Craig Lindsay, of Totara Park, asks :-

Will the oxygen ever be depleted or nearly depleted, because of all the deforestation, births outnumber the deaths, more planes and cars using oxygen, burning of fossil fuels and everything else that uses oxygen?

Candace Martin, a geologist at the University of Otago, responded.

The oxygen gas (O2) content of Earth’s atmosphere is sustained by the process of photosynthesis by which plants consume carbon dioxide and water, build organic matter, and release oxygen to the atmosphere. Opposite processes that consume oxygen and release carbon dioxide include those you list and, most importantly, respiration by both plants and animals and oxidation of organic matter in soils and the ocean. On a global scale, there is a balance between these processes that has kept the oxygen content of the atmosphere around 20% (by volume) over the last billion years of Earth history in spite of vast changes in life and climate. As long as there are plants, there will be oxygen gas in the atmosphere.

Oxygen is the most abundant element in Earth - and the other rocky planets of the solar system: Mercury, Venus, and Mars. However, it is almost always combined with another element in various oxide compounds including H2O and CO2. Only on Earth is oxygen gas present as a major constituent of the atmosphere. However, it was not always so. Early in Earth’s history, when the first plants starting releasing oxygen, the first thing that happened was the oxidation of iron in rocks exposed at Earth’s surface. Iron oxidation is what causes the rusty appearance of some rocks and soils. Oxygen gas did not begin to accumulate in the atmosphere until about 2.4 billion years ago.

Today the average oxygen concentration of the atmosphere is 21%. This value fluctuates slightly up and down with the seasons, in the opposite sense to carbon dioxide. Careful measurements have revealed that the average concentration of oxygen in the atmosphere has decreased since the start of the Industrial Revolution, and over the last several decades by exactly the amount expected for the observed increase in carbon dioxide due to burning of fossil fuels. This decrease is very small in comparison to the total oxygen content of the atmosphere, however.

Some of the oxygen gas in the upper atmosphere undergoes reactions to become ozone, O3. Ozone in the upper atmosphere is important because it helps to block ultraviolet rays from the Sun that are so intense that they could destroy life on Earth. Only a few years after the “ozone hole� was discovered, the human-produced fluoro-carbon gases (used in refrigeration) that caused it were outlawed by international treaty.

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