How do greenhouse gases cause climate change, and what pollutants come from burning fuels?
The greenhouse effect and human causes of climate change, the products of combustion, and the pollutants from burning fuels and their effects.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on climate and air quality, covering the greenhouse effect and human-caused climate change, the products of complete and incomplete combustion, and the pollutants from burning fuels.
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What this dot point is asking
WJEC Double Award Unit 2 wants you to explain the greenhouse effect and human-caused climate change, give the products of combustion, and describe the pollutants from burning fuels and their effects.
The greenhouse effect
The main greenhouse gases are carbon dioxide, methane and water vapour. Some greenhouse effect is natural and necessary, but too much causes problems.
Human-caused climate change
The evidence comes from measurements of rising carbon dioxide levels and global temperatures over recent decades.
Products of combustion
When a hydrocarbon fuel burns:
- Complete combustion (plenty of oxygen) produces carbon dioxide and water, and releases the most energy.
- Incomplete combustion (limited oxygen) produces carbon monoxide (a toxic gas) and carbon (soot), and releases less energy.
Pollutants from burning fuels
- Carbon dioxide: a greenhouse gas contributing to climate change.
- Carbon monoxide: toxic, colourless and odourless; binds to haemoglobin and reduces oxygen transport.
- Soot (carbon particles): causes breathing problems and blackens buildings.
- Sulfur dioxide: from sulfur impurities in the fuel; dissolves in rain to form acid rain, which damages plants, lakes and buildings.
- Oxides of nitrogen: form at high temperatures in engines; also contribute to acid rain and breathing problems.
Reducing the pollution
There are ways to reduce these problems, and exam questions often ask for them. Burning less fossil fuel, using renewable energy sources, and improving efficiency all cut carbon dioxide. Removing sulfur from fuels before burning, or removing sulfur dioxide from waste gases ("flue-gas desulfurisation"), reduces acid rain. Catalytic converters on cars change carbon monoxide and oxides of nitrogen into less harmful gases. Planting trees absorbs carbon dioxide. Being able to suggest a measure and say which pollutant it tackles is a common higher-mark task.
Why carbon dioxide levels are rising
Measurements show that the carbon dioxide concentration in the atmosphere has risen sharply since the industrial revolution. This is because humans burn huge amounts of fossil fuels (coal, oil and gas) for electricity, transport and industry, releasing stored carbon as carbon dioxide far faster than photosynthesis and the oceans can remove it. At the same time, deforestation removes trees that would take in carbon dioxide. The result is a build-up of carbon dioxide that enhances the greenhouse effect, which is the scientific basis for concern about climate change.
Try this
Q1. Name the two products of the complete combustion of a hydrocarbon. [2 marks]
- Cue. Carbon dioxide and water.
Q2. Which pollutant gas causes acid rain? [1 mark]
- Cue. Sulfur dioxide (oxides of nitrogen also contribute).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain how human activities are increasing the greenhouse effect and what this is causing.Show worked answer →
A Unit 2 explain question worth 4 marks. Reward: greenhouse gases such as carbon dioxide and methane trap heat radiated from the Earth, keeping the planet warm (1); human activities such as burning fossil fuels and deforestation increase the amount of carbon dioxide (1); this enhances the greenhouse effect and causes global warming (1); leading to climate change such as rising sea levels and more extreme weather (1). Markers credit how greenhouse gases work, the human cause, the warming and the consequences. A common error is to confuse the greenhouse effect with the ozone hole.
WJEC style3 marksWhen a fuel burns with too little oxygen, carbon monoxide can form. Explain why carbon monoxide is dangerous.Show worked answer →
A Unit 2 explain question. Reward: carbon monoxide is a toxic gas (1); it is colourless and odourless, so it cannot be detected easily (1); it binds to haemoglobin in red blood cells, reducing the oxygen the blood can carry, which can cause unconsciousness or death (1). Markers credit it being toxic, hard to detect, and reducing oxygen transport. A common error is to say it is dangerous only because it is flammable.
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