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Where does Atmospheric Oxygen Come from?

Mary McMahon
Updated Mar 05, 2024
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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.

Frequently Asked Questions

What is the primary source of atmospheric oxygen?

The primary source of atmospheric oxygen is photosynthesis, a process conducted by plants, algae, and cyanobacteria. During photosynthesis, these organisms use sunlight to convert carbon dioxide and water into glucose and oxygen. According to the National Oceanic and Atmospheric Administration, phytoplankton contribute between 50 to 85 percent of the oxygen in Earth's atmosphere.

How much oxygen do the world's oceans contribute to the atmosphere?

The world's oceans are a significant contributor to atmospheric oxygen, primarily through the photosynthetic activity of marine phytoplankton. It's estimated that phytoplankton are responsible for producing about 50% of the Earth's oxygen each year, making the oceans a vital component of the global oxygen cycle, as reported by the National Aeronautics and Space Administration.

Can atmospheric oxygen come from sources other than photosynthesis?

While photosynthesis is the dominant source of atmospheric oxygen, there are other minor sources. For example, some oxygen can be produced through the photodissociation of water vapor in the upper atmosphere by ultraviolet light. However, this process contributes a negligible amount compared to the vast quantities generated by photosynthesis on Earth's surface.

How do forests impact the level of atmospheric oxygen?

Forests play a crucial role in maintaining atmospheric oxygen levels. They are often referred to as the "lungs of the planet" because of their extensive photosynthetic activity. Mature forests, in particular, store significant amounts of carbon and continually recycle carbon dioxide into oxygen, helping to stabilize the atmospheric oxygen concentration. Deforestation, however, poses a threat to this balance by reducing the number of trees available to perform photosynthesis.

Is the oxygen level in the atmosphere constant?

The oxygen level in the atmosphere is not constant and can be influenced by various factors, including human activities, natural events, and the balance of photosynthesis and respiration. However, over geological timescales, the oxygen concentration has remained relatively stable, with current levels at about 21% of the Earth's atmosphere, as stated by the United States Geological Survey.

What would happen if the rate of photosynthesis on Earth decreased?

If the rate of photosynthesis on Earth decreased significantly, it would lead to a reduction in atmospheric oxygen levels over time. This could have dire consequences for aerobic life, which depends on oxygen for respiration. Additionally, it could disrupt ecological balances, affect climate patterns, and lead to increased carbon dioxide levels, exacerbating global warming and climate change.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon350982 — On Oct 09, 2013

Before you get to the "brilliant" conclusion that you can clear cut every forested area you know please consider:

1. Plants absorb CO2 from the atmosphere and so significantly reduce the greenhouse effect.

2. A tree is not just a tree, and a forest is not just a clump of trees. They are a very important part of complex ecosystems that we need to keep in order to keep living.

3. By retaining CO2, a tree is contributing to slow down the acidification ratios of the ocean.

4. The acidification of the ocean will reduce the oxygen production.

5. The shadow produced by a tree reduces water evaporation.

6. We all need three essential things to exist: drinkable water, oxygen and food. For that, we must have minimal survival conditions for ourselves and for the animals we will need to eat;

7. In the last 100 years, more than 95 percent of the existing forests have been cut, contributing to the reduction of earth's biodiversity and desertification.

8. Inside the earth's forests resides the potential for the recovery of what we really need to survive in the future as a species.

9. The recovery of the soil after a clear cut may take several hundreds of years. By taking down the species that keep the soil's health you will be creating desert areas contributing to the loss of natural resources.

By anon317932 — On Feb 04, 2013

We are all dead already -- dead in the way someone who has fallen off a skyscraper is as good as dead before he hits the ground, and then really dead afterwards. We are debating the effects of falling, while falling to our deaths.

By anon275714 — On Jun 19, 2012

A few billion years ago, before life evolved on earth, the atmosphere didn't have any oxygen. The only reason there is oxygen now is because plants, cyanobacteria and plankton evolved to produce it. Creatures like ourselves use that oxygen (when we breathe) for the energy of life, and breathe out CO2, a byproduct of that process. Meanwhile the CO2 gets used by the plants, etc., all in an endless oxygen / CO2 / oxygen cycle that makes life on earth possible.

If trees, cyanobacteria and plankton disappeared from the earth or became drastically reduced, it wouldn't be long before we and other creatures like us would use up most of any remaining oxygen and life as we know it would end. Or, if we lost only half the oxygen, it would be as though we all lived at 19,000 feet or above.

Note: Humans burning fossil fuels is adding more CO2 than is found in the above cycle, upsetting the balance that has evolved over the millennia and beginning to produce human generated climate change.

By profdrake — On Aug 29, 2011

my studies suggest that plants actually adjust the ratio of oxygen and co2 used and produced according to ratios present in our environment temp and humidity also has an effect which contributes to keeping 02 and C02 levels constant. any thoughts?

By anon210129 — On Aug 29, 2011

The earth’s atmosphere is about 20 percent O2 (though I’ve seen other figures). Estimates on how atmospheric O2 is supplied: 1-2 percent by the sun splitting water molecules via UV. Around 1/3 to 1/2 by photosynthesis employed by oceanic cyanobacteria and plankton (c&p). Around 2/3 to 1/2 by photosynthesis employed by trees.

If the available CO2 is the limiting ingredient mediating the balance between the trees and c&p, then as trees decrease, the c&p population should increase. If that isn’t the case, then O2 should drop to 7.7 – 11.1 percent. (If there’s historical data on tree and c&p numbers, these can be compared for an inverse relationship.) Or, if that’s too much the case (as such an ecological equation may not be as linear as a chemical reaction, then O2 could exceed 20 percent.

How much is too little? I’ve seen estimates that humans require 16-18 percent O2. How much is too much?

By anon163379 — On Mar 27, 2011

what does high/low levels of oxygen mean to life on earth?

By bodryn — On Jan 29, 2011

anon134098: I'm no expert on this, but it seems to me the constant winds all over the earth would do a good job of keeping oxygen evenly supplied to all parts of the earth.

By anon134098 — On Dec 13, 2010

"planktons and plants are produces oxygen." i agree that but what does keep oxygen level in deserts, since there is no plants in desert and now a days oxygen consumption is high due to industrial growth.

By anon133932 — On Dec 12, 2010

It seems as though this is a complicated topic to explain. Organisms that perform photosynthesis that lived about 1.1 billion years after the formation of the earth are the reason we have an oxygen environment, but from which molecule, CO2 or H2O, did the water come.

H2O is clearly the right answer, but how can one prove it? By looking at the thermophilic reactions that take place near vents of the ocean, we see c6h1206 and h2s made. With oxygen and sulfur both being in the same period, it is safe to say that it acts the same way as sulfur, as far as electron activity in the outer shell, that is. So, is it too far a jump to say that the oxygen came from H20 in the same nature as the Sulfur came from the H2S.

By anon127763 — On Nov 17, 2010

Over the entire life cycle of a plant, (including decomposition) total CO2 input is roughly equal to CO2 output. Through its life, it sequesters the carbon by turning it into the woody parts that make up a plant (bark, leaves, stems, and the rest).

This sequestering occurs at different rates through the plants life, for instance when it is young it is taking in more carbon than when it is very old. However, in the case of tress, the rate of CO2 sequestering goes down, but the total input of CO2 is high because old trees are also big trees.

In the case of deforestation, unless the tree is cut at the right time in its life, the deforestation will result in a net carbon output. It is also important to note that trees don't just grow, they produce fruits and nuts, which use a lot of carbon (not to mention the decomposition process wherein the fallen tree becomes food for bacteria, fungus, and animals) and are another way by which forests are net carbon sequesterer (they take in more than they release). And that's just terrestrial photosynthesis.

Ocean photosynthesis, primarily by phytoplankton, is more complex in its interconnections. Phytoplankton live near the surface of the ocean and live short lives, where they convert CO2 into oxygen through a process that takes about 2 hours to explain.

The short of it is CO2 enters the oceans through wave action, turning the water it interacts with into carbonic acid. That acid destroys the tiny calcium shells of the plankton. The resulting calcium carbonate is used by other phytoplankton to make shells.

Meanwhile, another type of plankton goes through a silicon driven process (derived from sand which is SiO2). The silicon ones fall to the bottom of the ocean when they die, but the calcium ones stay near the top unless eaten.

The material that gets to the bottom becomes part of a 990 year process before eventually coming up to the top again.

As you can see, things are more complicated than you may expect. Read up on ocean acidification and deforestation before making claims about environmentalists being alarmists.

By anon120775 — On Oct 22, 2010

Photosynthesis is reduced to an equation that says Carbon Dioxide and water are turned into carbohydrates and Oxygen storing the light energy in the carbohydrates. The Oxygen comes from the water. --Fred

By anon110878 — On Sep 13, 2010

If all the oxygen in the atmosphere came from the photosynthetic reduction of carbon dioxide, how can oxygen be "good" and carbon dioxide "bad"?

By anon92677 — On Jun 29, 2010

Byron, you're correct. plants are essentially carbon neutral. I can only assume that these numbers have been created by alarmist environmentalists to discourage deforestation etc. Seems like every figure we read in the news is pushing an agenda nowadays.

By Larry Lawhorn — On May 15, 2010

30 percent of our oxygen is from trees on land.

70 percent of our oxygen is from phytoplankton in the oceans.

30 percent of the ocean phytoplankton has been lost to ocean acidification since in the past 30 years.

And "big oil" is spraying the sky with chem-trails so we can subsidize the burning of more fossil fuels.

By anon79679 — On Apr 23, 2010

I missing something. CO2 is only .038 percent of the atmosphere by volume. How does .038 percent CO2 get converted to 20 percent oxygen by plants?

By anon78183 — On Apr 17, 2010

Good question, Sherlock. What keeps the oxygen stable?

By anon61775 — On Jan 22, 2010

What keeps the percent of oxygen stable?

By anon58335 — On Jan 01, 2010

OK so we all know plants produce oxygen and we cut down the plants.

So what will we be breathing in another say 20 years?

Why is this little fact not harped upon by the environmentalists!

By Katonoah — On Dec 14, 2009

By logical inference, since plants do produce a portion of our atmospheric oxygen, perhaps up to 30 pc, it then follows that they must consume less oxygen at night than they do during the day, since they produce a positive amount overall.

By anon43540 — On Aug 30, 2009

I'm confused. Does oxygen still come from plants when it is needed for burning coal?

By anon4536 — On Oct 22, 2007

The primary source of atmospheric oxygen is not plants, but plankton. The ratio is about 70:30.

By Dayton — On Jul 25, 2007


I did a bit of research into your question, and here's what I discovered:

During the day, photosynthesis uses carbon dioxide and sunlight to produce energy for the plant, and water as a byproduct. Oxygen is a byproduct of a small portion of this process.

At night, when photosynthesis is not occurring, the process doesn't exactly "reverse" itself, as you say. The plant does consume oxygen, but at a far slower rate than it produces during the day. The net result is that plant processes are the primary contributor of oxygen to our atmosphere.

By Byron — On Jul 22, 2007

Plants produce oxygen and consume carbon dioxide during the day. At night this process is reversed. How, then can they provide a significant amound of oxygen? Seems to me that the this overall process contributes to about zero.

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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