Revision notes for AQA GCSE Chemistry Human activities which contribute to an increase in greenhouse gases in the atmosphere. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Human activities which contribute to an increase in greenhouse gases in the atmosphere
What you'll learn
- How carbon dioxide and methane are linked to the greenhouse effect.
- Which human activities increase carbon dioxide and methane in the atmosphere.
- Why many scientists link these increases to global climate change.
- How to evaluate the quality of evidence in climate-change reports.
The starting point: the atmosphere
The atmosphere is the layer of gases surrounding the Earth. It contains nitrogen, oxygen, carbon dioxide, water vapour, methane and other gases.
Some gases in the atmosphere are called greenhouse gases because they absorb energy that the Earth gives out.
Definition
Greenhouse gas
A greenhouse gas is a gas in the atmosphere that absorbs infrared radiation given off by the Earth’s surface. Carbon dioxide, CO2, and methane, CH4, are important greenhouse gases in GCSE Chemistry.
Infrared radiation is energy transferred by radiation from warm objects. The Earth’s surface warms up in sunlight, then gives out infrared radiation. Greenhouse gases absorb some of this radiation, then emit energy in all directions, including back towards the surface.
Key Idea
Natural vs enhanced greenhouse effect
The natural greenhouse effect helps keep Earth warm enough for life. The problem is the enhanced greenhouse effect: human activities increase greenhouse gas concentrations, so more infrared radiation is absorbed and the average temperature near Earth’s surface may rise.
This diagram summarises the main human activities you need to know for carbon dioxide and methane.
Carbon dioxide: human activities that increase it
Carbon dioxide, CO2, is produced when carbon-containing substances burn. It is also removed from the atmosphere by plants during photosynthesis, so activities that reduce plant growth can increase the amount of CO2 left in the air.
Burning fossil fuels
Fossil fuels are fuels formed from ancient living organisms over millions of years. Examples include coal, crude oil and natural gas.
Combustion means burning a substance in oxygen. When fossil fuels burn completely, carbon atoms in the fuel are converted into carbon dioxide.
For example, methane in natural gas burns like this:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
Human activities that burn fossil fuels include:
- generating electricity in power stations
- using petrol and diesel in cars, lorries, ships and aircraft
- heating homes and buildings using gas, oil or coal
- industrial processes, such as making metals and cement
Deforestation
Deforestation means the large-scale removal of trees.
Deforestation increases carbon dioxide in two main ways:
- Fewer trees means less photosynthesis, so less CO2 is removed from the atmosphere.
- If trees are burned or decay, carbon stored in their wood is released, often as CO2.
Common Mistake
Only thinking about what is released
Deforestation is not just about releasing carbon dioxide. It also reduces the number of trees available to remove carbon dioxide by photosynthesis.
Methane: human activities that increase it
Methane, CH4, is a greenhouse gas. There is less methane than carbon dioxide in the atmosphere, but methane is important because it is very effective at absorbing infrared radiation.
Farming livestock
Livestock farming, especially cattle and sheep farming, increases methane. These animals have microorganisms in their digestive systems that break down food and produce methane, which is released into the atmosphere.
Rice fields
Rice fields are often flooded with water. This creates anaerobic conditions, meaning conditions without oxygen. Microorganisms break down plant material in the flooded soil and produce methane.
Landfill waste
A landfill site is a place where waste is buried. Food waste and other organic waste can decompose without much oxygen underground. This anaerobic decomposition can produce methane.
Natural gas leaks
Natural gas is mainly methane. If methane leaks during extraction, transport or storage, it enters the atmosphere directly.
Tip
The core recall pairs
For GCSE, make sure you can quickly give two activities for each gas:
- Carbon dioxide: burning fossil fuels; deforestation.
- Methane: livestock farming; landfill waste or rice farming.
Example
Linking activities to greenhouse gases
A question says: “A farm clears woodland, keeps more cattle, and sends food waste to landfill. Explain how these actions can increase greenhouse gases.”
- Clearing woodland increases carbon dioxide because fewer trees photosynthesise, so less CO2(g) is removed from the air.
- If the trees are burned or left to decay, carbon stored in the wood can be released as CO2(g).
- Keeping more cattle increases methane because microorganisms in the animals’ digestive systems produce CH4(g).
- Sending food waste to landfill can increase methane because buried waste may decompose anaerobically, producing CH4(g).
From extra greenhouse gases to climate change
Global warming means an increase in the Earth’s average surface temperature. Global climate change means long-term changes in climate patterns, such as temperature, rainfall, storms, droughts and sea level.
Based on peer-reviewed evidence, many scientists believe that human activities are causing greenhouse gas concentrations to increase. They also believe this will cause the temperature of the Earth’s atmosphere at the surface to increase, resulting in global climate change.
Common Mistake
Confusing weather and climate
Weather is short-term and local, such as “it snowed this week”. Climate is the long-term pattern over many years. One cold winter in one country does not disprove global climate change.
Evidence and peer review
Scientists use evidence from many sources, including:
- direct measurements of atmospheric CO2 and CH4
- temperature records from land, sea and satellites
- ice cores, which contain trapped ancient air bubbles
- measurements of sea level, ice cover and ocean temperature
- climate models tested against real observations
Definition
Peer review
Peer review is the checking of scientific work by other experts before it is published. It helps identify errors, weak methods, unsupported conclusions and possible bias.
Peer review is important because climate change is a complex topic with major consequences. Scientists need other experts to check that the methods, data and conclusions are reliable.
Scientists also need to communicate results to a wide range of audiences, including other scientists, governments, businesses and the public. Good communication should be clear, accurate and honest about uncertainty.
Evaluating climate-change reports
In the exam, you may be given a report, graph, article or claim and asked to evaluate it. Evaluate means judge how good it is, using evidence.
A strong report is more likely to:
- use peer-reviewed sources
- use large, long-term, global data sets
- explain where the data came from
- show uncertainty ranges or limitations
- compare several independent lines of evidence
- avoid relying on one unusual event
- separate evidence from opinion
- mention possible conflicts of interest, such as funding sources
A weak report may:
- use only a short time period
- focus on one place rather than global data
- cherry-pick only the evidence that supports its claim
- ignore uncertainty
- use emotional language instead of evidence
- be written by someone with a clear bias
Bias means a tendency to favour one viewpoint, sometimes by selecting only part of the evidence.
Example
Evaluating a climate-change report
A report claims: “Climate change is not linked to human activity because one UK city had a colder-than-average winter.” The report is a company blog, is not peer-reviewed, and gives no data sources.
- Compare the scale of the evidence with the claim: one city and one winter is local weather, so it is not enough to judge long-term global climate change.
- Check whether the data can be trusted: the report gives no data sources or method, so other people cannot easily check or repeat the analysis.
- Consider review and bias: because the report is not peer-reviewed, experts may not have checked it; because it is a company blog, it may present only selected evidence.
- Make a balanced judgement: this is weak evidence for the claim, so a better conclusion would need long-term global data from peer-reviewed and independent sources.
Uncertainty and models
A model is a simplified representation of a real system. A climate model uses scientific ideas, measurements and calculations to predict how the climate may change.
Global climate change is difficult to model because the Earth is a very complex system. Models may include the atmosphere, oceans, ice, clouds, plants, soil and human emissions. Scientists must simplify some parts, because no model can include every detail perfectly.
Uncertainties can come from:
- future human choices, such as how much fossil fuel is burned
- natural variation in climate
- limits in old data, especially before modern instruments
- difficulty modelling clouds, oceans and ice accurately
- different assumptions used in different models
Common Mistake
Thinking uncertainty means no evidence
Uncertainty does not mean “scientists have no idea”. It means there is a range of possible values or outcomes. Good scientists state uncertainties clearly instead of pretending predictions are exact.
Media reports can sometimes simplify the science too much. Some reports may be based on only part of the evidence, or may mix scientific findings with opinions. Your job in a GCSE answer is not to decide based on emotion, but to judge the evidence given.
Exam technique
In the exam
- For recall questions, name human activities clearly: “burning fossil fuels” and “deforestation” for carbon dioxide; “livestock farming” and “landfill/rice fields” for methane.
- For explain questions, link the activity to the gas: for example, burning carbon-containing fuels produces CO2(g), while anaerobic decomposition can produce CH4(g).
- For evaluate questions, comment on evidence quality: peer review, sample size, time period, global vs local data, uncertainty, bias and whether the conclusion follows from the data.
Self review
Check yourself
- Can you name two human activities that increase carbon dioxide and two that increase methane?
- Can you explain why deforestation can increase atmospheric carbon dioxide in two different ways?
- Can you spot why a non-peer-reviewed report using one cold winter is weak evidence about global climate change?
Carbon dioxide and methane as greenhouse gases
Guide 2 of 4
You've reached the end
Test yourself on this topic, or move on to the next guide.
How was this guide?