What is 'Green House Effect'?

The correct answer is A. The greenhouse effect is the natural process by which atmospheric gases such as water vapour, carbon dioxide, methane and nitrous oxide absorb outgoing long-wave infrared radiation from the Earth's surface and re-radiate part of it back down. Without this natural effect, the average surface temperature would be far colder than today. The problem arises when human emissions from fossil fuels, deforestation and agriculture thicken this blanket, which is why we speak of an enhanced greenhouse effect and the resulting global warming.

Why the others fail:

• B is the opposite of what the effect does. Cooling is linked to factors like volcanic aerosols or reduced solar output, not to greenhouse gases.
• C and D describe localised effects. Changes in sea, river and lake temperatures are consequences of warming, not the definition of the phenomenon.

Core concept: radiative forcing and the greenhouse gas balance. For Mains GS-III (climate change, mitigation and adaptation, India's commitments under the Paris Agreement), this is the base concept to build on.

Trap: Students pick C or D because the question sounds like a water-body topic, or they confuse the greenhouse effect with ozone depletion, which is a separate problem.