Changes in the Atmosphere

Early Atmosphere Composition

The Earth's early atmosphere was very different from the one we have today. It mainly consisted of:

• Carbon dioxide (CO 2)
• Water vapour (H 2O)
• Nitrogen (N 2)

There was very little or no oxygen (O 2) in the early atmosphere.

This atmosphere formed mainly due to intense volcanic activity. Volcanoes released large amounts of gases trapped inside the Earth, including carbon dioxide, water vapour, and nitrogen. This process is called outgassing.

Volcanic eruptions also released other gases like ammonia and methane, but these were present in much smaller amounts and gradually disappeared over time.

Development of the Atmosphere

As the Earth cooled, the water vapour in the atmosphere condensed to form oceans. This was a crucial step in changing the atmosphere's composition.

Much of the carbon dioxide dissolved in the oceans. Here, it reacted with minerals to form sedimentary rocks such as limestone, which contain calcium carbonate (CaCO 3). This process removed a large amount of CO 2 from the atmosphere.

For instance, when carbon dioxide dissolves in seawater, it can react with calcium ions to form calcium carbonate:

$$\text{Ca}^{2+} + \text{CO}_3^{2-} \rightarrow \text{CaCO}_3$$

This calcium carbonate then settles to the ocean floor, forming sedimentary rock layers over millions of years.

Increase in Oxygen Levels

The atmosphere began to change significantly when simple plants and algae evolved and started photosynthesising. Photosynthesis uses carbon dioxide and water to produce glucose and oxygen:

$$\text{6CO}_2 + \text{6H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{6O}_2$$

This process released oxygen into the atmosphere for the first time.

As oxygen levels increased, some of it reacted with ultraviolet (UV) radiation from the Sun to form ozone (O 3) in the upper atmosphere. The ozone layer formed, which protects living organisms by absorbing harmful UV rays.

The increase in oxygen allowed more complex life forms to develop and survive.

For example, early algae in the oceans produced oxygen through photosynthesis, gradually increasing atmospheric oxygen over millions of years.

Current Atmosphere Composition

Today, the Earth's atmosphere is made up of approximately:

• 78% nitrogen (N 2)
• 21% oxygen (O 2)
• About 1% other gases including argon (Ar), carbon dioxide (CO 2), and trace gases

This stable mixture supports life on Earth by providing oxygen for respiration and nitrogen for building proteins.

Carbon dioxide remains a trace gas but plays an important role in processes like photosynthesis and the carbon cycle (covered in other topics).

The atmosphere's composition has changed dramatically over billions of years, from mostly carbon dioxide and water vapour to the nitrogen-oxygen mix we have today.

Quick Check: If oxygen is 21% and other gases are 1%, what percentage is nitrogen? (Answer: 78%)

In recent times, human activities like burning fossil fuels have increased carbon dioxide levels, affecting the atmosphere's composition and contributing to climate change (covered in other topics).