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Greenhouse gases, global warming and the ozone layer

Climate change · Part 1 / 3 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

How the greenhouse effect works; the main greenhouse gases, their sources and global warming potentials; observed warming, feedbacks and impacts; the stratospheric ozone layer, the chemicals that destroy it and the Antarctic ozone hole; the Vienna Convention, the Montreal Protocol and the Kigali Amendment, with India's commitments.

Climate change is taught in three parts. This first part is the science: how a few gases keep the Earth warm, which of them human activity is adding, how much the planet has already warmed and what that does. It also covers a second, separate problem, the thinning of the ozone layer, and the treaty that turned it around. Parts 2 and 3 cover the climate treaties and India's plans. global warming is about heat trapped in the lower air by gases such as carbon dioxide. Ozone loss happens much higher, in the stratosphere, and is caused by chlorine and bromine. A few chemicals connect the two stories, and Prelims questions like to test exactly those links.

The greenhouse effect begins with sunlight. Most solar energy arrives as short-wave radiation that passes easily through the air and warms the ground and the sea. A warm surface then gives off its own energy as long-wave infrared. A greenhouse gas absorbs part of this outgoing infrared and sends much of it back towards the surface, so the lower atmosphere holds more heat than it otherwise would. Without this natural blanket the Earth's average surface temperature would be roughly minus 18 °C rather than about 15 °C. Nitrogen and oxygen, which together make up about 99 per cent of dry air, hardly absorb infrared at all; the trapping is done by gases present in small amounts. The problem today is the enhanced effect: extra gases added by people thicken the blanket, and that drives global warming.

Water vapour is the largest natural contributor to the greenhouse effect, but people do not control it directly: its amount follows temperature, so it acts as an amplifier rather than a starting push. Carbon dioxide is the main gas released by human activity, chiefly from burning coal, oil and gas, from making cement and from clearing forests. Before the industrial era its concentration was about 280 parts per million; NOAA's annual average at Mauna Loa in Hawaii passed 420 parts per million for the first time in 2023. Methane comes from livestock digestion, flooded rice fields, landfills, wetlands, coal mines and leaking gas pipes. Nitrous oxide comes mostly from fertilised soils and manure. The fluorinated gases, namely hydrofluorocarbons (HFCs), perfluorocarbons, sulphur hexafluoride and nitrogen trifluoride, are purely industrial and released in small quantities, yet each tonne traps a great deal of heat. Ozone near the ground also acts as a greenhouse gas.

Because the gases differ so much in strength, scientists compare them with a single yardstick, the global warming potential. It measures how much heat one tonne of a gas traps over a chosen period, usually 100 years, relative to one tonne of carbon dioxide, which is fixed at 1. In the IPCC's Fifth Assessment Report (2013) methane scores about 28, nitrous oxide about 265 and sulphur hexafluoride about 23,500. Multiplying each gas by its score gives emissions in carbon dioxide equivalent, which is how national targets add different gases together. The time window matters: methane breaks down in about twelve years, so over 20 years its score rises to about 84. That is why cutting methane, HFCs and black carbon (soot) is seen as the fastest way to slow warming; black carbon also darkens snow and ice and lowers their albedo.

How much has the world warmed? The IPCC's Sixth Assessment Report (Working Group I, 2021) found that global surface temperature in 2011–2020 was about 1.1 °C higher than in 1850–1900, and called it unequivocal that human influence has warmed the atmosphere, ocean and land. Warming is uneven. Land heats faster than the ocean, and the Arctic has warmed at more than twice the global rate. Part of the reason lies in feedbacks, changes that a little warming sets off and that then add more warming. When bright sea ice melts, dark water beneath it absorbs more sunlight, so a falling albedo feeds the heat. Thawing permafrost can release stored carbon as carbon dioxide and methane. Warmer air holds more water vapour, itself a heat-trapping gas.

The effects reach well beyond hotter days. Sea level rises for two reasons: seawater expands as it warms, and melting glaciers and ice sheets add water. The IPCC (2021) put the rise in global mean sea level at about 0.20 metres between 1901 and 2018, with the pace speeding up. The ocean also soaks up roughly a quarter of the carbon dioxide people emit, which makes seawater more acidic; ocean acidification makes it harder for corals, shellfish and some plankton to build their shells and skeletons, while marine heatwaves bleach coral reefs. Heatwaves and heavy downpours grow more intense, and Himalayan and other mountain glaciers are shrinking. For India this means harsher summers, a less predictable monsoon, risk to crop yields and greater danger to low-lying coasts. The burden falls hardest on those who contributed least, the root of the fairness debate in Part 2.

Now the second problem. The ozone layer is the part of the stratosphere, mostly between about 15 and 35 kilometres up, where ozone (O3) is most concentrated. It absorbs nearly all of the Sun's most dangerous ultraviolet rays and most UV-B, shielding living things from sunburn, skin cancer and cataracts and protecting crops and ocean plankton. Column ozone is measured in Dobson units, and scientists speak of an ozone hole where the total falls below 220 Dobson units. The same molecule plays two opposite roles depending on where it is. High up it is a shield; near the ground, formed from traffic fumes in sunlight, it is a lung-damaging pollutant and a heat-trapping gas.

An ozone-depleting substance is any chemical that, once it reaches the stratosphere, frees chlorine or bromine there. The best known are chlorofluorocarbons (CFCs), once used in refrigerators, air-conditioners, aerosol sprays and foams; halons in fire extinguishers; carbon tetrachloride and methyl chloroform used as solvents; methyl bromide, a fumigant for soil and stored grain; and the milder hydrochlorofluorocarbons (HCFCs). They are very stable: they survive for years, drift slowly upwards, and are finally broken apart by strong ultraviolet light, setting chlorine or bromine free. A single chlorine atom then destroys many thousands of ozone molecules in turn, and bromine is even more destructive atom for atom. The worst damage is over Antarctica. In the dark polar winter a cold whirl of winds, the polar vortex, lets polar stratospheric clouds form; on them chlorine turns into active forms, and when sunlight returns in September and October the ozone hole opens quickly.

The world answered in two steps. The Vienna Convention for the Protection of the Ozone Layer (1985) was a framework: countries agreed to cooperate on research and to act, without fixing cuts. News of the Antarctic hole that same year, from British Antarctic Survey scientists, added urgency, and the Montreal Protocol was adopted on 16 September 1987 and came into force on 1 January 1989. It set deadlines to phase out the production and use of each ozone-depleting substance and gave developing countries extra years plus money from a Multilateral Fund created by the London Amendment of 1990. In 2009 the Vienna Convention and the Montreal Protocol became the first treaties in UN history to be ratified by every country. India joined the Protocol in 1992 and enforces it through the Ozone Depleting Substances (Regulation and Control) Rules, 2000, made under the Environment (Protection) Act. The WMO–UNEP assessment of 2022 expects the layer to recover to 1980 levels by about 2066 over Antarctica, 2045 over the Arctic and 2040 elsewhere.

Success created a new problem. Many replacements for CFCs and HCFCs were hydrofluorocarbons, which contain no chlorine and so leave ozone alone, but which are powerful greenhouse gases, and demand for cooling was soaring. The Kigali Amendment to the Montreal Protocol, agreed in Kigali, Rwanda, in October 2016 and in force from 1 January 2019, brought HFCs under the Protocol for a phase-down of about 80–85 per cent, not a total phase-out. Countries move in three groups. Most developed countries began cutting in 2019; China and most developing countries froze consumption in 2024; India, together with Pakistan, Iran, Iraq and the Gulf states, freezes in 2028 against a 2024–2026 baseline and must reach an 85 per cent cut by 2047. India ratified the amendment in 2021, and its India Cooling Action Plan (March 2019) looks twenty years ahead at cooling needs across buildings, transport, cold chains and refrigerants.

Putting the part together: global warming and ozone depletion are different problems in different layers, caused by different gases and handled by different treaties, yet they meet at two points. CFCs did double harm, thinning the ozone layer and trapping heat, so the Montreal Protocol also slowed warming. HFCs, the cure for ozone loss, became a climate problem that the Kigali Amendment now handles. The ozone story succeeded because substitutes existed and only a few industries had to change, while carbon dioxide comes from almost everything an economy does. That contrast explains why the climate treaties in the next two parts were much harder to negotiate, and why the question of who should cut first became central.

Exam traps

• Ozone depletion is not the cause of global warming, and the ozone hole does not let heat in. The two are separate problems in separate layers; they meet only because some chemicals (CFCs, HFCs) affect both.
• Nitrogen and oxygen make up about 99 per cent of dry air but are not greenhouse gases. Water vapour is the largest natural contributor, yet it responds to temperature rather than being directly emitted in amounts that matter.
• Carbon dioxide has the lowest global warming potential (1, by definition) but causes the most warming, because so much of it is emitted and it lasts so long. Sulphur hexafluoride has the highest GWP among the common gases.
• Methane's GWP is higher over 20 years (about 84) than over 100 years (about 28), because it breaks down in about twelve years. A question that reverses this is wrong.
• HFCs do not harm the ozone layer. They were brought under the Montreal Protocol by the Kigali Amendment because they warm the climate, and Kigali is a phase-down of about 80–85 per cent, not a phase-out.
• India's HFC freeze year is 2028, with an 85 per cent cut by 2047. The 2024 freeze applies to China and most other developing countries.
• The Antarctic ozone hole is deepest in the southern spring, September–October, not in midwinter darkness: it needs both the winter's polar stratospheric clouds and the returning sunlight.
• The Montreal Protocol (1987) sits under the Vienna Convention (1985), not under the UNFCCC. In 2009 these two became the first treaties ratified by every country.

Recap

• Greenhouse effect: short-wave sunlight in, long-wave heat out; greenhouse gases send part of it back. Natural effect keeps Earth near 15 °C instead of about minus 18 °C.
• Main human-added gases: carbon dioxide (fossil fuels, cement, forests), methane (livestock, rice, landfills), nitrous oxide (fertiliser), F-gases (industry). CO2 passed 420 ppm at Mauna Loa in 2023 (NOAA).
• Global warming potential compares gases with CO2 = 1 over 100 years: methane about 28, nitrous oxide about 265, SF6 about 23,500 (IPCC AR5).
• IPCC AR6 (2021): 2011–2020 about 1.1 °C warmer than 1850–1900; human influence unequivocal; Arctic warming more than twice as fast; feedbacks amplify.
• Impacts: sea level up about 0.20 m in 1901–2018, ocean acidification, coral bleaching, glacier retreat, heat and rain extremes; poorest hit hardest.
• Ozone layer: stratosphere, about 15–35 km, absorbs UV; ozone hole below 220 Dobson units; ground-level ozone is a pollutant.
• Chlorine and bromine from CFCs, halons and others destroy ozone; Antarctic hole opens in September–October.
• Vienna Convention 1985, Montreal Protocol 1987 (in force 1989), universal ratification 2009; India joined 1992, ODS Rules 2000; recovery over Antarctica about 2066.
• Kigali Amendment 2016 (in force 2019): HFC phase-down; India freezes 2028, 85 per cent cut by 2047; India Cooling Action Plan 2019.

Practice questions

Sources

• NASA Earth Observatory and NASA Science: Earth's energy budget, the greenhouse effect and the evidence for climate change (US government) — https://science.nasa.gov/climate-change/
• NOAA Global Monitoring Laboratory: trends in atmospheric carbon dioxide, methane and nitrous oxide (Mauna Loa record, annual means to 2024) — https://gml.noaa.gov/ccgg/trends/
• IPCC Sixth Assessment Report, Working Group I, Summary for Policymakers (2021); IPCC Fifth Assessment Report, Working Group I, Chapter 8 (2013), global warming potentials — https://www.ipcc.ch/report/ar6/wg1/
• Vienna Convention for the Protection of the Ozone Layer (1985); Montreal Protocol on Substances that Deplete the Ozone Layer (1987) and its amendments, including the Kigali Amendment (2016), UNEP Ozone Secretariat — https://ozone.unep.org/treaties
• WMO and UNEP, Scientific Assessment of Ozone Depletion: 2022 (released January 2023), executive summary — https://csl.noaa.gov/assessments/ozone/2022/
• Ministry of Environment, Forest and Climate Change, Ozone Cell: Ozone Depleting Substances (Regulation and Control) Rules, 2000; India Cooling Action Plan (March 2019); ratification of the Kigali Amendment (2021) — https://ozonecell.nic.in