The Earth’s atmosphere consists of approximately 78% nitrogen and 21% oxygen, with trace amounts of other gases. Oxygen is essential to all animal life, and to many other organisms. Since the gas is used up by oxygen-breathing life forms, and also tends to react with many rocks and minerals, it must be constantly replenished. About 98% of atmospheric oxygen comes from photosynthesis, the process by which plants produce sugars from carbon dioxide and water. The remainder results from the breakup of water by ultraviolet radiation.
Plants and some bacteria use photosynthesis to manufacture food in the form of sugars and other energy rich substances. Water and carbon dioxide are taken up by the organism, and sunlight provides energy that powers the process. Oxygen happens to be a highly useful by-product. As far as scientists can tell, oxygen levels on the Earth have remained fairly stable for several hundred million years. This indicates that oxygen production by photosynthesis has been more or less balanced by its consumption by other processes, such as oxygen-breathing, or aerobic, life forms and chemical reactions.
The sources of atmospheric oxygen through photosynthesis are phytoplankton, such as cyanobacteria in the ocean, and trees and other green plants on land. The amount that each source contributes is under debate: some scientists suggest that over half comes from oceans, for example, while others put the number at closer to one third. What is clear is that the numbers have fluctuated over geological time, depending on the balance of life on Earth. When the atmosphere was first developing, for example, cyanobacteria contributed most of the oxygen.
The Rise in Oxygen Levels
It is thought that, initially, oxygen produced by cyanobacteria was used up reacting with iron in soils, rocks, and the ocean, forming iron oxide compounds and minerals. Geologists can estimate the amount of oxygen in the atmosphere in ancient times by looking at the kinds of iron compounds in rocks. In the absence of oxygen, iron tends to combine with sulfur, forming sulfides such as pyrites. When it is present, however, these compounds break down and the iron combines with oxygen, forming oxides. As a result, pyrites in ancient rocks indicates low oxygen levels, whereas oxides indicate the presence of significant amounts of the gas.
Once most of the available iron had combined with oxygen, the gas was able to accumulate in the atmosphere. It is thought that by about 2.3 billion years ago, levels had risen from a tiny trace to about 1% of the atmosphere. Things then seemed to balance out for a long period as other organisms evolved to use oxygen to provide energy by the oxidation of carbon, producing carbon dioxide (CO2). They achieved this by eating carbon-rich organic plant material, either living or dead. This created a balance, with oxygen production through photosynthesis matched by its consumption by oxygen-breathing organisms.
It seems that, because of this balance, photosynthesis alone cannot account for the initial rise in oxygen. One explanation is that some dead organic matter became buried in mud or other sediment and was not available to aerobic organisms. This matter could not combine with atmospheric oxygen, so not all the element produced was used up in this way, allowing levels to rise.
At some point later in the Earth’s history, oxygen levels rose dramatically to around their present level. Some scientists believe this may have happened around 600 million years ago. Around this time, a great many relatively large, complex, multicellular organisms appeared that would have required much higher oxygen levels. It is not clear what caused this change, however. Interestingly, it occurred as the Earth seemed to be emerging from a massive ice age, during which most of the planet was covered by ice.
One theory is that the action of glaciers, when advancing and retreating, ground up rock rich in phosphorus and released huge amounts of it into the oceans. Phosphorus is an essential nutrient for phytoplankton, so this may have caused an explosion of this form of life. This would, in turn, lead to increased production of oxygen, with probably very little land-based life to use it up. Not all scientists agree with this theory, however, and as of 2012, the issue remains unresolved.
Threats to Atmospheric Oxygen Levels
A study has shown that oxygen levels declined steadily between 1990 and 2008 by about 0.0317% overall. This is mostly attributed to the burning of fossil fuels, which use up oxygen in combustion. The decline, however, is less than expected, given the quantity of fossil fuels burned during that period. One possibility is that increased levels of carbon dioxide, possibly combined with use of fertilizers, has encouraged faster plant growth and more photosynthesis, partly compensating for the loss. It is estimated that even if all the world’s fossil fuel reserves were to be burned, it would have only a very small direct impact on oxygen levels.
Deforestation is another popular concern. Although the destruction of large areas of rainforest has many other serious environmental effects, it is considered unlikely to significantly reduce oxygen levels. In addition to trees and other green plants, rainforests support a whole range of oxygen-breathing life. It seems that these forests contribute very little to atmospheric oxygen levels overall, as they consume almost as much oxygen as they produce.
A more serious threat may be the impact of human activities on phytoplankton, which, according to some sources, make the biggest contribution to global oxygen levels. There is concern that increased carbon dioxide in the atmosphere from the burning of fossil fuels could make the oceans warmer and more acidic, which could reduce the amount of phytoplankton. As of 2012, the evidence is unclear, as different types of phytoplankton are affected differently. Some may decline in numbers, while others may grow and photosynthesize faster.