystalline solids where methane molecules are trapped within water molecule cages, stable under high pressure and low temperature.
• Major Locations: Arctic permafrost regions and seafloor sediments on continental margins (300-500m depth), particularly offshore India (Krishna-Godavari basin).
• Climate Risk: Methane is 25-30 times more potent as a greenhouse gas than CO₂ over a 100-year period; release creates positive feedback loop.
• Oxidation Process: Atmospheric methane oxidizes via hydroxyl radicals (OH) to CO₂ with an average lifetime of ~12 years.
• Potential Resource: Estimated global reserves exceed conventional natural gas; India conducted successful extraction trial in 2019.
• Destabilization Triggers: Rising ocean temperatures, pressure changes, and permafrost thawing can release stored methane.

Detailed Explanation:

Answer: Option 1 — Increased plant growth due to increased concentration of carbon dioxide in the atmosphere

Carbon fertilization refers to the phenomenon where elevated atmospheric CO₂ concentrations enhance the rate of photosynthesis in plants, leading to increased plant growth and biomass production. Since CO₂ is a key substrate for photosynthesis, higher concentrations allow plants to synthesize more sugars (glucose), thereby accelerating growth rates. This effect is distinct from global warming (temperature increase), ocean acidification, or adaptive responses to climate change.

📝 Short Notes: Carbon Fertilization and Plant Physiology

• Carbon Fertilization: The enhancement of photosynthetic rates and plant growth due to elevated atmospheric CO₂ levels.
• Photosynthesis Basics: Plants absorb CO₂, water, and sunlight to produce glucose (C₆H₁₂O₆) and release oxygen (O₂) as a byproduct.
• Current CO₂ Levels: Atmospheric CO₂ concentration has reached approximately 420 ppm (as of recent measurements), the highest in millions of years, primarily due to fossil fuel combustion and deforestation.
• Growth Response: Many C3 plants (wheat, rice, soybeans) show significant growth enhancement with increased CO₂, though C4 plants (maize, sugarcane) show less response.
• Limitations: Carbon fertilization benefits are limited by other factors like nutrient availability (nitrogen, phosphorus), water stress, and temperature extremes.
• Ecosystem Impacts: While increased plant growth may sequester more carbon, this effect is offset by increased respiration, decomposition, and other climate change impacts.

Detailed Explanation:

Carbon dioxide (CO₂) and other greenhouse gases absorb infrared radiation (heat) emitted by Earth's surface after it absorbs solar energy.

This trapped infrared radiation causes the greenhouse effect, gradually raising atmospheric temperature—not UV or all solar radiation, but specifically the long-wave infrared re-radiated from Earth.