Edexcel IGCSE Biology · Spec 4.12-4.15
Air Pollution and Global Warming
The causes and effects of air pollution, acid rain, and the greenhouse effect and global warming.
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Air Pollution and Global Warming
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Explained
Air pollution, acid rain, and the greenhouse effect
Three separate problems get muddled together here, and the first job is to keep them apart. Sulfur dioxide causes acid rain. Carbon monoxide poisons people. Carbon dioxide and methane cause global warming. They are different gases doing different things.
Sulfur dioxide and acid rain
Fossil fuels contain sulfur as an impurity. When they burn, the sulfur burns too, forming sulfur dioxide, which rises into the atmosphere.
There it dissolves in the water in clouds and reacts to form sulfuric acid, which falls as acid rain. Nitrogen oxides, formed in vehicle engines where the heat makes nitrogen and oxygen react, do the same thing and form nitric acid.
Acid rain damages trees, kills fish by lowering the pH of lakes and rivers, and corrodes buildings and statues made of limestone.
Carbon monoxide
Carbon monoxide is produced by incomplete combustion, which happens when there is not enough oxygen for the fuel to burn fully.
It is toxic because it binds to haemoglobin in red blood cells, and it binds more strongly than oxygen does. Haemoglobin carrying carbon monoxide cannot carry oxygen, so less oxygen reaches the tissues.
It is odourless and colourless, which is why faulty heaters are dangerous and why the warning is a detector rather than your senses.
The greenhouse effect
Short wavelength radiation from the Sun passes through the atmosphere and warms the Earth's surface. The surface then re-radiates energy at longer wavelengths, as infrared.
Greenhouse gases absorb that outgoing infrared and re-radiate some of it back towards the surface, so less escapes to space. The planet stays warmer than it otherwise would.
This is a natural process, and without it the Earth would be far too cold for life. The problem is that burning fossil fuels and clearing forests have raised the concentrations of carbon dioxide and methane, so more energy is trapped and average temperatures rise. That is the enhanced greenhouse effect, and it drives global warming.
The consequences include melting ice and rising sea levels, changing rainfall patterns and more frequent extreme weather, and species having to shift their range or die out.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks how acid rain forms and its effects, for three marks, and sets out the chain in four steps: sulfur dioxide comes from burning fossil fuels, it reacts with or dissolves in water in clouds, this forms sulfuric acid, and the acid kills or damages organisms, plants, animals or forests.
Each arrow in that chain is a separate mark. Writing that burning fuel causes acid rain compresses four steps into one and earns a fraction of what is available.
On a neighbouring question about the benefits of electric vehicles, the same mark scheme lists the credited points separately: less burning of fossil fuels, less carbon dioxide, less global warming or climate change, less release of sulfur dioxide or nitrous oxide, less noise pollution, less particulate pollution, and greater efficiency. It then allows the converse for petrol and diesel.
Notice how many distinct pollutants are named there. A three mark question of this kind is asking for three different effects, not for one effect explained three ways.
Keeping the gases straight
If a question mentions lakes, forests or limestone buildings, it is about acid rain, so the gas is sulfur dioxide. If it mentions the blood or a faulty heater, it is carbon monoxide. If it mentions temperature, ice or sea level, it is carbon dioxide and methane.
Reading the effect first and working back to the gas is more reliable than trying to recall the whole topic at once.
Spec 4.12-4.15
What you need to know
- Describe pollution by sulfur dioxide and carbon monoxide
- Name the greenhouse gases
- Explain global warming and its effects
Active recall
Quick check
Answer each question before opening the answer.
Name two greenhouse gases and their effect.
Carbon dioxide and methane — they trap heat in the atmosphere, causing global warming.
State two consequences of global warming.
Any two of: climate change, rising sea levels, more extreme weather, loss of habitats and species.
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