The greenhouse effect and greenhouse gases

Detail

1. How the greenhouse effect works

• Greenhouse effect means the lower air lets sunlight in but holds back much of the heat that the Earth sends out. This keeps the surface warm.
• Step by step (cause → process → result):
• 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 ground and oceans absorb this light, warm up and send energy back out as longwave (infrared) radiation, because the Earth is much cooler than the Sun.

• Certain gases absorb most of this outgoing longwave radiation and send part of it back down. 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.

• Greenhouse gases (GHGs) are the gases that absorb longwave radiation. The warming they cause is the greenhouse effect.

2. 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 stays about 33 °C warmer: about 14–15 °C on average instead of –18 °C	Global warming: average temperatures rise above the natural level
Value	Makes life possible	Changes the climate (heat waves, melting ice, rising seas)

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

3. Why some gases matter more: lifetime and GWP

• 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 effect 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): how much heat 1 kg of a gas traps over 100 years, compared with 1 kg of CO2 (CO2 = 1).

• Share of warming, 2024 (radiative forcing, meaning the extra heat energy a gas traps per square metre of the Earth's surface; 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 in 2024 than in 1990. All the gases together equalled 539 ppm of CO2 (CO2-equivalent) in 2024 [2].

4. The main greenhouse gases

Gas	Main sources	Lifetime	GWP (100 yr)	Level, 2024	Special point
Carbon dioxide (CO2)	Burning coal, oil and gas; deforestation; cement	Centuries (part of it)	1	423.9 ppm [4]	Largest single driver
Methane (CH4)	Paddy fields, livestock, wetlands, landfills, gas leaks	~12 yr (WMO gives ~9 yr [4])	~28	1,942 ppb, 166% above pre-industrial [4]	Short-lived but strong
Nitrous oxide (N2O)	Nitrogen fertilisers, manure	>100 yr	~270	338.0 ppb, 25% above pre-industrial [4]	Also destroys ozone
Chlorofluorocarbons (CFCs)	Refrigerants, aerosol sprays	Long (decades to a century)	In the thousands	Falling since the Montreal Protocol	Caused the Antarctic ozone hole
Ozone (O3), lower air	Formed from vehicle and factory pollution in sunlight	Days to weeks	Not given as a single GWP	Varies by place	GHG and pollutant

Carbon dioxide

• CO2 is the largest GHG that humans add. It comes mainly from fossil fuels and deforestation (cutting forests releases their stored carbon and removes a sink).
• How fast it is rising:
• NCERT: CO2 rises about 0.5% a year.
• In ppm (parts per million), the average rise was 0.8 ppm a year in the 1960s and 2.4 ppm a year in 2011–2020, three times as fast [4].
• The rise from 2023 to 2024 was 3.5 ppm, the largest since modern measurements began in 1957 [4][5]. NOAA measured 3.72 ppm from 1 January 2024 to 1 January 2025. That is 27% more than the earlier record of 2.94 ppm in 2015 [3].

• Why 2024 was a record: continued fossil emissions, a surge in wildfire emissions, and land and ocean sinks taking up less CO2 in the warmest year on record, which also had a strong El Niño [4][5].

• Level: 423.9 ppm in 2024 [4], against about 280 ppm before the Industrial Revolution, which is about 150% of the pre-industrial level. It was 377.1 ppm in 2004 [4].

• Global fossil CO2 emissions were 36.3 Gt CO2 in 2024, 0.9% more than in 2023 [7].

Methane

• Methane (CH4) is the main part of natural gas. Microbes also make it when they break down organic matter without oxygen.
• Indian sources: flooded paddy fields (for example in West Bengal, Uttar Pradesh, Punjab and Odisha), livestock (India has the world's largest cattle population; cows and buffaloes release methane while digesting food), landfills and coal mines.
• Natural source: wetlands. Leaks from oil and gas systems are a fast-growing human source.
• It lasts about 12 years, with a GWP of about 28. So cutting methane slows warming quickly.
• It is up over 160% since 1750 (WMO: 166% in 2024) and causes about 16% of the warming from long-lived GHGs [4].

Nitrous oxide

• Nitrous oxide (N2O) comes mainly from nitrogen fertilisers (urea) and manure. Soil microbes convert the extra nitrogen into N2O.
• It lasts over 100 years, with a GWP of about 270.
• It does two kinds of harm:
• It traps heat. It is the third most important long-lived GHG [4].
• In the stratosphere it destroys ozone.

Chlorofluorocarbons and HFCs

• Chlorofluorocarbons (CFCs) are factory-made gases. They were used in refrigerators, air conditioners and aerosol sprays. Their GWPs run into the thousands.
• How they destroy ozone:
• CFCs are very stable, so they rise slowly into the stratosphere (the air layer above about 10–50 km).
• There, strong ultraviolet light breaks them up and releases chlorine.

• Chlorine destroys ozone molecules in a chain reaction. Over Antarctica, very cold polar stratospheric clouds speed this up every spring (September–October). The result is the Antarctic ozone hole.

• The global response:

• The Montreal Protocol (1987) phased out CFCs.
• CFCs were replaced by HFCs (hydrofluorocarbons). HFCs do not harm ozone, but they are strong GHGs.

• The Kigali Amendment (2016) to the Montreal Protocol phases down HFCs.

• India and the Kigali Amendment:

• India ratified it in September 2021 [6].
• India freezes HFC use in 2028.
• India then cuts HFCs in four steps from 2032: 10% by 2032, 20% by 2037, 30% by 2042 and 85% by 2047 [6].
• India finalised a National Strategy for HFC phase-down in 2023 [6].

Ozone: the two faces

Layer	Height	Role
Stratospheric ozone ("good ozone")	About 15–35 km	Absorbs harmful ultraviolet-B rays and protects life
Tropospheric ozone ("bad ozone")	Near the ground	A greenhouse gas and a pollutant; harms lungs and crops

• How tropospheric ozone forms: vehicles and factories release nitrogen oxides and hydrocarbons → in strong sunlight these react together → ozone forms near the ground.
• This is the cause of photochemical smog (smog made by sunlight acting on pollution), seen in Delhi, Los Angeles and other cities in summer.

Global Methane Pledge

• The Global Methane Pledge was launched at COP26 (Glasgow, 2021). Its members aim to cut methane emissions by 30% from 2020 levels by 2030.
• India has not joined. India's methane comes mostly from paddy farming and livestock, which support the livelihoods of small farmers. India argues these are survival emissions, not luxury emissions.

5. Carbon sinks

• A carbon sink is anything that absorbs more CO2 than it releases, such as forests, oceans and soils.
• A carbon source does the opposite: it releases more CO2 than it absorbs.

• Deforestation turns a sink into a source: cut and burnt trees release their carbon, and the land stops absorbing CO2.

• How much the sinks take:

• About half of the CO2 humans emit stays in the air. The other half goes into land ecosystems and the ocean, but these sinks are becoming less effective [4].

• The oceans take up about a quarter of emissions and land about 30%.

• The ocean pays a cost: ocean acidification.

• CO2 dissolves in seawater and forms carbonic acid.
• The water's pH falls.

• Corals and shell-forming animals find it harder to build their skeletons and shells. Coral reefs at risk include those in the Lakshadweep, the Andaman and Nicobar Islands and the Gulf of Mannar.

• How long CO2 lasts:

• NCERT: CO2 takes 20–50 years to adjust to changes in its sources and sinks.
• However, a large part of emitted CO2 affects the climate for hundreds of years [4]. So past emissions keep warming the planet.

6. Climate sensitivity

• Climate sensitivity is how much the Earth would finally warm if the amount of CO2 in the air doubled from the pre-industrial level (from about 280 to about 560 ppm).
• NCERT notes that this doubling is the standard measure used in climate models.
• IPCC AR6 (Sixth Assessment Report, 2021): the best estimate is 3 °C, with a likely range of 2.5–4 °C.
• The warming is larger than CO2 alone would cause, because of feedbacks. These include more water vapour, and less snow and ice, which means less sunlight is reflected back to space (albedo feedback; albedo is how much light a surface reflects).

7. Carbon budget

• A carbon budget is the total CO2 that can still be emitted while keeping warming below a chosen limit, such as 1.5 °C or 2 °C. Warming rises roughly in step with the total CO2 emitted, which is why a fixed budget works.
• IPCC AR6: about 500 Gt CO2 from 2020 gives a 50% chance of staying under 1.5 °C.
• How fast it is running out:
• Global fossil emissions were 36.3 Gt CO2 in 2024 [7]. With land-use emissions, the yearly total is about 40 Gt CO2.
• At that rate, roughly 200 Gt of the 500 Gt was used in 2020–24.

• Recent Global Carbon Budget updates show only a few years of current emissions remain for 1.5 °C, so the budget could run out before 2030.

• The equity question: developing countries, including India, argue that rich industrialised countries have already used most of the budget through their past (historical) emissions. So they should cut faster and pay for finance and technology transfer. This is the principle of Common But Differentiated Responsibilities (CBDR).

Prelims Hooks

• The atmosphere is mostly transparent to incoming shortwave radiation and absorbs outgoing longwave radiation. Trap: the effect is not caused by blocking incoming sunlight.
• Without the natural greenhouse effect, Earth's average would be about –18 °C; with it, 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.
• Lowest to highest GWP (100 yr): 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 [4].
• Nitrous oxide is both a greenhouse gas and an ozone-depleting gas. HFCs harm the climate but not the ozone layer.
• Montreal Protocol, 1987 (CFCs) → Kigali Amendment, 2016 (HFCs). India ratified Kigali in Sept 2021, will freeze HFC use in 2028 and cut it 85% by 2047 [6].
• Global Methane Pledge (COP26, Glasgow, 2021): a 30% cut by 2030 from 2020 levels. India is not a member.
• IPCC AR6 climate sensitivity: best estimate 3 °C (likely range 2.5–4 °C).

Mains Points

• Drivers of the enhanced greenhouse effect (GS-I/III):
• Burning fossil fuels and deforestation raised CO2 from about 280 to 423.9 ppm (2024) [4].
• Farming adds methane and N2O (paddy, livestock, fertilisers).

• Weakening land and ocean sinks show that warming can speed itself up: the record 2024 rise came partly from lower sink uptake [4][5].

• India's position on gases:

• India joined Kigali, a phase-down that is industrial and backed by technology [6].
• India stays out of the Global Methane Pledge, to protect farm livelihoods and food security.

• India's options to cut methane without hurting farmers: direct-seeded rice and alternate wetting and drying in paddy, better cattle feed, and biogas under GOBAR-Dhan.

• Carbon budget and equity:

• Only a few years of the 1.5 °C budget remain at current emissions (36.3 Gt CO2 fossil, 2024) [7].
• This strengthens the case for CBDR, climate finance and fair shares of the remaining carbon space.

• India can argue for a carbon-space view that includes historical emissions, rather than counting only current emissions.

• Ocean acidification: absorbing CO2 harms marine life. This threatens Indian coral reefs and fisheries, which links climate policy with coastal livelihoods (GS-III environment).