What Would Earth Be Like With Zero Oxygen Levels?

What Would Earth Be Like With Zero Oxygen Levels?

This question originally appeared on Quora, a knowledge sharing network.
D.J. Brenner, who has worked with strong oxidisers including pure oxygen and fluorine answers this as the following:

"The most likely answer is that Earth would be a lot like Venus. Oxygen is abundant in our neighbourhood of the solar system, being found making up the majority (by mass) of the CO2 in Venus’s atmosphere. It also combined with the iron on Mars to form rust, giving Mars its characteristic red color. Earth has plenty of carbon to combine with, so you would expect elevated CO2 levels in our atmosphere and oceans. The oceans will be able to absorb a lot of CO2 so we would not end up with the extreme greenhouse effect that exists in Venus." 

A real-colour image of Venus taken by Mariner 10 processed from two filters. The surface is obscured by thick sulfuric acid clouds.

A real-colour image of Venus taken by Mariner 10 processed from two filters. The surface is obscured by thick sulfuric acid clouds.


Venus is the second planet from the Sun and is the second brightest object in the night sky after the Moon. Named after the Roman goddess of love and beauty, Venus is the second largest terrestrial planet and is sometimes referred to as the Earth’s sister planet due the their similar size and mass. The surface of the planet is obscured by an opaque layer of clouds made up of sulfuric acid. The surface temperature on Venus can reach 471 °C.

So what is Carbon?

Carbon is a chemical element, like hydrogen, oxygen, lead or any of the others in the periodic table.

Carbon is a very abundant element. It exists in pure or nearly pure forms – such as diamonds and graphite – but can also combine with other elements to form molecules. These carbon-based molecules are the basic building blocks of humans, animals, plants, trees and soils. Some greenhouse gases, such as CO2 and methane, also consist of carbon-based molecules, as do fossil fuels, which are largely made up of hydrocarbons (molecules consisting of hydrogen and carbon).

In the context of climate change, "carbon" is commonly used as a shorthand for carbon dioxide, the most important greenhouse gas released by humans. Technically, however, this isn't accurate. Carbon only becomes carbon dioxide when each atom of carbon joins with two atoms of oxygen (hence the chemical formula of carbon dioxide, CO2).

This shorthand can sometimes cause confusion, because although "a tonne of carbon" will often be used to mean "a tonne of CO2", in a scientific context the same phrase could mean "CO2 containing a tonne of carbon" (which is a much smaller amount, as oxygen accounts for most of the weight of each CO2 molecule).

The term carbon also crops up in the phrase carbon footprint, which describes the total amount of greenhouse gases released as the result of a given activity. In this context, "a tonne of carbon" may mean something else still: "a mix of greenhouse gases with a combined warming impact equivalent to that of a tonne of CO2".

Carbon molecules move around the Earth system in the carbon cycle.

Why does earth have oxygen?

The reason Earth has free oxygen in its atmosphere is because of life. Early in the Earth’s history the atmosphere would be exactly like we described above. Ancient microbes (think algae-like and bacteria-like) lived on this world without oxygen. They eventually evolved to find the process of photosynthesis. For them it was just a way of storing energy. They needed a lot of carbon for this to work, which they got from the C in CO2. To them free oxygen was just a byproduct, but it eventually built up to the point where the majority of the planet went from an anaerobic (without oxygen) environment to an aerobic (with oxygen) environment.

Could life have evolved to the point where we are now without oxygen? Maybe, but it would be much more difficult. You would need an alternate energy source as abundant as the sun. We know whole ecosystems pop up near hydrothermal vents, and these systems use disulfide bonds as their primary energy source. The distance between hydrothermal vents can be quite large, so you don’t get the migrations and adaptations that you get with animals that rely on light and the carbon cycle to exist. It will take a lot longer for these ecosystems to develop into complex life, if they ever get there.

You could also interpret this as “what would Earth look like with no oxygen, not even in molecules.” That is a question that is almost impossible to answer, but from the perspective of Brenner the answer would be “strange.”

We exist today because of algae.

It is estimated that marine plants produce between 70 and 80 percent of the oxygen in the atmosphere. Nearly all marine plants are single celled, photosynthetic algae. Even marine seaweed is many times colonial algae. They are a bunch of single cells trying to look like a big plant (see seaweed photo), but they are really individuals.


Seaweed are not plants, but are algae. Not only do algae provide much of the Earth's oxygen, they are also the base for almost all marine life. Green algae get their color from chlorophyll and exist on or near the surface where there is plenty of sunlight. Green algae are not as common in the ocean as brown and red seaweed. It is also more closely related to land plants than any other type of algae. 

We need marine algae a whole lot more than they need us. Trees and other land plants are very important, but to survive we need algae. Why does so much of our oxygen come from algae? First of all, remember that the oceans cover about 71 percent of this planet and land is only about 29 percent. If we assume that every square mile of the ocean produces as much oxygen as every square mile of land, then this makes sense. The oceans would produce about 71 percent and the land 29 percent of the oxygen we breathe. 

Are oceans as productive as land?

Now the question is, “Are the oceans, indeed, as productive as the land?” At first you might not think so, after all when you look at the land there are trees, bushes and grass and all kinds of plants growing. They must produce a lot of oxygen! They do, but also remember that there are many places on land that don’t have much in the way of plants. How about Antarctica or the Sahara Desert along with many others? These are good-sized chunks of real estate where plants are rare. How much oxygen is being pumped out in these areas? Some areas on land have an abundance of plants and produce a large quantity of oxygen, while others have very few plants and produce very little.


Rainforests are the oldest living ecosystems on the planet. They cover 2 percent of the Earth's surface or 6 percent of land, and are an important source of oxygen. They also are home to many of the Earth's plant and animal species, but they also play a vital role in sustaining life beyond their ecosystems as they help regulate global climate and water cycle. Pictured above is a small section of the Brazilian rainforest, the Earth's largest.

The same can be said for the oceans. There are some areas that have an abundance of algae living in the waters and other areas that don’t. In the ocean, there are areas of upwelling where cold, nutrient rich bottom water moves toward the surface. These upwelling waters mix with the surface water and produce an area that is like liquid fertilizer for plants. There are billions of these critters pumping out oxygen. Other areas of the oceans don’t have much in the way of nutrients in the water and they are like the deserts on land with very few plants.

We can conclude that algae are equally important for producing oxygen as plants and trees are on land.

Plants on land are easy to spot. Plants in the ocean are a bit more difficult to see since they are single cells floating in the water. Even though you may not see them, they are there. Remember, these little cells go down to over 300 feet below the surface so they have lots of room to spread out.

Did you know that there are more than 7,000 different species of algae? Most live in the oceans, but they also live in fresh water and even on land. Also, algae produce about 330 billion tons of oxygen each year.

So that breath of fresh air you are getting is due for the most part to our friend, the algae.
If we kill them by polluting the oceans, we are also killing our vital lifeline. Now what are we going to do about it?



Article: Elizabeth Plokker