How much oxygen do plants produce?

The hard-to-believe answer

The short answer is: plants produce little to no excess of breathable oxygen. I know that sounds like an unbelievable statement. A little-known fact is that plants also breathe oxygen. This is called the respiration cycle, and it is the exact opposite chemical formula as photosynthesis. (See below.) A plant spends half its day making fuel (sugar) and the other half of the day burning the fuel with oxygen to create energy, much like an animal or a car. And when a plant dies, bacteria in the decomposition process consume almost all of the excess oxygen the plant produced during its lifetime. It’s the same as if the tree was burned. 

Many scientists say that all the breathable oxygen in the atmosphere comes from phytoplankton in the ocean. And interestingly, if the phytoplankton decomposes, it consumes oxygen. So, it is primarily when the dead phytoplankton sinks to the bottom of the ocean and gets buried, escaping decomposition, that there is an excess of oxygen leftover. However, it is such a small amount that it took billions of years for the phytoplankton to fill the atmosphere with oxygen. 

As a side note, when we are talking about producing or making oxygen, we are talking about breaking water (H₂O) and recombining the oxygen into molecular or breathable oxygen (O₂). By the way, a lot of the remaining hydrogen often floats away into outer space. The only thing that can actually make oxygen is the fusion that powers a star. All the oxygen in our solar system is the result of a supernova.

Complicating things even further, humans are deforesting and polluting the planet quickly. So, we are killing the things that make the air we breathe. To make matters worse, less oxygen also means fewer trees. Yes, trees breathe oxygen. (By the way, I use the word “tree” to mean all plants, especially those on land.)

Science hasn’t yet fully explained photosynthesis. It’s still a mystery and — quite literally — a miracle of life. In this article, my goal is to demonstrate how fragile the oxygen cycle is. As we know, carbon dioxide emissions are a problem, but so too is oxygen depletion. Plants can’t make up for all the oxygen that we’re freely burning in our combustion engines as if there’s an endless supply. I also hope to attract some attention from the scientific community to investigate this issue. 

Let’s explore these ideas in detail. If you want to skip the math, I’ve highlighted the answers below.

Myths/Misconceptions

I want to start by correcting some common misconceptions. It took me a long time to unbury them from all the garbage misinformation on the internet. 

One) Trees do not permanently sequester carbon dioxide. I’m all for planting more trees. And while a tree is alive, it will capture and store carbon in the form of glucose or cellulose (wood), but when the tree rots or burns, it releases all of its carbon dioxide back into the environment, though it is possible to be stored for thousands or millions of years. Coal, for example, is compressed plant matter.  

Two) Trees do not turn carbon dioxide into oxygen. Photosynthesis breaks the bond of water, which releases oxygen into the atmosphere. It is basically the same as electrolysis. So, in order to breathe, some water is broken. In theory, if this went on long enough, the Earth would run dry. 

Three) Trees do not recycle oxygen. Trees permanently sequester oxygen by bonding it to carbon to make glucose. This bond, practically speaking under Earth-like conditions, is permanent.** Eventually, the oxygen molecule gets taken out of the food chain and becomes something like compost (dirt), limestone or coal. 

Four) Almost all breathable oxygen comes from phytoplankton in the ocean, which grows more than a thousand times faster than land plants. A lot of phytoplankton escape rotting, which consumes oxygen, by sinking to the bottom of the ocean. But sometimes the opposite happens: if there are too many bacteria present in the water, the phytoplankton decompose and cause hypoxic dead zones.

Photosynthesis and cellular respiration — a break-even process

Respiration is used by all living humans and animals, to make energy for movement, heat and to keep vital organs running, without it we’d be dead. The difference between plants and animals is that plants use sunlight (energy) to synthesize food from inorganic matter. Plants are self-sufficient; whereas, animals need to eat plants as their source of food. (Fancy word alert! Plants are photoautotrophs.) Below are the chemical equations for what I’m talking about. 

The word equation for respiration is: 
Glucose + Oxygen = Carbon Dioxide and Water

The chemical equations are:

Photosynthesis (food production): 
6CO2 + 6H2O + Energy (sunlight) → C6H12O6+ 6O2

Cellular respiration (food consumption): 
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Notice that the equation for photosynthesis is the direct opposite of cellular respiration. You can see that the oxygen produced is then consumed again. So, according to this equation, photosynthesis produces no extra oxygen. But, it’s not that simple. Keep reading to find more detailed answers.

How much oxygen do plants make according to scientists?

How much excess oxygen do plants make? I’ve spent days researching this one thing alone. Short answer: hardly any. The oxygen built up in our atmosphere is the product of billions of years. 

The longer answer is that this process is not well understood by science. While it is relatively easy to measure the oxygen one leaf produces by putting it in a jar or underwater, calculating the oxygen the whole biosphere produces is just an estimate based on theory and satellite observations. (A study on CO2 fertilization describes this process.)

Here are some of the best answers that I have found. Further below, I will demonstrate how to arrive at our own answer using math.

• Professor of Ecosystem Science, University of Oxford, Yadvinder Malhi, says, “The net contribution of the Amazon ecosystem (not just the plants alone) to the world’s oxygen is effectively zero. The same is pretty much true of any ecosystem on Earth, at least on the timescales that are relevant to humans (less than millions of years).” 
• And professor of atmospheric science, Colorado State University, Scott Denning, cites 0.0001% of oxygen being leftover in the process —  a “vanishingly tiny fraction.”*
• Assuming our oxygen comes from the ocean, it would seem to be easy to estimate how much the Earth is producing. This research letter about “Increasing Escape of Oxygen From Oceans Under Climate Change” cites the oceans as outgassing 1.6 gigatonnes of oxygen per year, which matches some other sources. However, they attribute almost all of this outgassing due to global warming and not photosynthesis. As the oceans warm, the oxygen boils off. Already there are huge oxygen-depleted dead zones in the ocean. 
• Doing my due diligence, I must cite one contrary opinion. This article overall is great, however, I respectfully disagree with this number. The global oxygen budget and its future projection lists land, including plants as outputting 16.01 gigatonnes of oxygen/year. 

Calculating oxygen production based on historical evidence of photosynthesis

To reverse engineer the excess oxygen the Earth produces, take a look at this graph. As you can see, it took about 1.5 billion years for the Earth to produce enough excess oxygen in the atmosphere to sustain life on land. (By the way, it took an additional 1– 2 billion years for the oceans to become saturated with oxygen.)

Per my calculations of how much oxygen is in the atmosphere, there are 1,184,090 gigatonnes of oxygen in the atmosphere now. Roughly speaking, according to our graph, there was half as much oxygen in the atmosphere when life on land began. That equals about 592,045 gigatonnes.

So if we divide 1.5 billion years by 592,045 gigatonnes of oxygen, we get an estimate of the annual excess oxygen produced per year — before any animals existed to suck it all up and long before machines began breathing. This equals 2534 gigatonnes of excess oxygen produced per year prior to animal life.

Double-checking my answer: photosynthesis yields 0.002% excess oxygen. It’s not far off the number that Professor Denning estimated (see above) at 0.0001%. My answer seems too good. Like, my estimate of the amount of oxygen in the atmosphere, all I used was some basic math. As I said, photosynthesis is not well understood. We are all just guesstimating.

So, we can see that photosynthesis contributes little excess oxygen. If it were, after billions of years, the atmosphere would be a gigantic ball of oxygen. 

Why is photosynthesis so inefficient?

We’ve calculated how much oxygen plants produce in three different ways. None of them yield a lot. Here’s one reason why. Photosynthesis only converts about 3% of light energy into glucose.  

Take a look at this PBS video about the enzyme rubisco. It’s what makes photosynthesis happen. But it is very inefficient. About one in every four or five tries, it goes wrong. Instead of taking a carbon dioxide molecule out of the atmosphere, it accidentally uses oxygen. So, potentially 20–25% of the excess oxygen a plant produces gets reabsorbed and lost in this poor reaction. And these accidental compounds that rubisco creates shut down photosynthesis. 

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Double jeopardy — Are the plants suffocating too?

Plants can suffocate. And if they are, we should notice a drop in the tree line worldwide as oxygen levels would drop faster at higher altitudes.  

I used to think this was happening, but a recent scientific study in Nature reports an increase in photosynthesis by 11.85 ± 1.4%. This is the result of CO2 fertilization. Increased plant growth is due to the increased carbon emissions in our atmosphere. More carbon dioxide means more plant growth and more plant growth means more oxygen. Ironically, global warming is offsetting itself. But maybe not for long. 

That being said…

Why isn’t the Earth greener? 

If global warming is occurring and creating wetter and warmer environments, then shouldn’t the Earth be greener and lusher than ever? The above article thinks so. Yet, it seems a brown and dirty world. On land, I see deforestation, desertification, urbanization and pollution have killed or displaced plants. And in the sea, we have hypoxic dead zones due to runoff from cities and farms, ocean acidification, changing current patterns, and warming oceans destroying the plants. This NASA study reports a 6% decrease in the ocean’s net primary productivity (NPP), including phytoplankton, one of the most important organisms on the planet. 

Summary

Above I made the case that photosynthesis is a prolonged and inefficient process. The oxygen in our atmosphere is the result of billions of years of excess build-up. So, as humans burn more and more, I think it would be a mistake to rely on plants to make more oxygen. 

And in my opinion, I believe the Earth’s ecosystems are dying, and, therefore, its ability to produce oxygen may suddenly drop. Currently, it appears oxygen depletion is offset by some factors like CO2 fertilization, ocean warming and glacial melt, which I talk about in other articles. In the future, I believe this missing oxygen will play as much of a role in climate change as too much carbon dioxide.

Please forward this article to someone who you think might be able to advance the research. It will take all of us to solve the challenges of climate change. In the meantime, may I recommend riding a bicycle and planting a garden? 

Postscript. Is there another source of oxygen?

Since photosynthesis is so inefficient, and the oxygen and carbohydrates should be equally balanced but aren’t, some scientists are looking at other sources of atmospheric oxygen on Earth. One theory is that the ionizing radiation of the sun can sometimes split apart a water molecule.† This is called radiolysis. It is similar to electrolysis. And if this theory is true, it is also a very slow process, and a process that predominately happened billions of years ago when the sun was stronger and before the Earth had a protective magnetic field.