Climate Change and Greenhouse Gases

This lesson covers the greenhouse effect, greenhouse gases, and climate change as required by the Edexcel GCSE Chemistry specification (1CH0). You need to understand how greenhouse gases keep the Earth warm, how human activities are increasing their concentrations, the evidence for climate change, and why there is debate about the extent of human impact.

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The Greenhouse Effect

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

How It Works

1. Short-wavelength 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 energy and warms up.
3. The warm surface re-emits the energy as long-wavelength 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 out to space, but some is radiated back towards the Earth.
6. This keeps the Earth warmer than it would otherwise be.

Exam Tip: The greenhouse effect is a natural and essential process. The problem is the enhanced greenhouse effect — the increase in greenhouse gas concentrations due to human activities, which causes additional warming.

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The Main Greenhouse Gases

The three main greenhouse gases you need to know are:

Greenhouse gas	Formula	Natural sources	Human sources
Carbon dioxide	CO₂	Respiration, volcanic activity, decomposition	Burning fossil fuels, deforestation
Methane	CH₄	Wetlands, termites, natural gas seepage	Agriculture (rice paddies, cattle), landfill, fossil fuel extraction
Water vapour	H₂O	Evaporation from oceans, lakes, rivers	Not directly increased by human activity (but warming increases evaporation)

How Human Activities Increase Greenhouse Gases

Carbon dioxide (CO₂):

Activity	How it increases CO₂
Burning fossil fuels	Coal, oil and natural gas release CO₂ when burned for energy, transport and industry
Deforestation	Cutting down and burning forests releases the carbon stored in trees; fewer trees mean less CO₂ is removed by photosynthesis
Cement production	Heating limestone (CaCO₃) to make cement releases CO₂: CaCO₃ → CaO + CO₂

Methane (CH₄):

Activity	How it increases CH₄
Agriculture	Rice paddies (anaerobic conditions) and cattle (digestive systems of ruminants produce methane)
Landfill	Decomposition of organic waste under anaerobic conditions
Fossil fuel extraction	Natural gas (methane) leaks during extraction and transport

Exam Tip: Although methane is present in much smaller concentrations than CO₂, it is a more potent greenhouse gas — each molecule of methane absorbs significantly more infrared radiation than a molecule of CO₂.

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The Enhanced Greenhouse Effect

The enhanced greenhouse effect refers to the additional warming caused by the increase in greenhouse gas concentrations due to human activities.

• Since the Industrial Revolution (~1750), CO₂ levels have risen from about 280 ppm to over 420 ppm (parts per million).
• Methane concentrations have more than doubled.
• This increase means the atmosphere traps more infrared radiation, causing the Earth to warm.

This additional warming is the primary cause of climate change.

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Evidence for Climate Change

Multiple independent lines of evidence show that the Earth's climate is changing:

Evidence	Detail
Rising global temperatures	Average global surface temperature has risen by about 1.1 °C since pre-industrial times; the last decade was the warmest on record
Ice core data	Air bubbles trapped in ancient ice show a strong correlation between CO₂ levels and temperature over the last 800,000 years
Melting ice caps and glaciers	Arctic sea ice extent has decreased significantly; glaciers worldwide are retreating
Rising sea levels	Global sea levels have risen by about 20 cm since 1900, due to thermal expansion of water and melting of land ice
Changes in weather patterns	More frequent and severe heatwaves, storms and flooding in many regions
Earlier spring events	Plants flowering earlier; bird migration patterns changing

Exam Tip: If asked for evidence of climate change, give specific, measurable data — e.g. "global temperatures have risen by approximately 1.1 °C since pre-industrial times" is better than "it's getting warmer."

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Consequences of Climate Change

Consequence	Detail
Rising sea levels	Melting of ice sheets (Greenland, Antarctica) and thermal expansion of water → flooding of low-lying coastal areas and islands
Extreme weather	More frequent and intense heatwaves, droughts, storms and heavy rainfall events
Habitat loss	Changing temperatures and rainfall patterns threaten ecosystems; coral bleaching due to warmer oceans
Food security	Changed growing conditions affect crop yields; droughts and floods reduce food production in vulnerable regions
Species migration and extinction	Species may not be able to adapt quickly enough to changing conditions
Water availability	Altered rainfall patterns affect freshwater supplies; melting glaciers threaten water sources for billions of people

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Why Is There Debate?

While the vast majority of climate scientists agree that human activities are the dominant cause of recent warming, there is some debate about certain aspects:

Area of debate	Detail
Extent of human impact	Some question how much of the warming is due to humans versus natural cycles (e.g. solar activity, volcanic eruptions)
Data interpretation	Different models may give different predictions about future temperature rises; uncertainty in measurements
Media and politics	The scientific consensus may be misrepresented; economic interests can influence the debate
Peer review	Scientific findings are published in peer-reviewed journals, where they are scrutinised by other experts. This process helps ensure reliability but takes time, and early results may later be revised

The Scientific Consensus

It is important to understand that the scientific consensus (as represented by the IPCC — the Intergovernmental Panel on Climate Change) is very clear:

• The Earth is warming.
• Human activities (primarily burning fossil fuels) are the dominant cause of warming observed since the mid-20th century.
• The evidence is overwhelming and comes from multiple independent sources.

Exam Tip: The Edexcel specification expects you to understand that peer review is essential for validating scientific claims. Be prepared to explain what peer review is (independent experts checking the methods and conclusions of a study) and why it matters (ensures reliability and identifies errors).

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The Carbon Cycle — Brief Link

The movement of carbon between the atmosphere, oceans, rocks and living organisms is called the carbon cycle. Human activities (burning fossil fuels, deforestation) are adding carbon to the atmosphere faster than natural processes can remove it — this is why CO₂ levels are rising.

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