CC7.3 — Earth's atmosphere (Edexcel 1SC0)

Current atmosphere composition

• Nitrogen (N₂): ~78%
• Oxygen (O₂): ~21%
• Argon (Ar): ~1%
• Carbon dioxide (CO₂): ~0.04%
• Water vapour, trace gases.

Evolution of Earth's early atmosphere

Early Earth: mainly CO₂, water vapour, ammonia, methane (from volcanic activity). No free oxygen.

• Oceans formed as water vapour condensed → CO₂ dissolved in water.
• Photosynthesis by early algae/plants → CO₂ reduced; O₂ increased.
• Nitrogen accumulated from volcanic activity and from denitrifying bacteria.

Greenhouse effect

Greenhouse gases (CO₂, CH₄, water vapour, N₂O) absorb infrared radiation emitted from Earth's surface and re-emit it back → warms Earth.

Natural greenhouse effect: keeps Earth ~33°C warmer than without an atmosphere — essential for life.

Enhanced greenhouse effect: human activities increasing CO₂ (burning fossil fuels, deforestation) and CH₄ (livestock, landfill, rice paddies) → global warming.

Effects of global warming

Rising sea levels (melting ice), more extreme weather, habitat loss, ocean acidification (CO₂ + H₂O → H₂CO₃), species extinction.