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The percentage of oxygen in our atmosphere has remained almost constant for thousands of years. This is due to three important processes that occur simultaneously and balance out any changes.

• Respiration – nearly all living cells use oxygen to obtain energy from their food using respiration. This process takes in oxygen from the atmosphere and releases carbon dioxide.
• Combustion – whenever organic material, such as wood or grass, burns, oxygen reacts with the carbon and hydrogen in the material, releasing heat and light in the form of fire. This process takes in oxygen from the atmosphere and releases carbon dioxide.
• Photosynthesis – plants produce their own food in the form of glucose using the process of photosynthesis. This process uses sunlight (and so only occurs during the day) to remove carbon dioxide from the atmosphere and release oxygen.

In the last few thousand years, these three processes have balanced each other out, maintaining levels of oxygen at around 21 per cent.

Increasing carbon dioxide levels

In the last hundred years or so, a number of factors have contributed to increasing carbon dioxide levels in our atmosphere. These include:

• the burning of fossil fuel – this process is adding carbon dioxide to the atmosphere faster than it can be removed by photosynthesis
• the growth of human and animal populations – this has increased the overall level of respiration, leading to increased carbon dioxide levels
• deforestation – cutting down large areas of forest and rainforest for resources such as wood, so that the land can be used as farmland reduces the amount of photosynthesis occurring, and so less carbon dioxide is removed from the atmosphere

The increase in carbon dioxide levels due to these factors has been linked to the rise in global temperatures, an effect known as global warming.