The key idea
The Earth's atmosphere is approximately 78% nitrogen, 21% oxygen and small amounts of other gases including CO₂ and water vapour.Greenhouse gases trap heat and maintain surface temperature, but increasing their concentration causes global warming.
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Composition of the atmosphere
04%) and water vapour.The early atmosphere of the Earth contained much more CO₂ and very little oxygen, similar to the current atmospheres of Mars and Venus.Over billions of years, photosynthesis produced oxygen and locked carbon away.
The greenhouse effect
Greenhouse gases (CO₂, CH₄, water vapour, N₂O) in the atmosphere absorb infrared radiation emitted by the Earth's warm surface and re-emit it in all directions, including back towards Earth.This natural greenhouse effect is essential for maintaining a temperature that supports life.Without it, Earth's surface temperature would be around −18°C.
Climate change
Human activities — burning fossil fuels, deforestation and intensive farming — have significantly increased the concentrations of CO₂ and CH₄ in the atmosphere.This enhances the greenhouse effect, causing global temperatures to rise.Consequences include melting ice caps, rising sea levels, more extreme weather, and disruption of ecosystems.Most scientists agree this is largely human-caused, but the full extent is still debated.
Development of the atmosphere over time
In the first billion years, volcanic activity released mainly CO₂, water vapour and nitrogen.As the Earth cooled, water vapour condensed to form oceans, and CO₂ dissolved in seawater to form carbonates.5 billion years ago steadily increased O₂ levels and reduced CO₂.The deposition of carbon in fossils and rocks locked away carbon dioxide permanently.
Definitions to learn
Greenhouse gas
A gas that absorbs and re-emits infrared radiation, contributing to the greenhouse effect.
Greenhouse effect
The trapping of infrared radiation by atmospheric gases that keeps Earth's surface warm enough for life.
Global warming
The observed increase in average global temperature attributed largely to increased greenhouse gas concentrations.
Carbon footprint
The total amount of CO₂ and equivalent greenhouse gases emitted by an individual, product or activity.
Fossil fuel
A fuel (coal, oil, natural gas) formed from the remains of ancient organisms over millions of years.
Explain the greenhouse effect and why it is important for life on Earth.
Short-wavelength radiation from the Sun passes through the atmosphere to warm the Earth's surface.
The surface re-emits longer-wavelength infrared radiation.
Greenhouse gases (CO₂, CH₄, water vapour) absorb some of this infrared radiation.
The gases re-emit energy in all directions, some back towards the Earth.
This keeps the Earth warmer than it would otherwise be, making life possible.
Greenhouse gases trap infrared radiation and maintain Earth's temperature.
For greenhouse effect questions, always include both absorption of infrared radiation by greenhouse gases AND re-emission back towards the Earth.
Do not confuse the greenhouse effect with ozone depletion. They are different atmospheric issues.
How to score full marks
- 1The greenhouse effect is NATURAL and ESSENTIAL for life — distinguish this from the ENHANCED greenhouse effect caused by human activity.
- 2State the full mechanism: greenhouse gases absorb infrared radiation from Earth AND re-emit it back towards Earth.
- 3When describing the early atmosphere, state that CO₂ was very high and O₂ was very low — opposite to today.
Try these yourself
Open each answer only after you have explained the full chemical process.
1State the approximate percentage of nitrogen and oxygen in the atmosphere.[2 marks]
- 1.Recall the two main components.
2Name three greenhouse gases.[3 marks]
- 1.Recall gases that absorb infrared radiation.
3Explain why burning fossil fuels increases the greenhouse effect.[4 marks]
- 1.Link combustion to CO₂ and then to warming.
4Explain how the evolution of photosynthesising organisms changed the composition of the early atmosphere.[3 marks]
- 1.Describe the changes in CO₂ and O₂ over time.
5State the origin of nitrogen in the early atmosphere.[2 marks]
- 1.Describe what produced it.
6Give two consequences of global warming for the planet.[2 marks]
- 1.Think about sea level and weather patterns.
7Explain why it is difficult to reach conclusions about the extent of human influence on climate change.[3 marks]
- 1.Consider the complexity of the evidence.
8Describe two ways that carbon dioxide has been removed from the atmosphere over geological time.[2 marks]
- 1.Identify natural processes that remove CO₂.