Climate Change and Greenhouse Gases

Summary: Greenhouse gases (CO₂, CH₄, water vapour) trap infrared radiation, causing global warming. Carbon footprint measures total greenhouse gas emissions from an activity or product. Tags: igcse chemistry environmental-chem Created: 2026-07-14 Last Updated: 2026-07-14

Content

The Greenhouse Effect

The greenhouse effect is a natural process by which certain gases in the Earth’s atmosphere trap heat, keeping the planet warm enough to sustain life. Without it, the average surface temperature of the Earth would be approximately −18 °C rather than the current +15 °C — a difference of about 33 °C.

The mechanism operates as follows:

1. Incoming solar radiation: The Sun emits electromagnetic radiation across a broad spectrum. Much of the energy reaching the Earth is in the form of short-wavelength radiation — visible light and ultraviolet (UV) radiation. The atmosphere is largely transparent to these wavelengths, so a significant proportion reaches the Earth’s surface.

2. Absorption and re-emission: The Earth’s surface absorbs this short-wavelength radiation and warms up. The warm surface then re-emits energy in the form of longer-wavelength infrared (IR) radiation (heat radiation).

3. Absorption by greenhouse gases: Greenhouse gas molecules in the atmosphere absorb some of this outgoing IR radiation. The molecules vibrate more energetically (the absorbed IR energy corresponds to the natural vibrational frequencies of the bonds in molecules such as C=O, O–H, and C–H), and they re-radiate the energy in all directions — including back down toward the Earth’s surface.

4. Warming effect: This trapping and re-radiation of heat keeps the lower atmosphere and the Earth’s surface warmer than they would otherwise be — analogous to how the glass of a greenhouse admits visible light but traps heat inside.

The Natural vs Enhanced Greenhouse Effect

It is important to distinguish between two concepts:

The natural greenhouse effect is the baseline warming produced by naturally occurring levels of greenhouse gases (primarily water vapour, CO₂, and methane from natural sources). This effect has operated for billions of years and makes Earth habitable.

The enhanced (or anthropogenic) greenhouse effect refers to the additional warming caused by human activities that have increased the atmospheric concentration of greenhouse gases beyond natural levels. The burning of fossil fuels, deforestation, intensive agriculture, and industrial processes have elevated CO₂, CH₄, and N₂O concentrations significantly since the Industrial Revolution (c. 1750). This enhanced effect is driving the current global warming trend.

Major Greenhouse Gases

The Global Warming Potential (GWP) compares the heat-trapping ability of a given mass of gas to the same mass of CO₂ over a specific time period (commonly 100 years). For example, methane has a GWP of 28–34, meaning that 1 kg of CH₄ traps as much heat over 100 years as approximately 28–34 kg of CO₂.

Carbon Dioxide (CO₂)

CO₂ is the most significant anthropogenic greenhouse gas by total warming contribution. Despite its relatively low GWP, its sheer volume of emissions and long atmospheric lifetime make it the dominant driver of long-term climate change. Key sources:

• Combustion of fossil fuels: coal, oil, and natural gas burned for electricity, heating, and transport
• Deforestation: removal of forests (which act as carbon sinks) and often burning of the cleared wood
• Cement manufacture: calcination of limestone (CaCO₃ → CaO + CO₂) releases CO₂ as a by-product

Pre-industrial CO₂ concentration was approximately 280 parts per million (ppm). As of 2024, atmospheric CO₂ exceeded 420 ppm — an increase of over 50%.

Methane (CH₄)

Methane is produced by anaerobic decomposition of organic matter — that is, decomposition in the absence of oxygen. Key sources:

• Enteric fermentation: Methane-producing microorganisms (methanogens) in the digestive systems of ruminant animals (cattle, sheep) ferment plant material, releasing CH₄ through belching
• Rice paddies: Flooded rice fields create anaerobic conditions in the soil, where methanogens decompose organic matter
• Landfills: Organic waste buried in landfill sites decomposes anaerobically, producing methane (landfill gas)
• Natural gas leaks: Natural gas is primarily methane; leaks during extraction (fracking), transport, and distribution release it into the atmosphere
• Melting permafrost: As Arctic permafrost thaws, ancient organic matter decomposes, releasing both CO₂ and CH₄ — a dangerous climate feedback loop

Although methane’s atmospheric lifetime is relatively short (~12 years), its high GWP makes it a potent short-term warming agent. Reducing methane emissions is considered one of the fastest ways to slow the rate of warming.

Water Vapour (H₂O)

Water vapour is the most abundant greenhouse gas and contributes the largest share to the natural greenhouse effect. However, the atmospheric concentration of water vapour is not directly controlled by human emissions — it is governed by temperature through the Clausius-Clapeyron relationship (warmer air holds more moisture).

Water vapour acts as a positive feedback mechanism: as CO₂ and other greenhouse gases warm the atmosphere, the atmosphere can hold more water vapour, which in turn traps more heat, amplifying the initial warming. This feedback approximately doubles the sensitivity of the climate to CO₂ increases.

Nitrous Oxide (N₂O)

Nitrous oxide is released from agricultural soils treated with nitrogen-based fertilisers. Soil bacteria carry out nitrification and denitrification processes that produce N₂O as a by-product. Additional sources include industrial production of nylon and nitric acid, and biomass burning. N₂O has a long atmospheric lifetime and also contributes to stratospheric ozone depletion.

Carbon Footprint

The carbon footprint of a product, service, organisation, or individual is the total mass of greenhouse gases (expressed as CO₂ equivalent, CO₂e) emitted over the full life cycle of that entity. It includes all stages from raw material extraction, through manufacturing and transport, to use and final disposal.

For example, the carbon footprint of a plastic bottle includes:

• Extraction and refining of crude oil to produce the plastic feedstock
• Energy used in manufacturing the bottle
• Transport of the empty bottle to the filling plant
• Transport of the filled bottle to retailers
• Refrigeration at the retail location
• Disposal or recycling of the used bottle

Carbon footprints can be calculated for individuals (lifestyle choices — diet, travel, home energy), organisations (corporate emissions), products (life cycle analysis), and activities (e.g. the carbon footprint of a single flight).

Effects of Climate Change

The enhanced greenhouse effect is driving a range of interconnected environmental changes:

Rising global temperatures: The global average surface temperature has risen by approximately 1.1–1.2 °C since the pre-industrial period (1850–1900). The rate of warming has accelerated in recent decades, with the warmest years on record concentrated in the most recent period.

Sea level rise: Two mechanisms contribute:

• Thermal expansion: As ocean water warms, it expands in volume. This is currently the dominant contributor to sea level rise.
• Melting of land-based ice: Glaciers and ice sheets (Greenland, Antarctica) are losing mass at an accelerating rate. The meltwater adds to ocean volume. (Melting of floating sea ice does not directly raise sea level — it already displaces its own weight in water — but it does contribute indirectly by reducing albedo, as darker ocean water absorbs more solar radiation than reflective ice.)

Global mean sea level has risen by approximately 21–24 cm since 1880, and the rate of rise is increasing.

More frequent and intense extreme weather events: A warmer atmosphere holds more energy and more moisture. This increases the frequency and severity of:

• Heatwaves and droughts
• Heavy rainfall and flooding events
• Intense tropical cyclones (hurricanes, typhoons)
• Wildfires (drier vegetation provides more fuel)

Changes in precipitation patterns: Some regions are becoming wetter (higher latitudes, some tropical regions), while others are becoming drier (subtropics, including the Mediterranean, southern Africa, and parts of Australia). This affects freshwater availability and agricultural productivity.

Desertification: Rising temperatures and reduced rainfall in already arid and semi-arid regions accelerate the degradation of land into desert. Overgrazing and deforestation compound the problem by removing vegetation that stabilises soil.

Loss of biodiversity: Species must either adapt, migrate to more suitable climates, or face extinction. Many species cannot move or adapt quickly enough to keep pace with the rate of climate change. Coral reefs are particularly vulnerable: elevated water temperatures cause coral bleaching (expulsion of symbiotic algae), and ocean acidification impairs the ability of corals to build calcium carbonate skeletons.

Ocean acidification: Approximately 25–30% of anthropogenic CO₂ emissions have been absorbed by the oceans. Dissolved CO₂ reacts with water to form carbonic acid:

[ \text{CO}_2\text{ (g)} + \text{H}_2\text{O (l)} \rightleftharpoons \text{H}_2\text{CO}_3\text{ (aq)} ]

Carbonic acid dissociates, releasing H⁺ ions and lowering ocean pH:

[ \text{H}_2\text{CO}_3\text{ (aq)} \rightleftharpoons \text{H}^+\text{ (aq)} + \text{HCO}_3^-\text{ (aq)} ]

Since the Industrial Revolution, the average pH of ocean surface waters has fallen from approximately 8.2 to 8.1, corresponding to a roughly 30% increase in hydrogen ion concentration (acidity). This acidification reduces the availability of carbonate ions (CO₃²⁻), which marine organisms — including corals, molluscs, and some plankton — require to build their calcium carbonate (CaCO₃) shells and skeletons. In sufficiently acidified water, CaCO₃ structures may even begin to dissolve.

Reducing Carbon Footprints

Strategies to reduce greenhouse gas emissions fall into several broad categories:

Key Concepts from Past Papers

• carbon dioxide
• mark each for any 2 of: carbon monoxide carbon water
• calcium oxide carbon dioxide
• carbon dioxide water
• mark each for any two of: plastics sewage microbes
• carbon carbon dioxide
• mark each for any two of: catalytic converters low sulfur fuels flue gas desulfurisation
• mark each for any 2 of:

Keywords from Past Papers

carbon, dioxide, monoxide, global, warming, climate, change, water, increased, gas, ice, melt, rise, sea, desertification

Related Notes

• Composition of Air
• Air Pollution
• Water Treatment

Sources

• OpenStax, Chemistry 2e, Chapter 9: Gases — Section 9.1, gas pressure and the greenhouse effect; Chapter 5: Thermochemistry — energy and climate. https://openstax.org/books/chemistry-2e/
• BBC Bitesize, GCSE Chemistry (Single Science), “The Greenhouse Effect” and “Climate Change.” https://www.bbc.co.uk/bitesize/topics/z9k3qyc
• Cambridge IGCSE Chemistry 0620 Syllabus, Topic 11: Air and Water — greenhouse gases, carbon footprint, effects of climate change.
• CK-12 Foundation, Chemistry, Chapter: Atmosphere — greenhouse effect, global warming, and climate. https://www.ck12.org/chemistry/

Past Paper Sources

• 0620/31 May/June 2015: Q11(c)(ii) (0m)
• 0620/32 Feb/March 2015: Q66(c)(i) (2m), Q11(a)(ii) (1m)
• 0620/32 Feb/March 2017: Q33(b)(iii) (1m), Q22(e)(ii) (0m), Q33(b)(iii) (1m)
• 0620/32 Feb/March 2018: Q11(a)(iv) (1m)
• 0620/32 Feb/March 2019: Q77(b)(ii) (2m), Q55(d)(iii) (1m)
• 0620/32 Feb/March 2022: Q55(b)(ii) (4m), Q22(a)(ii) (2m)
• 0620/32 May/June 2018: Q11(a)(iv) (1m)
• 0620/32 May/June 2020: Q11(a)(i) (1m), Q88(e)(ii) (0m)
• 0620/33 May/June 2016: Q88(c)(i) (1m), Q55(b)(ii) (0m)
• 0620/33 May/June 2019: Q33(b)(i) (1m)
• 0620/33 May/June 2022: Q22(d)(ii) (2m), Q44(c)(i) (1m)
• 0620/33 May/June 2024: Q44(a)(ii) (2m), Q66(e)(ii) (1m), Q88(b)(i) (0m)

Common Misconceptions

• The greenhouse effect is a bad thing caused by humans. The natural greenhouse effect is essential for life on Earth — without it, the planet would be frozen. The problem is the enhanced greenhouse effect caused by the rapid increase in greenhouse gas concentrations from human activities.
• The ozone hole and the greenhouse effect are the same thing. They are distinct phenomena. Ozone depletion involves the breakdown of stratospheric O₃ by CFCs, increasing UV radiation reaching the surface. The greenhouse effect involves the trapping of IR radiation by gases such as CO₂ and CH₄. While CFCs are both ozone depleters and potent greenhouse gases, the two environmental problems have different causes and mechanisms.
• CO₂ is the most powerful greenhouse gas. CO₂ is the most significant by total warming contribution because of its volume, not its potency per molecule. Methane, N₂O, and CFCs all have far higher GWPs per kg than CO₂. Conversely, water vapour is the largest contributor to the natural greenhouse effect but acts as a feedback rather than a direct driver.
• If global warming is real, why are some winters so cold? Climate refers to long-term trends (decades to centuries), not individual weather events. A cold winter in one region does not contradict a global warming trend. Additionally, climate change can disrupt atmospheric circulation patterns (e.g. the polar vortex), causing extreme cold outbreaks in some regions even as the global average rises.
• Melting sea ice raises sea levels. Floating ice displaces its own weight in water (Archimedes’ principle). When floating sea ice melts, the water level does not rise. Sea level rise is driven by thermal expansion of the oceans and the melting of land-based ice (glaciers, ice sheets). Melting sea ice contributes indirectly by reducing albedo.
• Ocean acidification means the oceans will become acidic (pH below 7). Ocean pH has fallen from ~8.2 to ~8.1 — still alkaline (above 7). The term “acidification” refers to the direction of pH change (becoming less alkaline, i.e. moving toward neutral), not that the oceans are expected to cross into the acidic range. Nevertheless, this shift in carbonate chemistry has profound biological effects.
• Carbon footprint only includes CO₂ emissions. A carbon footprint includes all greenhouse gases, expressed as CO₂ equivalents (CO₂e). Methane, N₂O, and fluorinated gases are included and weighted by their GWP.
• Planting trees alone can solve climate change. Reforestation is important but cannot offset current fossil fuel emissions alone. The amount of CO₂ released annually by fossil fuel combustion far exceeds what could be captured by reforestation on available land. Emission reductions at source remain essential.