Greenhouse gases

Also called: GHGs · Topic: Climate Change: Causes, Evidence and the Global Response · NCERT: Class 7, Ch 3 "Climates of India"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 4 "Climate"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

Greenhouse gases (GHGs) are gases in the air that let sunlight pass through but absorb the heat (longwave or infrared radiation) that the Earth gives out, and send part of it back down. This warms the lower air and the surface. NCERT lists the main ones as carbon dioxide (CO2), methane, nitrous oxide, chlorofluorocarbons (CFCs) and ozone. Water vapour is the largest natural greenhouse gas.

• Without them, the Earth would be frozen. With too much of them, it warms too fast.
• The extra GHGs that humans add cause global warming. That is why every climate treaty, from Montreal to Paris, is really about these gases.

Explanation

How greenhouse gases trap heat

• Sunlight comes in as shortwave radiation.
• 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 Earth sends heat out as longwave radiation.

• The ground and oceans absorb sunlight and warm up.

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

• GHGs catch the outgoing heat.

• They absorb most of this longwave radiation and send part of it back down.

• Result: the lower air and the surface get warmer.

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

• Natural greenhouse effect: the gases that are already in the air keep the Earth about 33 °C warmer. The average is about 14–15 °C instead of –18 °C. This makes life possible.
• Enhanced greenhouse effect: humans have added extra gases "in just a few centuries" (NCERT, Class 7). This pushes temperatures above the natural level. That is global warming.

Why some gases matter more

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

• GWP order: CO2 (1) < methane (~28) < nitrous oxide (~270) < CFCs (thousands).
• 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, the shares are:
• 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. So the warming push from these gases was 54% stronger than in 1990. All the gases together equalled 539 ppm of CO2 (CO2-equivalent) [2].

• Water vapour is a feedback, not a driver.
• Warmer air holds more water vapour, and more water vapour traps more heat.
• 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. Water vapour then amplifies (makes bigger) the change they start.

• Nitric oxide (NO) and carbon monoxide (CO) are not strong GHGs themselves. But they react with GHGs and change how much of them is in the air.

The main greenhouse gases

Gas	Main sources	Lifetime	GWP (100 yr)	Level, 2024
CO2	Burning coal, oil and gas; deforestation; cement	Centuries (part of it)	1	423.9 ppm [3]
Methane (CH4)	Paddy fields, livestock, wetlands, landfills, gas leaks	~12 yr (WMO gives ~9 yr [3])	~28	1,942 ppb, 166% above pre-industrial [3]
Nitrous oxide (N2O)	Nitrogen fertilisers (urea), 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
Ozone (lower air)	Vehicle and factory pollution reacting in sunlight	Days to weeks	No single GWP	Varies by place

• Carbon dioxide is the largest single driver.
• It was about 280 ppm before the Industrial Revolution and 423.9 ppm in 2024 (about 150% of the pre-industrial level). It was 377.1 ppm in 2004 [3].
• The yearly rise was 0.8 ppm in the 1960s and 2.4 ppm in 2011–2020 [3].
• The 2023–24 rise of 3.5 ppm was the largest since modern measurements began in 1957 [3][4].
• Why 2024 broke the record: fossil fuel emissions continued, wildfire emissions surged, and land and ocean sinks took up less CO2 in the warmest year on record, which also had a strong El Niño [3][4].

• Global fossil CO2 emissions were 36.3 Gt CO2 in 2024, up 0.9% on 2023 [6].

• Methane is short-lived but strong. It lasts only about 12 years, so cutting it slows warming quickly.

• Nitrous oxide does harm in two ways:
• It traps heat. It is the third most important long-lived GHG [3].

• In the stratosphere, it destroys ozone.

• CFCs also do harm in two ways:

• They are very stable, so they rise slowly into the stratosphere (about 10–50 km up).
• There, ultraviolet light breaks them up and releases chlorine. Chlorine destroys ozone in a chain reaction.

• Their replacements, HFCs (hydrofluorocarbons), do not harm ozone but are strong GHGs.

• Lower-air (tropospheric) ozone forms in a chain:

• Vehicles and factories release nitrogen oxides and hydrocarbons.
• Strong sunlight makes these react, and ozone forms near the ground.
• The result is a GHG, a pollutant and photochemical smog (smog made by sunlight acting on pollution).

Sinks, lifetime and the limits on emissions

• A carbon sink absorbs more CO2 than it releases. Forests, oceans and soils are sinks.
• About half of the CO2 humans emit stays in the air. The rest goes into land and ocean, but these sinks are becoming less effective [3].
• The oceans take up about a quarter of emissions and the land about 30%.

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

• CO2 lingers. NCERT says CO2 takes 20–50 years to adjust to changes in its sources and sinks. But a large part of it affects the climate for hundreds of years [3].

• Climate sensitivity is how much the Earth would finally warm if CO2 doubled from about 280 to about 560 ppm. The IPCC AR6 best estimate is 3 °C (likely range 2.5–4 °C).

• Feedbacks make the warming bigger: more water vapour, and less snow and ice (albedo feedback, where albedo means how much light a surface reflects).

• Carbon budget is the total CO2 that can still be emitted while staying below a chosen warming limit. IPCC AR6 gives about 500 Gt CO2 from 2020 for a 50% chance of staying under 1.5 °C.

• About 200 Gt of it was used in 2020–24.
• At current rates, the budget could run out before 2030.

Where it occurs

• Methane from Indian paddy fields: flooded rice fields in West Bengal, Uttar Pradesh, Punjab and Odisha. Microbes break down organic matter without oxygen and release methane.
• Methane from Indian livestock: India has the world's largest cattle population. Cows and buffaloes release methane while digesting food. Landfills and coal mines add more.
• Natural methane: wetlands around the world. Leaks from oil and gas systems are a fast-growing human source.
• Nitrous oxide: farms that use heavy nitrogen fertiliser (urea) and manure. Soil microbes turn the extra nitrogen into N2O.
• CFC damage over Antarctica: very cold polar stratospheric clouds speed up ozone destruction every spring (September–October). This creates the Antarctic ozone hole.
• Ground-level ozone and photochemical smog: Delhi, Los Angeles and other cities in summer.
• Ocean acidification from absorbed CO2:
• CO2 dissolves in seawater and forms carbonic acid, so the water's pH falls.
• Corals and shell-forming animals then struggle to build skeletons and shells.

• Indian reefs at risk are in the Lakshadweep, the Andaman and Nicobar Islands and the Gulf of Mannar.

• Global agreements on these gases:

• Montreal Protocol (1987) phased out CFCs.
• Kigali Amendment (2016) phases down HFCs. India ratified it in September 2021, will freeze HFC use in 2028, and will cut it 10% by 2032, 20% by 2037, 30% by 2042 and 85% by 2047. India finalised a National Strategy for HFC phase-down in 2023 [5].
• Global Methane Pledge, launched at COP26 (Glasgow, 2021), aims to cut methane by 30% from 2020 levels by 2030. India has not joined.

Don't confuse with

• Greenhouse effect vs global warming: the greenhouse effect is natural and keeps the Earth at about 14–15 °C. Global warming is the enhanced effect caused by the extra GHGs that humans add.
• Stratospheric ozone vs tropospheric ozone: stratospheric ozone ("good ozone", about 15–35 km up) absorbs harmful ultraviolet-B rays. Tropospheric ozone ("bad ozone", near the ground) is a GHG and a pollutant.
• CFCs vs HFCs: CFCs destroy ozone and warm the climate. HFCs warm the climate but do not harm the ozone layer. CFCs come under the Montreal Protocol (1987) and HFCs under the Kigali Amendment (2016).
• Ozone hole vs greenhouse warming: the ozone hole comes from chlorine destroying stratospheric ozone, which lets in more ultraviolet light. Greenhouse warming comes from gases trapping outgoing longwave heat. Trap: the greenhouse effect is not caused by blocking incoming sunlight.

Prelims Hooks

• The air is mostly transparent to incoming shortwave radiation and absorbs outgoing longwave radiation. Without GHGs, the average would be about –18 °C instead of 14–15 °C.
• Water vapour is the largest natural GHG but acts as a feedback, not a driver.
• GWP, lowest to highest (100 yr): CO2 (1) < methane (~28) < nitrous oxide (~270) < CFCs (thousands). Share of 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 the largest since 1957 [3].
• Nitrous oxide is both a GHG and an ozone-depleting gas. HFCs harm the climate but not the ozone layer.
• Global Methane Pledge (COP26, Glasgow, 2021): a 30% cut by 2030 from 2020 levels. India is not a member. India has ratified Kigali (Sept 2021), with an 85% HFC cut by 2047 [5].

Mains Points

• What drives the rise in GHGs (GS-I/III):
• Fossil fuels and deforestation pushed CO2 from about 280 to 423.9 ppm (2024) [3].
• Farming adds methane and N2O through paddy, livestock and fertilisers.

• Weaker 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:

• India joined Kigali, an industrial phase-down backed by technology [5].
• India stays out of the Global Methane Pledge. It argues that methane from paddy and cattle is a survival emission, not a luxury emission, because small farmers depend on it.

• India can still 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:

• With fossil emissions at 36.3 Gt CO2 (2024) [6], only a few years of the 1.5 °C budget remain.
• This strengthens the case for Common But Differentiated Responsibilities (CBDR), climate finance, technology transfer, and a fair share of carbon space that counts historical emissions.
• Ocean acidification also threatens Indian coral reefs and fisheries, which links GHG policy to coastal livelihoods.

Related concepts

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

Read more

• The greenhouse effect and greenhouse gases