The Earth's atmosphere
The atmosphere today
For about the last 200 million years, the proportions of gases in the atmosphere have been much the same as now:
| Gas | Proportion |
|---|---|
| Nitrogen | about 78% (roughly four-fifths) |
| Oxygen | about 21% (roughly one-fifth) |
| Argon | about 0.9% |
| about 0.04% |
There are also small amounts of water vapour and other noble gases.
Key terms
The early atmosphere
The Earth formed about 4.6 billion years ago. Scientists aren't certain what its early atmosphere was like, because there is little evidence from so long ago.
One theory:
- In the first billion years, intense released gases, mainly , along with water vapour and nitrogen. Small amounts of methane and ammonia may also have been present.
- There was little or no oxygen. This is like the atmospheres of Mars and Venus today.
- As the Earth cooled, the water vapour condensed to form the oceans.
Nitrogen, which is unreactive, gradually built up in the atmosphere.
Key terms
How oxygen increased
About 2.7 billion years ago, algae began to produce oxygen by :
carbon dioxide + water → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Over the next billion years, plants evolved, and the amount of oxygen gradually rose to a level that allowed animals to evolve.
Key terms
How carbon dioxide decreased
Carbon dioxide was removed from the atmosphere in several ways:
- It dissolved in the oceans, and were then precipitated, forming sediments.
- Algae and plants used it for .
- Carbon was locked up in and :
- formed from the shells and skeletons of marine organisms
- coal formed from thick layers of plant material
- formed from plankton buried in mud
Burning fossil fuels today releases this stored carbon back into the atmosphere as carbon dioxide.