Question
Explain the greenhouse effect. Name the major greenhouse gases in order of their contribution. How does the enhanced greenhouse effect lead to global warming?
(NCERT Class 11 — asked in CBSE boards and NEET)
Solution — Step by Step
The greenhouse effect is a natural process where certain atmospheric gases trap infrared radiation (heat) emitted by the Earth’s surface, keeping the planet warm enough to support life.
Without this effect, Earth’s average temperature would be about instead of the current .
- Solar radiation (mostly visible light and UV) passes through the atmosphere and heats the Earth’s surface
- The warm surface re-emits energy as infrared radiation (long-wavelength heat)
- Greenhouse gases absorb this infrared radiation and re-emit it in all directions — some goes back toward the surface
- This “trapping” of heat raises the surface temperature above what it would be otherwise
The key point: greenhouse gases are transparent to incoming short-wave solar radiation but absorb outgoing long-wave infrared radiation.
| Gas | Formula | Contribution to greenhouse effect | Main source |
|---|---|---|---|
| Water vapour | ~60% (natural) | Evaporation | |
| Carbon dioxide | ~25% | Fossil fuels, respiration | |
| Methane | ~6% | Rice paddies, cattle, wetlands | |
| Nitrous oxide | ~3% | Fertilisers, combustion | |
| CFCs | s | ~5% | Refrigerants, aerosols |
| Ozone | ~1% (tropospheric) | Vehicle emissions |
gets the most attention because human activities have increased its concentration dramatically — from ~280 ppm (pre-industrial) to over 420 ppm today.
The enhanced greenhouse effect is the additional warming caused by human-generated greenhouse gases. Burning fossil fuels, deforestation, and industrial processes have increased and concentrations far above natural levels.
Consequences: rising global temperatures, melting polar ice, sea level rise, extreme weather events, ocean acidification, and disruption of ecosystems.
Why This Works
Greenhouse gases have molecular structures that can vibrate at infrared frequencies. When an IR photon hits a molecule, the molecule absorbs the energy and vibrates (bending and stretching modes). It then re-emits the energy in a random direction. Since roughly half goes back toward Earth, heat accumulates near the surface.
Simple diatomic molecules like and don’t absorb IR because their symmetric stretching doesn’t change the dipole moment. That’s why they make up 99% of the atmosphere but aren’t greenhouse gases.
Alternative Method — The Blanket Analogy
Think of greenhouse gases as a blanket around the Earth. A thin blanket (natural greenhouse effect) keeps you comfortable. Adding extra blankets (more , ) makes you overheat. The blanket doesn’t add heat — it just prevents heat from escaping. That’s exactly what greenhouse gases do to outgoing infrared radiation.
For NEET: remember that water vapour is the most abundant greenhouse gas by contribution, but is the most significant human-caused one. Questions often ask “which gas contributes most to global warming” — the answer they expect is , since the question context is about anthropogenic (human-caused) climate change.
Common Mistake
Students frequently write that “the greenhouse effect is harmful.” The natural greenhouse effect is essential for life — without it, Earth would be frozen. What’s harmful is the enhanced greenhouse effect caused by excess emissions. In CBSE boards, making this distinction clearly can earn you extra marks in the value-based questions.