Greenhouse gases, climate change and air pollution explained
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Burning fuels, farming and cutting down forests change the gases in the air. Some of these gases keep the Earth warm. Others are pollutants that harm people and the environment.
This matters because climate change and air pollution affect where people can live, what they can grow and how healthy they are. Scientists and governments use chemistry to understand the problem and decide what to do.
By the end you will be able to describe the greenhouse effect, give causes and effects of climate change, explain a carbon footprint, and explain how the main pollutants from burning fuels are made.
The greenhouse effect
Greenhouse gases keep the Earth warm enough for life. The main ones are water vapour, carbon dioxide and methane.
The Sun gives out short wavelength radiation. It passes through the atmosphere and warms the surface of the Earth. The warm Earth gives out long wavelength radiation, which is infrared.
Greenhouse gases absorb some of this infrared radiation and then release it again in all directions. Some goes back down to the Earth, so the surface stays warmer than it would be without them. This is the greenhouse effect.
A flow of five stages: short wavelength radiation from the Sun, absorbed by the Earth surface, long wavelength infrared radiation given out, greenhouse gases absorb and release it, and some returns to the surface so the Earth stays warm.
Human activities
Human activities increase the amounts of carbon dioxide and methane in the air. Carbon dioxide rises when we burn fossil fuels for transport, heating and electricity. It also rises when forests are cut down and burned or left to rot, because there are fewer trees to take in carbon dioxide by photosynthesis.
Methane rises with cattle and sheep farming, because the animals release it when they digest food. It also rises with rice paddy fields and with rubbish rotting in landfill sites.
Many scientists believe these extra gases are causing the average temperature of the Earth's atmosphere to rise. This is called global warming and it leads to climate change.
Climate change: effects and evidence
A rise in average global temperature is a major cause of climate change. Possible effects include: melting ice caps and a rise in sea level, which causes flooding and coastal erosion; more extreme weather such as storms, droughts and heatwaves; changes in rainfall that affect water supplies and crops; and changes in the places where wild species can live.
Evidence includes records showing that atmospheric carbon dioxide, fossil fuel use and temperature have all gone up together. This is a correlation. It suggests, but does not prove on its own, that one causes the other.
The evidence is not perfect. Measurements are taken in different places, and older records are less accurate. Climate is very complex to model, so predictions are uncertain. Scientists have their work checked by other scientists. This is peer review. Newspapers may report only part of the evidence.
Carbon footprint
The carbon footprint is the total amount of carbon dioxide and other greenhouse gases given out over the full life cycle of a product, a service or an event. It includes making, transporting, using and getting rid of something.
You reduce a carbon footprint by reducing emissions of carbon dioxide and methane. Actions include using renewable energy instead of fossil fuels, saving energy, using public transport or walking, planting trees, and capturing carbon dioxide from power stations and storing it underground.
Actions can be limited by cost, by the technology available, by people's lifestyle choices and by countries not agreeing with each other. Good answers give an action and a reason that it might not work for everyone.
What comes out when fuels burn
Most fuels contain carbon and hydrogen, and some contain a little sulfur. With plenty of oxygen, complete combustion makes carbon dioxide and water vapour.
With too little oxygen the combustion is incomplete. Then carbon monoxide forms, and carbon is released as soot, which is made of small solid particles. Unburned hydrocarbons can also escape.
If a fuel contains sulfur, burning it makes sulfur dioxide. In a hot engine, nitrogen and oxygen from the air react together to make oxides of nitrogen. Soot and other tiny solid particles in the air are called particulates.
Pollutants and the harm they do
Carbon monoxide is a toxic gas. It is colourless and has no smell, so it is hard to detect. It joins to haemoglobin in red blood cells and reduces the oxygen they carry, so it can cause headaches, unconsciousness and death. Faulty heaters and boilers can make it in homes.
Sulfur dioxide and oxides of nitrogen cause breathing problems. They also dissolve in rain water and make acid rain. Acid rain damages stone buildings and statues, harms trees and makes lakes too acidic for fish.
Particulates cause lung problems and also global dimming, where less sunlight reaches the Earth's surface. Hydrogen is a possible clean fuel for cars because it makes only water when it burns, but making and storing hydrogen cheaply and safely is hard.
Recap
Greenhouse gases are water vapour, carbon dioxide and methane. They absorb infrared radiation from the Earth and release it again.
Burning fossil fuels and deforestation add carbon dioxide. Cattle, rice and landfill add methane. The carbon footprint is the total greenhouse gas emissions over the life cycle.
Incomplete combustion makes carbon monoxide and soot. Sulfur in a fuel makes sulfur dioxide. Hot engines make oxides of nitrogen. These cause toxic effects, acid rain and global dimming.
Key points
Greenhouse gases: water vapour, carbon dioxide, methane. They absorb infrared radiation from the Earth and release it again.
Slip: Using solar panels. Solar panels do not release carbon dioxide when they work.
Slip: Planting trees. Planting trees takes carbon dioxide out of the air.
Example 2
Coal is mainly carbon but contains a little sulfur. Predict the products of burning coal in a limited supply of oxygen.
Carbon burns to make carbon dioxide if there is plenty of oxygen.
With limited oxygen, incomplete combustion makes carbon monoxide and soot (carbon).
The sulfur burns to make sulfur dioxide.
Answer: Carbon monoxide and soot from incomplete combustion, and sulfur dioxide from the sulfur
Common slips
Slip: Only carbon dioxide and water. That is complete combustion. The oxygen supply was limited.
Slip: Carbon monoxide and oxides of nitrogen. Oxides of nitrogen need a very hot engine, and coal burning gives sulfur dioxide from sulfur.
Slip: Sulfur dioxide comes from nitrogen in the air. Sulfur dioxide comes from sulfur in the fuel.
Example 3
The concentration of carbon dioxide in the atmosphere rose from 280 parts per million to 420 parts per million. Calculate the percentage increase, and explain why a rise in carbon dioxide may increase the average temperature.
Increase = 420 − 280 = 140.
Percentage increase = 140 ÷ 280 × 100 = 50%.
Carbon dioxide is a greenhouse gas.
It absorbs infrared radiation from the Earth and releases it again, some back to the surface, so more carbon dioxide traps more heat.
Answer: 50%. More carbon dioxide absorbs and releases more infrared radiation, so the Earth gets warmer
Common slips
Slip: 420 ÷ 280 × 100 = 150% That is the new value as a percentage of the old one. The increase is 150% − 100% = 50%.
Slip: 140 ÷ 420 × 100 = 33% Divide the increase by the original value (280), not the new value.
Slip: More carbon dioxide absorbs short wavelength light and so cools the Earth. It absorbs the long wavelength (infrared) radiation, which warms the Earth.
Slip: A gap in the ozone layer lets in more heat. The greenhouse effect is separate. Greenhouse gases trap the heat radiated from the surface.
Questions and answers
Greenhouse gases, climate change and air pollution practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
Which greenhouse gas is released when cattle digest their food?
Show answer
Answer: methane
Cattle release a gas from digestion.
It is methane.
Question 2 Easy
Which greenhouse gas is made by burning fossil fuels?
Show answer
Answer: carbon dioxide
Burning a hydrocarbon makes carbon dioxide and water.
The greenhouse gas is carbon dioxide.
Question 3 Easy
Which toxic gas has no colour and no smell and forms in incomplete combustion?
Show answer
Answer: carbon monoxide
Incomplete combustion gives a toxic gas.
It is carbon monoxide.
Question 4 Easy
Which gas, formed from sulfur in a fuel, causes acid rain?
Show answer
Answer: sulfur dioxide
The sulfur in the fuel burns in oxygen.
It makes sulfur dioxide.
Question 5 Easy
What is the name for the total greenhouse gases emitted over the full life cycle of a product?
Show answer
Answer: carbon footprint
It measures the greenhouse gas emissions of a product or event.
It is the carbon footprint.
Question 6 Medium
What are the tiny black solid particles made when a fuel burns without enough oxygen?
Show answer
Answer: soot
They are made of carbon.
They are called soot.
Question 7 Medium
Which pollutants form when nitrogen and oxygen from the air react in a hot engine?
Show answer
Answer: oxides of nitrogen
The high temperature lets nitrogen react with oxygen.
The products are oxides of nitrogen.
Question 8 Medium
Where does the sulfur dioxide from burning coal come from?
Show answer
Answer: sulfur in the coal
Sulfur dioxide forms when sulfur burns.
The sulfur is an impurity in the coal.
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
(a) Describe the greenhouse effect. [3 marks] (b) Name one greenhouse gas. [1 mark]
Show answer and marking points
Answer: (a) short wavelength radiation from the Sun passes through the atmosphere and warms the Earth, the Earth gives out long wavelength infrared radiation, greenhouse gases absorb some of it and release it again so the Earth stays warm (b) methane
(a) short wavelength radiation from the Sun passes through the atmosphere to the surface (1)
(a) the Earth gives out long wavelength (infrared) radiation (1)
(a) greenhouse gases absorb it and release it, some back to Earth (1)
(b) carbon dioxide, methane or water vapour (1)
Exam-style question 24 marks
(a) Explain how carbon monoxide is made when petrol burns in a car engine. [2 marks] (b) Explain why carbon monoxide is dangerous to people. [2 marks]
Show answer and marking points
Answer: (a) there is not enough oxygen so the combustion is incomplete and the carbon forms carbon monoxide (b) it is toxic because it stops red blood cells carrying oxygen, and it has no colour or smell so people cannot detect it
(a) not enough (limited) oxygen (1)
(a) incomplete combustion of the carbon in the fuel (1)
(b) toxic, because it stops the blood carrying oxygen (1)
(b) colourless and odourless so cannot be detected (1)
Common mistakes
Saying the greenhouse effect is caused by a hole in the ozone layer
The greenhouse effect is greenhouse gases absorbing infrared radiation from the Earth. The ozone layer is a different topic.
Saying greenhouse gases trap light from the Sun
They absorb long wavelength (infrared) radiation given out by the warm Earth.
Saying carbon monoxide causes acid rain
Sulfur dioxide and oxides of nitrogen cause acid rain. Carbon monoxide is toxic.
Mixing up soot and sulfur dioxide
Soot is carbon particles from incomplete combustion. Sulfur dioxide comes from sulfur in the fuel.
Saying one hot summer or one cold winter proves or disproves climate change
You need long-term data. Weather is not climate.
Questions students ask
Is Greenhouse gases, climate change and air pollution on the Foundation or Higher tier?
Greenhouse gases, climate change and air pollution is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 5.
What is a common mistake on Greenhouse gases, climate change and air pollution?
Saying the greenhouse effect is caused by a hole in the ozone layer. The greenhouse effect is greenhouse gases absorbing infrared radiation from the Earth. The ozone layer is a different topic.
Where are the answers to the Greenhouse gases, climate change and air pollution questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.