How did the Earth's atmosphere evolve, and how do greenhouse gases affect the climate?
The Earth and atmosphere: the evolution of the atmosphere from volcanic gases to the present composition, the role of photosynthesis, the greenhouse effect, and human activities affecting greenhouse gas levels.
A focused answer to Edexcel GCSE Chemistry topic 8, covering how the Earth's early atmosphere formed from volcanic activity, how photosynthesis increased oxygen and reduced carbon dioxide, the present composition of the atmosphere, the greenhouse effect and the main greenhouse gases, and the human activities that increase greenhouse gas levels and affect the climate.
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What this dot point is asking
Edexcel wants you to describe how the Earth's atmosphere evolved from volcanic gases to its present composition, explain the role of the oceans and photosynthesis, give the present composition of the atmosphere, explain the greenhouse effect and name the main greenhouse gases, and describe the human activities that increase greenhouse gas levels and affect the climate. The evolution sequence and the greenhouse effect are the key marks.
The early atmosphere
The Earth formed about billion years ago. Its early atmosphere is thought to have come from intense volcanic activity:
How the atmosphere changed
Two main processes changed the atmosphere over billions of years:
- As the Earth cooled, the water vapour condensed to form the oceans. Carbon dioxide dissolved in the oceans, reducing its proportion in the air. Over time, this carbon became locked up in sedimentary rocks (such as limestone) and in fossil fuels.
- Plants and algae evolved and carried out photosynthesis, which removes carbon dioxide and releases oxygen: . This gradually increased the oxygen level until animals could evolve.
Nitrogen built up because it is unreactive and was released by volcanoes and other processes without being removed.
The present atmosphere
For the last million years the composition has been roughly constant:
- About 78 percent nitrogen (close to four-fifths).
- About 21 percent oxygen (close to one-fifth).
- Small amounts of carbon dioxide, water vapour and noble gases (mainly argon).
The greenhouse effect
Without the greenhouse effect the Earth would be too cold for life, but an increase in greenhouse gases enhances the effect and raises the global temperature.
Human activities and climate change
Human activities increase the levels of greenhouse gases:
- Burning fossil fuels releases carbon dioxide.
- Deforestation reduces the removal of carbon dioxide by photosynthesis.
- Farming livestock and growing rice release methane.
The increase in greenhouse gases is linked to global warming and climate change, which can cause rising sea levels, more extreme weather and changes to habitats.
Try this
Q1. State the main gas released by volcanoes that made up the early atmosphere. [1 mark]
- Cue. Carbon dioxide.
Q2. Explain how photosynthesis changed the early atmosphere. [2 marks]
- Cue. It removed carbon dioxide and released oxygen, lowering carbon dioxide and raising oxygen.
Q3. Name two greenhouse gases and one human activity that increases their levels. [3 marks]
- Cue. Carbon dioxide and methane; burning fossil fuels (or deforestation, or farming livestock).
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksThe Earth's early atmosphere was very different from today. Describe how the proportion of carbon dioxide decreased and the proportion of oxygen increased over time.Show worked answer →
A 4-mark evolution-of-the-atmosphere question.
The early atmosphere was rich in carbon dioxide from volcanic activity (1 mark). The proportion of carbon dioxide decreased because it dissolved in the oceans that formed as the Earth cooled, and was later locked up in sedimentary rocks and fossil fuels (1 mark for dissolving in oceans, 1 mark for being locked in rocks or by plants). Oxygen increased because plants and algae evolved and carried out photosynthesis, which removes carbon dioxide and releases oxygen (1 mark).
Markers reward carbon dioxide dissolving in the oceans and being locked away, and oxygen rising due to photosynthesis by plants and algae.
Edexcel 20214 marksExplain how the greenhouse effect keeps the Earth warm, and describe two human activities that increase the amount of greenhouse gases in the atmosphere.Show worked answer →
A 4-mark greenhouse-effect question.
Greenhouse gases such as carbon dioxide and methane absorb the heat (infrared radiation) that the warmed Earth radiates back out, and re-emit some of it back towards the surface, keeping the Earth warmer than it would otherwise be (1 mark for absorbing radiation, 1 mark for keeping the Earth warm). Two human activities (1 mark each, any two): burning fossil fuels releases carbon dioxide; deforestation reduces the removal of carbon dioxide by photosynthesis; farming livestock and growing rice release methane.
Markers reward the absorption and re-emission of radiation and two valid human activities.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)