Q.With which of the following, the 'Agenda 21' of Rio Summit, 1992 is related to?

Greenhouse Effect and Global Warming

The greenhouse effect is the natural warming of the Earth caused by certain atmospheric gases. Sunlight reaches the Earth's surface, which absorbs it and re-emits heat as infrared (long-wave) radiation. Greenhouse gases — chiefly carbon dioxide (CO₂) and methane (CH₄), along with water vapour, nitrous oxide (N₂O) and chlorofluorocarbons (CFCs) — absorb and trap this outgoing infrared radiation, preventing its escape to space and reheating the atmosphere.

Without this effect the Earth would be too cold for life; however, rising anthropogenic emissions intensify it, causing global warming — an increase in average global temperature.

Relative contribution to global warming (approximate): CO₂ ≈ 60%, methane ≈ 20%, CFCs ≈ 14%, and N₂O ≈ 6% (values vary between sources). Methane, though present in smaller amounts, is a far more potent greenhouse gas per molecule than CO₂.

Consequences include melting of polar ice, rising sea levels, and shifts in rainfall and agriculture. Mitigation involves cutting fossil-fuel use, reforestation and reducing emissions of these gases.

This concept covers CET questions on the mechanism of the greenhouse effect, identification of greenhouse gases, and their relative contribution to global warming.

The greenhouse effect and global warming are covered in the environmental-issues chapter of NCERT Class 12 Biology, and "greenhouse gases and their relative contribution to global warming" is a frequently searched revision topic. This is a dependable source of important questions in NEET and CET biology exams because of the precise percentage contributions attributed to gases like methane and carbon dioxide.