Explaining Change: The Earth's Atmosphere — AQA Combined Science: Synergy
This topic covers how the atmosphere developed, the carbon and water cycles, the greenhouse effect, climate change, air pollutants and potable water.
Development of the Earth's atmosphere
The early atmosphere came from intense volcanic activity, releasing carbon dioxide, water vapour and nitrogen. As Earth cooled, water vapour condensed to form oceans, and much CO₂ dissolved in them. Algae and plants evolved and, through photosynthesis, removed CO₂ and produced oxygen, allowing animals to evolve. Carbon became locked in sedimentary rocks and fossil fuels. Today's atmosphere is about 78% nitrogen, 21% oxygen and small amounts of CO₂, water vapour and noble gases.
The carbon cycle
Carbon is recycled: photosynthesis removes CO₂ from the air; respiration, combustion and decomposition return it. Carbon passes along food chains.
The greenhouse effect
Greenhouse gases (carbon dioxide, methane, water vapour) absorb the infrared radiation re-emitted by the Earth's surface and radiate it back, keeping the planet warm enough for life.
Human impacts on the climate
Human activities increase greenhouse gases: CO₂ from burning fossil fuels and deforestation; methane from cattle, rice fields and landfill. Based on peer-reviewed evidence, most scientists agree this is causing global warming and climate change.
Climate change: impacts and mitigation
Consequences include melting ice caps, rising sea levels and flooding, more extreme weather, changes in rainfall affecting food production, and loss of biodiversity. The carbon footprint (total greenhouse gases over a product's life) can be reduced by renewable energy, efficiency, carbon capture, taxes and lifestyle changes — though cost and cooperation make this difficult. Media reports can be biased or oversimplified.
Pollutants that affect air quality
Burning fuels releases:
- Carbon dioxide — greenhouse gas.
- Carbon monoxide — toxic; from incomplete combustion; reduces oxygen carrying in the blood.
- Soot (particulates) — respiratory problems and global dimming.
- Sulfur dioxide — from sulfur impurities; causes acid rain.
- Oxides of nitrogen — from high engine temperatures; acid rain and respiratory problems.
The water cycle
The Sun's energy drives evaporation; water vapour rises, cools and condenses into clouds, then falls as precipitation, providing fresh water before draining to the seas.
Sources of potable water
Potable water is safe to drink (low in dissolved salts and microbes), not chemically pure. In the UK it is produced by choosing a fresh source, filtering and sterilising (with chlorine, ozone or UV). Where fresh water is scarce, sea water is desalinated by distillation or reverse osmosis, which uses a lot of energy.
Exam tips
- Learn how oxygen increased (photosynthesis) and CO₂ decreased (oceans, photosynthesis, locked in rocks/fuels).
- Explain the greenhouse effect using infrared absorption and re-emission.
- Learn pollutants with their source and effect.
- Distinguish potable (safe to drink) from pure water, and describe how each is produced.