How the atmosphere evolved
Earth's atmosphere has changed dramatically over its 4.6 billion years. The current N₂/O₂ mix is the result of two huge processes: volcanic outgassing giving an early CO₂-rich atmosphere, and photosynthesis later producing oxygen.
Stage 1: Early atmosphere (~4 billion years ago)
The young Earth had intense volcanic activity. Volcanoes released:
- Carbon dioxide CO₂ (a lot — the dominant gas).
- Water vapour H₂O (which condensed when Earth cooled, forming the oceans).
- Nitrogen N₂.
- Small amounts of methane and ammonia.
There was little or no oxygen at this stage.
The early atmosphere was very similar to today's atmospheres on Mars and Venus (mostly CO₂).
Stage 2: Formation of oceans
As Earth cooled below 100 °C, water vapour condensed into liquid water → oceans. Most of the CO₂ dissolved into the oceans, then formed insoluble carbonate sediments (limestone, chalk) that locked carbon into rocks.
This greatly reduced atmospheric CO₂.
Stage 3: Rise of oxygen (~2.7 billion years ago)
Cyanobacteria (blue-green algae) evolved and began photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Oxygen accumulated in the atmosphere. Plants, then animals, evolved to exploit oxygen for respiration. Today's atmosphere is the result.
Where the carbon went
Carbon from early atmospheric CO₂ is now found in:
- Sedimentary rocks (limestone CaCO₃ — fossilised shells of marine organisms).
- Fossil fuels (coal, oil, gas — fossilised plants/plankton).
- Oceans (dissolved CO₂ and bicarbonate).
- Biomass (living plants, animals, soil).
Why the modern problem matters
Burning fossil fuels (C9.3, C9.5) releases this stored carbon back into the atmosphere as CO₂ — faster than natural processes can re-absorb it. This is the source of climate change.
Comparing with other planets
| Planet | Atmosphere |
|---|---|
| Mars | ~95% CO₂, very thin |
| Venus | ~96% CO₂, very thick (extreme greenhouse) |
| Earth (early) | mostly CO₂ |
| Earth (now) | 78% N₂, 21% O₂, 0.04% CO₂ |
⚠Common mistakes
- Saying oxygen has always been around — it's the product of photosynthesis, evolved over time.
- Confusing the order of events. Outgassing first, then oceans, then photosynthesis.
- Saying volcanoes still produce most of our CO₂ — fossil fuel burning is the dominant source today.
- Forgetting limestone is a CO₂ store — much of early CO₂ became carbonate rock.
Links
Builds on C9.1. Sets up C9.3 (greenhouse gases) and C9.4 (carbon footprint).
AI-generated · claude-opus-4-7 · v3-deep-chemistry