**Decrease in Carbon Dioxide:**
The early atmosphere had a high percentage of CO₂ (around 96.5%), similar to Venus today. This decreased for several reasons:
1. **Dissolving in Oceans:** As the Earth cooled, water vapour condensed to form oceans. Carbon dioxide dissolved in this water.
2. **Formation of Sedimentary Rocks:** Dissolved CO₂ reacted with minerals to form carbonate precipitates, which over millions of years formed sedimentary rocks like limestone, locking up vast amounts of carbon.
3. **Photosynthesis:** Algae and plants evolved and began to use CO₂ for photosynthesis, converting it into glucose and oxygen. This process further reduced atmospheric CO₂.
4. **Fossil Fuels:** Some of the carbon from dead organisms was trapped and formed fossil fuels.
**Increase in Oxygen:**
The early atmosphere had virtually no oxygen (0.00%). The significant increase to today's level (20.95%) is almost entirely due to:
1. **Photosynthesis:** The evolution of photosynthetic organisms (like algae and later, plants) released oxygen as a waste product. Over billions of years, this gradually built up the oxygen concentration in the atmosphere to the levels we have today.
This question requires an explanation for two major changes in Earth's atmosphere over billions of years.
**1. The Decrease in Carbon Dioxide:**
The early atmosphere, like that of Venus, was dominated by carbon dioxide (approx. 96.5%). The level has dropped to just 0.04% today. This happened through several key processes:
- **Formation of Oceans:** As the Earth cooled, water vapour in the atmosphere condensed and formed vast oceans. A large amount of carbon dioxide from the atmosphere dissolved into these oceans.
- **Formation of Sedimentary Rocks and Fossil Fuels:** In the oceans, the dissolved carbon dioxide reacted to form carbonate precipitates. These built up on the seabed and, over geological time, formed sedimentary rocks such as limestone (calcium carbonate). This process locked away huge quantities of carbon dioxide. Additionally, carbon from dead marine organisms was buried and compressed, forming fossil fuels.
- **Photosynthesis:** About 2.7 billion years ago, algae evolved and began to photosynthesise. Photosynthesis uses carbon dioxide from the atmosphere (or water) and converts it into organic compounds (glucose) for growth. This biological process was a major factor in reducing atmospheric CO₂.
**2. The Increase in Oxygen:**
The early atmosphere was virtually free of oxygen (0.00%). The rise to today's level of about 21% is due to the process of **photosynthesis**.
- Photosynthesis, carried out first by algae and later by plants, releases oxygen as a waste product. The equation is:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Over billions of years, the continuous release of oxygen by these organisms gradually changed the composition of the atmosphere, allowing for the evolution of aerobic life, including animals.
Expert