The Atmosphere — Composition and Climate Change

This lesson covers the composition of the Earth's atmosphere, how it has changed over time, and the evidence and science behind climate change. These topics are part of the AQA GCSE Combined Science Trilogy (8464) chemistry specification.

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Current Composition of the Atmosphere

The proportions of gases in the Earth's atmosphere have been roughly the same for the last 200 million years.

pie title Approximate Composition of the Atmosphere
    "Nitrogen (N₂)" : 78
    "Oxygen (O₂)" : 21
    "Argon and other gases" : 1

Gas	Approximate Percentage
Nitrogen (N₂)	78%
Oxygen (O₂)	21%
Argon (Ar)	0.93%
Carbon dioxide (CO₂)	0.04%
Water vapour	Variable (typically 1–4%)

Exam Tip: You must know the approximate percentages: ~78% nitrogen, ~21% oxygen, ~1% other gases (mainly argon). Carbon dioxide is only about 0.04%.

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How the Atmosphere Has Changed

The Early Atmosphere (First Billion Years)

The Earth's early atmosphere was very different from today. It was formed by volcanic activity and is thought to have contained:

• Mainly carbon dioxide (CO₂) — similar to the atmospheres of Mars and Venus today
• Water vapour (H₂O)
• Small amounts of methane (CH₄) and ammonia (NH₃)
• Little or no oxygen

Key Point: There is some uncertainty about the exact composition of the early atmosphere because there were no measurements. Scientists use evidence from volcanic gases, other planets, and rocks to make estimates.

How the Atmosphere Changed

Event	Effect on Atmosphere
Earth cooled	Water vapour condensed to form oceans
CO₂ dissolved in oceans	Reduced CO₂ in atmosphere
Marine organisms evolved	Used dissolved CO₂; shells and skeletons formed sedimentary rocks (limestone) and fossil fuels
Photosynthesising organisms appeared (~2.7 billion years ago)	Produced O₂; consumed CO₂
O₂ levels rose	Allowed evolution of aerobic organisms; formation of ozone layer

Photosynthesis: The Key Equation

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$6CO2​+6H2​Olight​C6​H12​O6​+6O2​

Algae and plants used CO₂ and released O₂, gradually transforming the atmosphere over billions of years.

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Greenhouse Gases and the Greenhouse Effect

Greenhouse gases in the atmosphere absorb and re-emit infrared radiation from the Earth's surface, keeping the Earth warm enough to support life. Without the greenhouse effect, the Earth would be too cold for liquid water to exist.

The main greenhouse gases are:

Greenhouse Gas	Source
Carbon dioxide (CO₂)	Burning fossil fuels, deforestation, respiration
Methane (CH₄)	Livestock farming, rice paddies, landfill, natural gas leaks
Water vapour (H₂O)	Evaporation from oceans and lakes

How the Greenhouse Effect Works

1. Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth's surface.
2. The warm surface emits long-wavelength infrared radiation.
3. Greenhouse gases absorb some of this infrared radiation.
4. The absorbed energy is re-emitted in all directions, including back towards the Earth.
5. This keeps the Earth warmer than it would otherwise be.

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Climate Change

Evidence for Climate Change

Climate change refers to long-term changes in global weather patterns, including a rise in average global temperature. Evidence includes:

• Rising global average temperatures (instrumental records since the 1800s)
• Shrinking ice sheets and glaciers
• Rising sea levels
• Changes in seasonal patterns and wildlife behaviour
• Ice core data showing correlation between CO₂ levels and temperature over hundreds of thousands of years

Human Activities and Climate Change

The scientific consensus is that human activities are causing the enhanced greenhouse effect by increasing concentrations of greenhouse gases:

Activity	Gas Released	Effect
Burning fossil fuels	CO₂	Increases atmospheric CO₂
Deforestation	CO₂	Less CO₂ removed by photosynthesis
Agriculture (livestock)	CH₄	Increases atmospheric CH₄
Landfill	CH₄	Decomposition releases CH₄

Consequences of Climate Change

Consequence	Detail
Rising sea levels	Thermal expansion of water and melting ice caps; flooding of low-lying areas
Extreme weather	More frequent storms, droughts, heatwaves
Loss of habitat	Coral bleaching, loss of Arctic ice, changing ecosystems
Food security	Changed rainfall patterns affect crop yields
Species migration	Animals and plants move to new areas or face extinction

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Carbon Footprint

A carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service, or event.

Ways to reduce carbon footprint:

• Use renewable energy sources (solar, wind, hydroelectric)
• Improve energy efficiency (insulation, LED lighting)
• Reduce, reuse, recycle materials
• Use public transport, cycle, or walk instead of driving
• Plant trees (carbon capture)

Exam Tip: AQA may ask you to evaluate actions to reduce carbon footprint. Consider effectiveness, cost, and practicality. Not all strategies are equally effective or easy to implement.

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Common Mistakes

• Confusing the greenhouse effect (natural and necessary) with the enhanced greenhouse effect (human-caused increase).
• Stating that the greenhouse effect is entirely bad — without it, Earth would be too cold for life.
• Confusing climate change with the ozone layer — these are different environmental issues.

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Summary

• The atmosphere is ~78% nitrogen, ~21% oxygen, ~1% other gases (mainly argon)
• The early atmosphere was mainly CO₂ and water vapour, with little oxygen
• Photosynthesis by algae and plants increased O₂ and decreased CO₂ over billions of years
• Greenhouse gases (CO₂, CH₄, H₂O) absorb and re-emit infrared radiation, warming the Earth
• Human activities are increasing greenhouse gas levels, causing the enhanced greenhouse effect and climate change
• Consequences include rising sea levels, extreme weather, habitat loss, and food insecurity