Greenhouse Gases and Climate Change

This lesson covers greenhouse gases, the greenhouse effect and climate change as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand which gases contribute to the greenhouse effect, how human activities are increasing their concentrations, and the potential consequences.

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What Is the Greenhouse Effect?

The greenhouse effect is a natural process that keeps the Earth warm enough to support life. Without it, the average surface temperature would be about −18 °C instead of the current +15 °C.

How It 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 surface re-emits energy as long-wavelength infrared (IR) radiation.
4. Greenhouse gases in the atmosphere absorb some of this outgoing infrared radiation.
5. The absorbed energy is re-emitted in all directions — some back towards the Earth's surface.
6. This keeps the Earth warmer than it would otherwise be.

graph TD
    SUN["☀️ Sun"] -->|"Short-wavelength<br/>radiation (UV/visible)"| ATM["Atmosphere"]
    ATM -->|"Passes through"| EARTH["🌍 Earth’s Surface"]
    EARTH -->|"Absorbs radiation<br/>and warms up"| IR["Emits long-wavelength<br/>infrared radiation"]
    IR -->|"Some escapes<br/>to space"| SPACE["Space"]
    IR -->|"Some absorbed by<br/>greenhouse gases"| GHG["Greenhouse Gases<br/>CO₂, CH₄, H₂O"]
    GHG -->|"Re-emitted in all<br/>directions, including<br/>back to Earth"| EARTH

    style SUN fill:#f9a825,color:#000
    style ATM fill:#90caf9,color:#000
    style EARTH fill:#4caf50,color:#fff
    style IR fill:#e53935,color:#fff
    style SPACE fill:#1a1a2e,color:#fff
    style GHG fill:#ff7043,color:#fff

Exam Tip: The greenhouse effect itself is not a bad thing — it is essential for life. The problem is the enhanced (increased) greenhouse effect caused by human activities increasing greenhouse gas concentrations.

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The Main Greenhouse Gases

Greenhouse Gas	Formula	Sources
Carbon dioxide	CO₂	Burning fossil fuels, deforestation, respiration
Methane	CH₄	Cattle farming (digestion), rice paddies, landfill sites, natural gas leaks
Water vapour	H₂O	Evaporation from oceans, lakes; not directly caused by human activity in the same way

Which Has the Biggest Effect?

• Water vapour is the most abundant greenhouse gas but its concentration is not directly controlled by human activity.
• Carbon dioxide is the most significant greenhouse gas linked to human activity because of the huge quantities released by burning fossil fuels.
• Methane is a more potent greenhouse gas than CO₂ (molecule for molecule) but is present in lower concentrations.

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Human Activities Increasing Greenhouse Gases

Carbon Dioxide (CO₂)

Activity	How It Increases CO₂
Burning fossil fuels	Combustion of coal, oil and gas releases CO₂ that was locked underground for millions of years
Deforestation	Fewer trees to absorb CO₂ by photosynthesis; burning/decomposing trees releases stored carbon
Industrial processes	Cement manufacturing releases CO₂ from thermal decomposition of limestone

Methane (CH₄)

Activity	How It Increases CH₄
Agriculture	Cattle and sheep produce methane during digestion; rice paddies release methane from waterlogged soil
Landfill sites	Decomposition of waste by anaerobic bacteria releases methane
Extraction of fossil fuels	Methane leaks during mining of coal and extraction of natural gas

Exam Tip: When explaining how human activities increase CO₂, always mention burning fossil fuels first — this is the single biggest contributor. Also mention deforestation (both the loss of photosynthesis and the release of stored carbon).

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Climate Change — Consequences

The enhanced greenhouse effect is causing global warming — a gradual increase in the Earth's average temperature. This leads to climate change, which has wide-ranging consequences:

Effects on the Environment

Consequence	Explanation
Sea level rise	Ice caps and glaciers melt; thermal expansion of water → flooding of low-lying areas
More extreme weather	Increased frequency of storms, hurricanes, droughts and floods
Changes in rainfall patterns	Some regions get more rain; others experience drought
Ocean acidification	More CO₂ dissolves in oceans → more carbonic acid → lower pH → harms coral and shellfish
Loss of biodiversity	Habitats change faster than organisms can adapt; species may become extinct
Glacier and ice cap retreat	Reduced ice coverage at poles and on mountains

Effects on Humans

• Food security — changing climates affect crop yields and farming.
• Water supply — droughts reduce water availability in some regions.
• Migration — people forced to leave areas affected by flooding or drought.
• Health — increased spread of tropical diseases to new areas; heat-related illness.

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The Scientific Consensus

• The vast majority of climate scientists agree that human activities are causing the current rapid increase in global temperatures.
• This is based on extensive evidence: ice-core data, temperature records, satellite measurements and climate modelling.
• However, the exact scale and timing of future changes involves some uncertainty — climate models vary in their predictions.
• The peer review process ensures that scientific claims are checked by other experts before being published.

Exam Tip: If asked about uncertainties, explain that while the scientific community is very confident that humans are causing climate change, the exact extent of future impacts is harder to predict because climate systems are complex and models make different assumptions.

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Summary

• The greenhouse effect is a natural process where greenhouse gases trap infrared radiation, keeping Earth warm.
• The main greenhouse gases are CO₂, CH₄ and water vapour.
• Human activities — mainly burning fossil fuels and deforestation — are increasing CO₂ and CH₄ levels.
• The enhanced greenhouse effect is causing global warming and climate change.
• Consequences include sea level rise, extreme weather, ocean acidification, habitat loss and threats to food/water security.
• There is strong scientific consensus that human activities are the main cause.

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Worked Examples

Worked Example 1 — Identifying greenhouse gases

Question: Which of the following are greenhouse gases? N₂, O₂, CO₂, H₂O (vapour), CH₄, Ar.

Answer: CO₂, H₂O (vapour) and CH₄ are greenhouse gases. They absorb infrared radiation because their molecules can vibrate in ways that interact with IR photons. N₂, O₂ and Ar are not greenhouse gases under normal conditions.

Worked Example 2 — Explaining the greenhouse effect in steps

Question: Explain, step by step, how the greenhouse effect keeps Earth warm.