Greenhouse Effect Chemistry

The greenhouse effect is a core topic in the Cambridge 0620 environmental chemistry section. Examiners test the mechanism, the distinction between the natural and enhanced greenhouse effects, the sources of greenhouse gases, and the consequences of climate change.

How the greenhouse effect works

1. Short-wavelength radiation (visible light and UV) from the Sun passes through the atmosphere and reaches the Earth’s surface
2. The Earth’s surface absorbs this radiation and warms up
3. The warm Earth re-emits energy as long-wavelength infrared (IR) radiation
4. Greenhouse gases in the atmosphere absorb some of this infrared radiation
5. The greenhouse gases re-emit the infrared radiation in all directions — including back towards the Earth’s surface
6. This traps heat in the atmosphere, keeping the Earth warm

Without the natural greenhouse effect, the Earth’s average temperature would be about -18 °C instead of the current +15 °C. Life as we know it would not exist.

Greenhouse gases

Gas	Formula	Main source	Relative greenhouse effect
Carbon dioxide	CO₂	Burning fossil fuels, deforestation, respiration	Moderate per molecule, but very high total contribution
Methane	CH₄	Agriculture (rice paddies, cattle), landfill sites, natural gas leaks	Strong per molecule (~25x CO₂ over 100 years)
Water vapour	H₂O	Evaporation from oceans, lakes	Largest natural greenhouse gas
Nitrous oxide	N₂O	Agriculture (fertilisers), combustion	Strong per molecule (~300x CO₂)

At IGCSE level, CO₂ and CH₄ are the two greenhouse gases you must know in detail. Water vapour is important but is part of the natural cycle and not directly increased by human activity in the same way.

The enhanced greenhouse effect

The enhanced greenhouse effect is the additional warming caused by human activities increasing the concentration of greenhouse gases above natural levels.

Sources of increased CO₂

• Burning fossil fuels (coal, oil, natural gas) for energy, transport, and industry
• Deforestation — reduces photosynthesis (less CO₂ removed) and releases stored carbon
• Cement production — thermal decomposition of limestone (CaCO₃ → CaO + CO₂)

Sources of increased CH₄

• Rice paddies — anaerobic decomposition of organic matter in flooded fields
• Cattle farming — methane produced during digestion
• Landfill sites — decomposition of organic waste
• Natural gas extraction and transport — methane leaks

Malaysia’s palm oil plantations and rice paddies are relevant local examples of agricultural methane and CO₂ sources.

Consequences of climate change

Consequence	Detail
Rising global temperatures	Average temperatures increase, leading to heatwaves
Melting ice caps and glaciers	Reduces ice cover in polar regions
Rising sea levels	Thermal expansion of water + meltwater from ice; threatens coastal cities and low-lying islands
More extreme weather	Stronger storms, heavier rainfall, prolonged droughts
Ocean acidification	More CO₂ dissolves in seawater, forming carbonic acid (H₂CO₃), lowering pH
Habitat loss	Coral bleaching, shift of climate zones, threat to biodiversity
Agricultural impacts	Changes in rainfall patterns affect crop yields

For Malaysia, rising sea levels threaten coastal areas and islands, while changing rainfall patterns could affect agriculture and water supply.

Reducing the enhanced greenhouse effect

Strategy	How it helps
Use renewable energy (solar, wind, hydroelectric)	No CO₂ produced during generation
Improve energy efficiency	Less fuel burned per unit of useful work
Plant trees (reforestation)	More CO₂ removed by photosynthesis
Reduce deforestation	Preserves existing carbon sinks
Carbon capture and storage	CO₂ from power stations trapped and stored underground
Use public transport / electric vehicles	Fewer individual emissions
Reduce methane emissions	Better waste management, rice farming techniques

Greenhouse effect vs ozone depletion

These are two separate issues that candidates frequently confuse:

	Greenhouse effect	Ozone depletion
Location	Troposphere (lower atmosphere)	Stratosphere (upper atmosphere)
Cause	CO₂, CH₄ (greenhouse gases)	CFCs releasing chlorine
Problem	Traps heat — global warming	Allows UV through — skin cancer
Solution	Reduce fossil fuel use	Phase out CFCs (Montreal Protocol)

Worked exam question

Explain how the burning of fossil fuels contributes to the enhanced greenhouse effect. [3]

Burning fossil fuels produces carbon dioxide [1]. CO₂ is a greenhouse gas — it absorbs infrared radiation re-emitted by the Earth’s surface [1]. The increased concentration of CO₂ in the atmosphere means more infrared radiation is trapped/re-emitted back to Earth, causing additional warming [1].

State two consequences of global warming. [2]

Rising sea levels due to melting ice caps and thermal expansion of water [1]. More extreme weather events / changes in rainfall patterns affecting agriculture [1].

Common exam mistakes

1. Saying the greenhouse effect is entirely bad — the natural greenhouse effect is essential for life. It is the enhanced greenhouse effect (caused by human activity) that is problematic.
2. Confusing the greenhouse effect with ozone depletion — they are different problems caused by different gases.
3. Saying CO₂ “traps heat like glass in a greenhouse” without explaining the mechanism — state that CO₂ absorbs and re-emits infrared radiation.
4. Writing “pollution” as a consequence of the greenhouse effect — pollution causes the enhanced greenhouse effect; the consequences are climate-related (temperature rise, sea level rise, extreme weather).