AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
How carbon dioxide decreased: describe the main changes in the atmosphere over time and some of the likely causes of these changes
You must describe how Earth's atmosphere changed from an early mixture of carbon dioxide, water vapour, methane and ammonia to today's atmosphere of 78% nitrogen and 21% oxygen.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
You must describe how Earth's atmosphere changed from an early mixture of carbon dioxide, water vapour, methane and ammonia to today's atmosphere of 78% nitrogen and 21% oxygen. Carbon dioxide decreased because it dissolved in oceans, forming carbonates that precipitated as sedimentary rocks. Photosynthesis by algae and plants removed more carbon dioxide and released oxygen, so oxygen gradually increased. The remains of these organisms later formed fossil fuels. Nitrogen increased because it was released by volcanoes, and being unreactive, it accumulated. When describing, give a clear sequence of changes; when explaining causes, link each change to a named process. Use data such as percentage composition to support your answer.
What earns the marks
- Describe the early atmosphere as mainly carbon dioxide with little or no oxygen, plus water vapour, methane and ammonia from volcanic activity.
- Describe the decrease in carbon dioxide as caused by dissolving in oceans to form sedimentary rocks, and photosynthesis by algae and plants.
- Explain that the remains of algae and plants that absorbed carbon dioxide later formed fossil fuels such as coal, oil and natural gas.
- Describe the increase in oxygen as caused by photosynthesis, leading to conditions that allowed more complex life to evolve.
- Describe the increase in nitrogen as caused by volcanic activity, and explain that nitrogen is unreactive so it accumulates.
Common mistakes and how to put them right
Saying carbon dioxide decreased only because plants used it.
Put it right: Dissolving in oceans and forming sedimentary rocks were also major causes.
Claiming oxygen increased before photosynthesis evolved.
Put it right: Oxygen was produced by photosynthesis, so it increased after algae and plants appeared.
Describing nitrogen as the main gas in the early atmosphere.
Put it right: The early atmosphere was mainly carbon dioxide, and nitrogen increased later.
Exam tips
- Structure your answer as a timeline: early atmosphere, then removal of carbon dioxide, then increase in oxygen, then increase in nitrogen.
- If a graph or table is given, quote at least two figures and link each to a named process to show you are using the data.
Try the lesson
The start of the How carbon dioxide decreased: describe the main changes in the atmosphere over time and some of the likely causes of these changes lesson, made from checked teaching notes for this specification point.
Have a go first
Earth's early atmosphere was formed mainly from gases released by volcanic activity. Which gas was present in the early atmosphere in only very small amounts?
Listen
You must describe how Earth's atmosphere changed from an early mixture of carbon dioxide, water vapour, methane and ammonia to today's atmosphere of 78% nitrogen and 21% oxygen. Carbon dioxide decreased because it dissolved in oceans, forming carbonates that precipitated as sedimentary rocks. Photosynthesis by algae and plants removed more carbon dioxide and released oxygen, so oxygen gradually increased. The remains of these organisms later formed fossil fuels. Nitrogen increased because it was released by volcanoes, and being unreactive, it accumulated. When describing, give a clear sequence of changes; when explaining causes, link each change to a named process. Use data such as percentage composition to support your answer. A strong answer earns marks for these points. Describe the early atmosphere as mainly carbon dioxide with little or no oxygen, plus water vapour, methane and ammonia from volcanic activity. Describe the decrease in carbon dioxide as caused by dissolving in oceans to form sedimentary rocks, and photosynthesis by algae and plants. Explain that the remains of algae and plants that absorbed carbon dioxide later formed fossil fuels such as coal, oil and natural gas. Describe the increase in oxygen as caused by photosynthesis, leading to conditions that allowed more complex life to evolve. A common slip: Saying carbon dioxide decreased only because plants used it. Instead, dissolving in oceans and forming sedimentary rocks were also major causes. Exam tip: Structure your answer as a timeline: early atmosphere, then removal of carbon dioxide, then increase in oxygen, then increase in nitrogen.
Key words
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Changes in atmospheric composition over geological time
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Causes of carbon dioxide decrease
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Causes of oxygen and nitrogen increase
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See every definition
- Changes in atmospheric composition over geological time
- Earth's atmosphere changed from an early mixture of carbon dioxide, water vapour, methane and ammonia to today's atmosphere of 78% nitrogen and 21% oxygen.
- Causes of carbon dioxide decrease
- Carbon dioxide decreased because it dissolved in oceans to form sedimentary rocks, and photosynthesis by algae and plants removed more carbon dioxide.
- Causes of oxygen and nitrogen increase
- Oxygen increased because photosynthesis by algae and plants released it, while nitrogen increased because it was released by volcanoes and being unreactive it accumulated.
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