How did the Earth's atmosphere form and change, and why is the climate changing now?
The evolution of the Earth's early atmosphere, the present composition of the atmosphere, the greenhouse effect, and the evidence and consequences of human-caused climate change.
A focused answer to Edexcel GCSE Combined Science Topic 8 (CC8), covering the evolution of the early atmosphere, the present composition of the atmosphere, the greenhouse effect, and the evidence and consequences of human-caused climate change.
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What this dot point is asking
Edexcel wants you to describe how the Earth's early atmosphere formed and changed, state the present composition of the atmosphere, explain the greenhouse effect, and describe the evidence and consequences of human activity on the climate.
The early atmosphere
How the atmosphere changed
Over billions of years:
- Carbon dioxide decreased because it dissolved in the new oceans, was used by photosynthesising organisms, and became locked in sedimentary rocks and fossil fuels.
- Oxygen increased because algae, and later plants, carried out photosynthesis, releasing oxygen. This allowed animals to evolve.
- Nitrogen increased and built up because it is unreactive.
The atmosphere today
The greenhouse effect and climate change
Greenhouse gases, mainly carbon dioxide and methane, absorb the infrared (heat) radiation that the Earth emits and re-radiate some back to the surface, keeping the planet warm enough for life (the greenhouse effect).
Human activities (burning fossil fuels, deforestation, farming) are increasing the concentration of greenhouse gases, which traps more heat and raises the average global temperature. The evidence comes from measurements of carbon dioxide levels and global temperatures. Consequences include melting ice, rising sea levels, more extreme weather, and changes to where species can live.
The main human sources are worth knowing. Carbon dioxide is released by burning fossil fuels in power stations, vehicles and industry, and by deforestation (which also removes the trees that would absorb it). Methane comes from farming, especially cattle and rice fields, and from rotting waste in landfill sites. Because these activities are increasing worldwide, the concentration of both gases has risen sharply since the industrial revolution.
The amount of greenhouse gas a person, product or activity releases over its lifetime is called its carbon footprint. It can be reduced by using renewable energy (such as wind and solar) instead of fossil fuels, improving the efficiency of vehicles and appliances, capturing and storing carbon dioxide from power stations, planting trees, and reducing waste. Scientists are confident, from the strong correlation between rising carbon dioxide and rising temperatures and from a clear physical mechanism, that human activity is the main cause of recent climate change, though predicting the exact future change is difficult because the climate system is so complex.
Try this
Q1. State the approximate percentages of nitrogen and oxygen in the atmosphere today. [2 marks]
- Cue. About 78% nitrogen and 21% oxygen.
Q2. Name two greenhouse gases. [1 mark]
- Cue. Carbon dioxide and methane.
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 20204 marksDescribe how the amount of carbon dioxide in the Earth's early atmosphere decreased and how the amount of oxygen increased over time.Show worked answer →
A 4-mark describe question on the evolving atmosphere.
Carbon dioxide decreased because it dissolved into the oceans as they formed, and because early photosynthesising organisms (algae and plants) used it up (2 marks). Some carbon became locked up in sedimentary rocks and fossil fuels. Oxygen increased because algae and then plants carried out photosynthesis, releasing oxygen as a product over billions of years (2 marks).
Markers reward carbon dioxide dissolving in the oceans and being used in photosynthesis (and locked in rocks), and oxygen being produced by photosynthesis.
Edexcel 20224 marksExplain how greenhouse gases keep the Earth warm, and describe one consequence of an increase in greenhouse gases due to human activity.Show worked answer →
A 4-mark question on the greenhouse effect and climate change.
Greenhouse gases such as carbon dioxide and methane absorb the heat (infrared radiation) that the Earth radiates back out, and re-emit some of it back towards the surface, keeping the Earth warm (2 marks). Increasing greenhouse gases from human activities (such as burning fossil fuels) traps more heat, raising the average global temperature (1 mark). A consequence is, for example, melting polar ice and rising sea levels, or more extreme weather (1 mark).
Markers reward greenhouse gases absorbing and re-emitting heat radiation, the link to a rising average temperature, and a valid named consequence.
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Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)