Greenhouse Gases and Climate Change

This lesson covers greenhouse gases, the greenhouse effect, and climate change, as required by AQA GCSE Chemistry specification (5.9.2). You need to understand which gases are greenhouse gases, how the greenhouse effect works, the evidence for climate change, and why it is a cause for concern. This is one of the most frequently examined topics in the Chemistry of the Atmosphere unit.

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What Are Greenhouse Gases?

Greenhouse gases are gases in the atmosphere that absorb and re-emit infrared (heat) radiation, trapping thermal energy in the atmosphere and keeping the Earth warm. Without greenhouse gases, the average temperature of the Earth would be about -18°C instead of the current average of about 15°C.

The main greenhouse gases are:

Greenhouse Gas	Chemical Formula	Main Sources
Carbon dioxide	CO₂	Burning fossil fuels, deforestation, respiration, volcanic activity
Methane	CH₄	Cattle farming (enteric fermentation), rice paddies, landfill sites, natural gas extraction
Water vapour	H₂O	Evaporation from oceans, lakes, and rivers; transpiration from plants

Relative Contributions

Although methane is a much more potent greenhouse gas than carbon dioxide (about 25 times more effective at trapping heat per molecule over 100 years), carbon dioxide contributes more to the total greenhouse effect because it is present in much larger quantities and persists in the atmosphere for much longer.

Exam Tip: The three greenhouse gases you must know are carbon dioxide, methane, and water vapour. Make sure you can give at least two sources of each. Examiners often ask for specific sources, so avoid vague answers like "pollution."

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The Greenhouse Effect

The greenhouse effect is a natural process that warms the Earth's surface. It works as follows:

1. Short-wave radiation (visible light and ultraviolet) 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 surface re-emits energy as long-wave infrared radiation (heat).
4. Greenhouse gases in the atmosphere absorb some of this infrared radiation.
5. The greenhouse gases then re-emit the infrared radiation in all directions — some goes back towards the Earth's surface, some goes out to space.
6. The radiation directed back towards the surface causes additional warming.

flowchart TD
    A["Sun emits short-wave<br/>radiation (visible light)"] --> B["Passes through<br/>atmosphere"]
    B --> C["Absorbed by<br/>Earth’s surface"]
    C --> D["Surface re-emits<br/>long-wave infrared radiation"]
    D --> E["Some IR escapes<br/>to space"]
    D --> F["Some IR absorbed by<br/>greenhouse gases"]
    F --> G["Re-emitted in<br/>all directions"]
    G --> H["Some IR directed back<br/>to Earth’s surface"]
    H --> I["Additional warming<br/>of Earth’s surface"]

    style A fill:#f39c12,color:#fff
    style F fill:#e74c3c,color:#fff
    style I fill:#c0392b,color:#fff

The Natural Greenhouse Effect

The natural greenhouse effect is essential for life on Earth. Without it, the planet would be too cold to support liquid water or most forms of life. The problem arises when human activities increase the concentration of greenhouse gases, enhancing the greenhouse effect and causing additional warming beyond the natural level.

Exam Tip: The greenhouse effect is NOT the same as global warming. The greenhouse effect is a natural, essential process. Global warming (or more accurately, climate change) is the enhancement of the greenhouse effect caused by increased greenhouse gas concentrations due to human activities.

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Evidence for Climate Change

Climate change refers to long-term changes in global temperatures and weather patterns. While the climate has changed naturally throughout Earth's history, the current period of rapid warming is attributed primarily to human activities.

Evidence That the Climate Is Changing

Type of Evidence	What It Shows
Temperature records	Global average temperatures have risen by approximately 1.1°C since pre-industrial times (1850–1900). The rate of warming has accelerated since the mid-20th century.
Ice core data	Air bubbles trapped in ice cores show that CO₂ and temperature have been closely correlated for at least 800,000 years. Current CO₂ levels are the highest in this period.
Sea level rise	Global sea levels have risen by approximately 20 cm since 1900, due to thermal expansion of water and melting of ice sheets and glaciers.
Glacier retreat	Glaciers around the world have been shrinking since the late 19th century.
Arctic sea ice decline	The extent and thickness of Arctic sea ice has decreased significantly since satellite measurements began in 1979.
Changes in wildlife	Many species are shifting their ranges towards the poles or to higher altitudes. Flowering times and migration patterns are changing.

The Link Between CO₂ and Temperature

Ice core data shows a strong correlation between atmospheric CO₂ concentration and global temperature over the past 800,000 years. During ice ages, CO₂ levels were low (about 180 ppm). During warmer interglacial periods, CO₂ levels were higher (about 280 ppm). The current CO₂ level is approximately 420 ppm — far higher than at any point in the past 800,000 years.

Exam Tip: Correlation does not prove causation. Scientists have established the mechanism by which CO₂ causes warming (the greenhouse effect), which is why the scientific consensus is that increased CO₂ is causing climate change. If an exam question asks about "peer-reviewed evidence," mention both the correlation in ice core data AND the mechanism.

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Human Activities That Increase Greenhouse Gases

Carbon Dioxide

Human activities have increased atmospheric CO₂ from about 280 ppm (pre-industrial) to about 420 ppm (present day). The main sources are:

1. Burning fossil fuels — coal, oil, and natural gas combustion for energy, transport, and industry.
2. Deforestation — removing trees reduces the amount of CO₂ absorbed by photosynthesis. Burning or decomposing the cleared vegetation also releases stored carbon.
3. Cement production — heating limestone (CaCO₃) to make cement releases CO₂: CaCO₃ → CaO + CO₂

Methane

Atmospheric methane levels have more than doubled since pre-industrial times. The main sources are:

1. Agriculture — cattle and other ruminants produce methane during digestion (enteric fermentation). Rice paddies also produce methane due to anaerobic decomposition of organic matter in waterlogged soil.
2. Landfill sites — decomposition of organic waste under anaerobic conditions produces methane.
3. Fossil fuel extraction — natural gas is primarily methane, and it can leak during extraction, processing, and transport.
4. Wetlands — natural wetlands produce methane through anaerobic decomposition, but drainage and land-use changes also affect emissions.

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Potential Consequences of Climate Change

Climate change could have severe consequences for the planet and for human societies: