Greenhouse effect

Topic: Climate Change: Causes, Evidence and the Global Response · NCERT: Class 7, Ch 3 "Climates of India"; Class 11, Ch 4 "Climate"; Class 11, Ch 7 "Composition and Structure of Atmosphere"; Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

The greenhouse effect is the warming of the Earth's surface and lower air because certain gases let incoming sunlight pass through but absorb much of the heat (longwave radiation) the Earth sends out and send part of it back down. The natural effect keeps the Earth about 33 °C warmer than it would otherwise be, which makes life possible. Extra gases from human activity strengthen it into the enhanced greenhouse effect, which causes global warming.

Explanation

How it works (cause → process → result)

• Step 1: sunlight comes in.
• The Sun is very hot, so it gives out shortwave radiation (visible light and near-ultraviolet).

• Most of it passes straight through the air. The atmosphere is mostly transparent (see-through) to it.

• Step 2: the Earth sends heat out.

• The ground and oceans absorb this light and warm up.

• The Earth is much cooler than the Sun, so it sends energy back out as longwave (infrared) radiation.

• Step 3: gases trap the heat.

• Greenhouse gases (GHGs) absorb most of this outgoing longwave radiation.
• They send part of it back down to the ground.

• The lower air and the surface get warmer.

• The analogy: glass in a greenhouse, or a car with its windows shut in summer, lets light in but traps the heat inside.

Natural effect vs enhanced effect

Feature	Natural greenhouse effect	Enhanced greenhouse effect
Cause	Gases present in the air naturally (water vapour, CO2, methane)	Extra gases released by humans "in just a few centuries" (NCERT, Class 7)
Result	Earth averages about 14–15 °C instead of –18 °C, about 33 °C warmer	Global warming: temperatures rise above the natural level
Value	Makes life possible	Changes the climate (heat waves, melting ice, rising seas)

• Water vapour is a feedback, not a driver.
• It is the largest natural GHG.
• Warmer air holds more water vapour, and more vapour traps more heat, so the air warms further.
• It stays in the air only for days, so humans cannot change its level directly. CO2 and other long-lived gases set the temperature. Water vapour then amplifies (makes bigger) the change they start.

The gases and why some matter more

• NCERT (Class 11) names CO2, CFCs, methane, nitrous oxide and ozone as the main GHGs. Nitric oxide (NO) and carbon monoxide (CO) are not strong GHGs themselves, but they react with GHGs and change how much of them there is.
• A gas's warming power depends on three things:
• how much its concentration rises
• its lifetime (how long it stays in the air)

• the wavelengths it absorbs. A gas that absorbs in a "window" where other gases do not absorb traps more extra heat.

• Global warming potential (GWP) is how much heat 1 kg of a gas traps over 100 years, compared with 1 kg of CO2 (CO2 = 1).

Gas	Main sources	Lifetime	GWP (100 yr)	Level, 2024
CO2	Coal, oil, gas; deforestation; cement	Centuries (part of it)	1	423.9 ppm [3]
Methane (CH4)	Paddy, livestock, wetlands, landfills, gas leaks	~12 yr	~28	1,942 ppb, 166% above pre-industrial [3]
Nitrous oxide (N2O)	Nitrogen fertilisers, manure	>100 yr	~270	338.0 ppb, 25% above pre-industrial [3]
CFCs	Refrigerants, aerosol sprays	Decades to a century	Thousands	Falling since the Montreal Protocol
Tropospheric ozone	Vehicle and factory pollution in sunlight	Days to weeks	No single GWP	Varies by place

• Share of warming, 2024. Radiative forcing means the extra heat energy a gas traps per square metre of the Earth's surface. Counting long-lived gases only: CO2 66% (2.33 W/m²), methane 16% (0.57 W/m²), CFCs and related gases about 8%, nitrous oxide about 6% [2].
• NOAA's Annual Greenhouse Gas Index (AGGI) was 1.54 in 2024. This means the warming push from these gases was 54% stronger than in 1990. All the gases together equalled 539 ppm of CO2 (CO2-equivalent) [2].

How fast it is strengthening

• CO2 level: about 280 ppm before the Industrial Revolution, 377.1 ppm in 2004 and 423.9 ppm in 2024 [3]. That is about 150% of the pre-industrial level.
• Rate of rise: 0.8 ppm a year in the 1960s and 2.4 ppm a year in 2011–2020 [3]. The 2023–24 rise of 3.5 ppm was the largest since measurements began in 1957 [3][4].
• Why 2024 set a record:
• fossil fuel emissions continued
• wildfire emissions surged

• land and ocean sinks took up less CO2 in the warmest year on record, which also had a strong El Niño [3][4]

• Carbon sinks are weakening. A carbon sink is something that absorbs more CO2 than it releases, such as forests, oceans and soils. About half of human CO2 stays in the air. The rest goes into land and ocean, but these sinks are becoming less effective [3].

• Climate sensitivity is how much the Earth would finally warm if CO2 doubled from the pre-industrial level (about 280 to about 560 ppm). IPCC AR6 gives a best estimate of 3 °C, with a likely range of 2.5–4 °C. Feedbacks make the warming bigger. These include more water vapour and the albedo feedback: less snow and ice means less sunlight is reflected back to space (albedo is how much light a surface reflects).

Where it occurs

• Worldwide: the natural greenhouse effect works over the whole planet and keeps the global average at about 14–15 °C.
• India: methane sources
• Flooded paddy fields in West Bengal, Uttar Pradesh, Punjab and Odisha
• Livestock. India has the world's largest cattle population, and cows and buffaloes release methane while digesting food.

• Landfills and coal mines

• Wetlands are the main natural methane source worldwide. Leaks from oil and gas systems are a fast-growing human source.

• Tropospheric ozone (a GHG near the ground): forms when nitrogen oxides and hydrocarbons from vehicles and factories react in strong sunlight. This causes photochemical smog (smog made by sunlight acting on pollution) in Delhi and Los Angeles in summer.
• Ocean impact: oceans absorb about a quarter of CO2 emissions, which causes ocean acidification (CO2 dissolves in seawater, forms carbonic acid and lowers the pH). This threatens coral reefs in Lakshadweep, the Andaman and Nicobar Islands and the Gulf of Mannar.
• Global responses to GHGs:
• Montreal Protocol (1987) phased out CFCs. The Kigali Amendment (2016) phases down HFCs. India ratified Kigali in September 2021, will freeze HFC use in 2028 and cut it 85% by 2047 [5].
• Global Methane Pledge, launched at COP26, Glasgow (2021), aims for a 30% cut from 2020 levels by 2030. India has not joined.

Don't confuse with

• Ozone hole / ozone depletion: this is the thinning of stratospheric ozone (about 15–35 km up) by chlorine from CFCs, which lets in more ultraviolet-B rays. The greenhouse effect is about trapping outgoing longwave heat, not letting in incoming UV. CFCs happen to cause both.
• Blocking incoming sunlight: the greenhouse effect does not work by stopping sunlight from coming in. The air lets shortwave light through and traps the outgoing longwave radiation.
• Global warming: this is the result of the enhanced (human-made) greenhouse effect. The natural greenhouse effect is useful, and without it Earth would average –18 °C.
• HFCs vs CFCs: CFCs destroy ozone and trap heat. HFCs do not harm the ozone layer, but they are strong GHGs, which is why Kigali regulates them.

Prelims Hooks

• Without the natural greenhouse effect, Earth's average temperature would be about –18 °C. With it, the average is about 14–15 °C, a difference of about 33 °C.
• Water vapour is the largest natural GHG but acts as a feedback, not a driver.
• GWP (100 yr), lowest to highest: CO2 (1) < methane (~28) < nitrous oxide (~270) < CFCs (thousands).
• Share of long-lived GHG radiative forcing, 2024: CO2 66% > methane 16% > CFCs ~8% > N2O ~6% [2].
• CO2 in 2024: 423.9 ppm. The 2023–24 rise of 3.5 ppm was a record since 1957 [3].
• Nitrous oxide is both a GHG and an ozone-destroying gas. It is the third most important long-lived GHG [3].

Mains Points

• Drivers of the enhanced greenhouse effect (GS-I/III):
• Burning fossil fuels and cutting forests raised CO2 from about 280 to 423.9 ppm (2024) [3]. Global fossil CO2 emissions were 36.3 Gt in 2024, up 0.9% from 2023 [6].
• Farming adds methane and N2O through paddy, livestock and fertilisers.

• Weakening land and ocean sinks show that warming can speed itself up. The record 2024 rise came partly from lower sink uptake [3][4].

• India's gas-by-gas position (GS-III):

• India joined Kigali, an industrial phase-down backed by technology [5].
• India stays out of the Global Methane Pledge because paddy and livestock methane are survival emissions (needed for farmers' livelihoods and food security), not luxury emissions.

• India can cut methane without hurting farmers through direct-seeded rice, alternate wetting and drying in paddy, better cattle feed and biogas under GOBAR-Dhan.

• Equity and the carbon budget:

• A carbon budget is the total CO2 that can still be emitted while keeping warming below a chosen limit. IPCC AR6 gives about 500 Gt CO2 from 2020 for a 50% chance of staying under 1.5 °C, and only a few years of current emissions remain.
• Rich countries used most of the budget through past (historical) emissions. This supports Common But Differentiated Responsibilities (CBDR), climate finance and technology transfer for developing countries like India.

Related concepts

• Greenhouse gases
• Global warming potential
• Methane
• Nitrous oxide
• Chlorofluorocarbons
• Tropospheric ozone
• Carbon sink
• Climate sensitivity
• Carbon budget

Read more

• The greenhouse effect and greenhouse gases