Greenhouse gases

Also called: GHGs · Topic: Environment and Sustainable Development · NCERT: Class 11, Ch 7 "Environment and Sustainable Development"

Meaning

Greenhouse gases (GHGs) are gases in the air that trap heat. They let sunlight in but absorb part of the heat that the earth sends back out. The main ones are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and the factory-made F-gases (HFCs, PFCs and SF6).

GHGs matter because their natural level keeps the earth warm enough for life. Human activity has pushed them far above that level. In 2024, CO2 was about 52% above its pre-1750 level and methane was 166% above pre-industrial levels [2]. This extra amount is the main cause of global warming.

Explanation

How GHGs trap heat: the greenhouse effect

• The natural greenhouse effect
• Sunlight passes through the air and warms the earth's surface.
• The warm surface gives off heat as infrared radiation.

• GHGs absorb part of this heat and send it back down, so the lower air stays warmer.

• The problem is the extra effect. Since the Industrial Revolution, humans have added more GHGs. More heat is trapped, so the average temperature of the lower atmosphere keeps rising. This is global warming.

• Global Warming Potential (GWP) tells you how much heat a gas traps compared with the same mass of CO2. CO2 has a GWP of 1. For HFCs, the GWP runs into hundreds or thousands.

The main gases and where they come from

Gas	Main human sources	Level in 2024 (WMO GHG Bulletin)
Carbon dioxide (CO2)	Burning coal, oil and gas; cutting forests	423.9 ppm, about 52% above pre-industrial [2]
Methane (CH4)	Animal waste, cattle digestion, rice paddies, landfills	1,942 ppb, 166% above pre-industrial [2]
Nitrous oxide (N2O)	Mainly fertilisers	338 ppb, 25% above pre-industrial [2]
F-gases (HFCs, PFCs, SF6)	Cooling (refrigerators, ACs) and industry	—

• ppm means parts per million. ppb means parts per billion.
• The older NCERT figures are CO2 +31% and CH4 +149% above 1750 levels. Use the WMO figures for current affairs questions.

What makes GHG levels rise (NCERT Box 7.1)

• Burning coal and petroleum in power plants, vehicles and industry releases CO2.
• Deforestation adds CO2 in two ways:
• Trees are cut or burnt, so the carbon stored in them goes into the air.

• Fewer trees are left to absorb CO2.

• More cattle production raises GHGs through a chain:

• More cattle need more grazing land, so forests are cleared.
• More cattle produce more methane.

• Rearing, feeding and transporting cattle burns more fossil fuel.

• Weaker natural sinks. The land and oceans absorb CO2, so they act as "sinks". In 2024 these sinks became weaker and wildfires increased. Both pushed up the record CO2 rise [2].

• CO2 is rising faster than before:
• It grew by about 0.8 ppm a year in the 1960s.
• It grew by about 2.4 ppm a year in 2011-20 [2].
• It rose by a record 3.5 ppm from 2023 to 2024. This is the biggest jump since measurements began in 1957 [2].
• Between 2004 and 2024, CO2 went from 377.1 ppm to 423.9 ppm [2].

What rising GHGs lead to

• The earth has already warmed:
• IPCC AR6 (2021-23): the average temperature in 2011-20 was about 1.1°C above 1850-1900.
• WMO (January 2025): 2024 was the warmest year on record, at about 1.55°C ± 0.13°C above the pre-industrial level. It was likely the first calendar year above 1.5°C [3].

• Record GHG levels were the main cause of the 2024 heat. El Niño played a smaller part [3].

• Impacts (NCERT Box 7.1):

• Polar ice melts, the sea level rises and coasts flood.
• Drinking water from snow-melt is disrupted, for example in Himalayan glacier-fed rivers.
• Species die out because their habitats (ecological niches) disappear.
• Tropical storms become more frequent.

• Diseases such as malaria and dengue spread more widely.

• Worked example: the carbon budget. The carbon budget is the total CO2 that can still be emitted if warming is to stay below a chosen limit.

• AR6 budget from 1 January 2020: about 500 GtCO2 for a 50% chance of staying at 1.5°C. (1 Gt = 100 crore tonnes.)
• World emissions: about 40 GtCO2 a year.
• Used in 2020-25: 6 years × 40 = 240 GtCO2.
• Left at the start of 2026: 500 − 240 ≈ 260 GtCO2.
• Years left at this rate: 260 ÷ 40 ≈ 6.5 years. These are round numbers, so check the current figure.

In India

• India controls a GHG through the ozone treaty. HFCs replaced CFCs because they do not harm the ozone layer. But HFCs are strong GHGs. The Kigali Amendment to the Montreal Protocol, adopted in October 2016, phases them down [4].
• The Union Cabinet approved India's ratification in August 2021. India ratified it in September 2021 [4].
• India's HFC path: a freeze in 2028, then cumulative cuts of 10% by 2032, 20% by 2037, 30% by 2042 and 85% by 2047 [4].

• Benefit: the global HFC phase-down may avoid up to 0.5°C of warming by 2100 [4].

• India Cooling Action Plan (ICAP, 2019)

• It was the world's first cooling action plan, with a 20-year outlook [5].
• Targets by 2037-38: cut cooling demand by 20-25%, energy used for cooling by 25-40% and refrigerant demand by 25-30% [5].

• It supports the HFC phase-down through better energy efficiency and climate-friendly refrigerants [5].

• Indian sources of GHGs from the NCERT list:

• Coal-based power plants and vehicles release CO2.
• Rice paddies and a large cattle population release methane.

• Fertiliser use releases N2O.

• India's equity stand: developed countries used most of the historical carbon budget. So India argues for CBDR (Common But Differentiated Responsibilities: every country must act, but not in the same way or to the same degree).

Don't confuse with

• Ozone-depleting substances (ODS): CFCs, halons, carbon tetrachloride and methyl chloroform thin the ozone layer. HFCs are GHGs but do not deplete ozone. Some gases, such as CFCs, are both ODS and GHGs. This is why Montreal also helped avoid about 0.5°C of warming [6].
• Stratospheric ozone vs ground-level ozone: ozone in the stratosphere is "good" because it blocks UV rays. Near the ground (the troposphere), ozone is a pollutant and a GHG.
• Greenhouse effect vs global warming: the greenhouse effect is natural and keeps the earth fit for life. Global warming is the extra warming caused by human GHG emissions.
• Carbon budget vs annual emission cap: the carbon budget is the total CO2 still allowed for a temperature limit. It is not a limit for each year.

Prelims Hooks

• WMO GHG Bulletin, 2024 figures: CO2 was 423.9 ppm (about 52% above pre-industrial), methane was 1,942 ppb (166% above) and N2O was 338 ppb (25% above) [2].
• CO2 rose by a record 3.5 ppm in 2023-24. This is the largest rise since measurements began in 1957 [2].
• Main sources: methane comes from cattle, animal waste, rice paddies and landfills. N2O comes mainly from fertilisers. F-gases = HFCs, PFCs and SF6.
• Trap: HFCs are GHGs, not ODS. They are controlled by the Kigali Amendment (2016) to the Montreal Protocol, not by the Vienna Convention or Kyoto.
• Trap: 2024 was about 1.55°C above pre-industrial [3]. This does not mean the Paris 1.5°C limit has been breached, because that limit refers to an average over about 20 years.
• India's HFC steps: freeze in 2028, then 10% (2032), 20% (2037), 30% (2042) and 85% (2047) [4]. ICAP (2019) targets 2037-38 [5].

Mains Points

• Carbon budget and equity: the remaining 1.5°C budget is small, and rich countries have already used most of the historical budget. So India argues for CBDR, climate finance and "carbon space" to develop. It still acts on its own through Kigali and ICAP [4][5].
• Montreal as a model for GHG deals: Montreal worked for four reasons:
• substitutes were available
• the Multilateral Fund (1990) paid developing countries' extra costs
• developing countries got delayed deadlines
• every country ratified it

It avoided about 0.5°C of warming [6]. Climate deals lack an equally credible package of money and technology.

• Linked problems, linked policy: the ozone fix (HFCs) created a GHG problem. As India's cooling demand rises with heat waves and cities, HFC cuts must not make cooling unaffordable for the poor [5][4]. This fits SDG 13 (Climate Action) and the Brundtland idea of sustainable development.

Related concepts

• Global warming
• Carbon budget
• Ozone depletion
• Chlorofluorocarbons

Read more

• Global environmental issues: global warming, greenhouse gases and ozone depletion