The oxygen we breathe. Where does it come from? Will it be available forever?

What is the great oxidation event?


 Earth's atmosphere wasn't always as rich in oxygen as it is today. Oxygen makes up 21% of the atmosphere now, but it represented only 0.001% of these levels during the first two billion years of Earth's history.


 It can be said that oxygen on Earth was relatively scarce for most of our planet’s history of 4.6 billion years, but at some point the Earth underwent what scientists call the Great Oxidation Event (GOE), where microbes evolved  The oceans produce oxygen through photosynthesis, and oxygen first accumulated in the Earth's atmosphere at this time is believed to have occurred between 2.5 and 2.3 billion years of Earth's history.


 Later, the largest plants on Earth caused an astonishing increase in oxygen in our atmosphere. Oxygen has stabilized relatively at a high level over the past 500 million years. Today, about half of photosynthesis takes place in the ocean and half on land.


 How much oxygen does the ocean produce?


 According to the National Oceanic and Atmospheric Administration (NOAA), scientists estimate that 50-80% of Earth's oxygen production comes from the ocean, with most of that production coming from ocean plankton such as floating plants, algae and some bacteria that can carry  process of photosynthesis.


 There is also a certain type of bacteria called Prochlorococcus, which is the smallest photosynthetic organism on Earth, which alone produces up to 20% of the oxygen in our entire biosphere, a percentage higher than all the tropical rainforests on Earth combined.  However, it is difficult to calculate the exact percentage of oxygen produced in the ocean because the amounts are constantly changing.


 Scientists can use satellite images to estimate the rate of photosynthesis that occurs in the ocean, but satellite images cannot tell the whole story, as the amount of plankton changes seasonally in response to changes in the amount of nutrients in the water, temperature and other factors, in addition to some studies  showed that the amount of oxygen at specific sites varies with time of day and tides.


Is the ocean really responsible for the oxygen we breathe?


 It's true that the ocean is responsible for about 50% of the oxygen produced on the planet, but it's not responsible for 50% of the air that we humans breathe, says Misha Kitchell, editor of The Conversation.


 Most of the oxygen produced directly by the ocean is consumed by microbes and marine animals, and oxygen is also used when dead plants and animals decompose in the ocean.  In fact, net oxygen production in the ocean is close to zero.


 A tiny fraction of the primary production of oxygen (about 0.1%) escapes and is stored as organic carbon in marine sediments, a process referred to as a 'biological carbon pump'. This organic carbon may eventually be converted into fossil fuels such as coal, oil and gas.


 The tiny amount of oxygen generated to produce this carbon store can later be released into the atmosphere, and a similar process occurs on Earth as well, with some of the carbon stored in the soil.


 Therefore, the oxygen we breathe today does not come from contemporary production, whether from the land or the biosphere, but rather from the slow accumulation of oxygen in the atmosphere supported by the burial of organic matter over very long periods of time, up to hundreds of millions of years, as the continuous burial of carbon  Essential to keep enough oxygen pumped to the atmosphere.


What about the future of oxygen in the atmosphere?


 Burning billions of tons of carbon stored in fossil fuels today removes large amounts of oxygen from the atmosphere, thus reversing the pattern that led to the rise in atmospheric oxygen in the first place, where the level of oxygen in the atmosphere decreases as the concentration of carbon dioxide rises.


 But, according to Conversation, as early as the 1970s, prominent geochemist Wally S. Brooker realized that if we burned all known fossil fuel reserves, we would use less than 3% of our oxygen tank.


 Also, if we were to cut down or burn all forests and oxidize all the organic carbon stored in vegetation and topsoil around the world it would only lead to a small depletion of atmospheric oxygen.


 And if photosynthesis in the ocean and on land stops producing oxygen, we can continue to breathe for thousands of years despite other problems we will surely face.


 The expected reduction in atmospheric oxygen even in a worst-case scenario with widespread burning of fossil fuels and deforestation would be negligible for a large atmospheric reservoir.


 Models show that the oxygen content in the atmosphere will change little over the next 100,000 years in response to fossil fuel use.



 So, while there are many things to worry about in the future of our climate that the availability of oxygen to air-breathing organisms - including humans - is not among them, this is not the case with oxygen in the oceans.


Why is the oxygen in the oceans declining?


 The ocean contains less than 1% of the oxygen stored in the atmosphere, so the ocean's oxygen reservoir is at risk, especially in areas with very low or completely absent oxygen, referred to as oxygen minimum areas, which expand as the temperature rises.  planet.


 Already, the open oceans lost 0.5 to 3.3% of their oxygen stocks in the top 1,000 meters between 1970 and 2010, and the oxygen minimum areas increased in size by 3 to 8%.


 This loss of oxygen is primarily due to the increase in ocean layers, where the mixing of warmer and lighter ocean surface layers is less efficient with deeper and denser ocean layers, which limits oxygen penetration, in addition to an increase in oxygen consumption by ocean organisms with high  its temperature.


 A study published in the American journal Science in 2018 found that oxygen minimum areas in the open ocean have expanded by several million square kilometers, and that hundreds of coastal sites now have oxygen concentrations low enough to limit animal numbers and alter the cycle of important nutrients.  The size of low-oxygen areas is expected to grow by about 7% by 2100 under a high carbon dioxide emissions scenario.


Deoxygenation of this kind affects biodiversity and food webs, as well as negatively impacting food security and the livelihoods of the people who depend on it.


 Misha Kitchell says that while it is wrong to say that the ocean provides 50% of the oxygen we breathe, it is correct to say that over geological time scales the ocean provides a large portion of the oxygen we breathe today, and it is quite correct to say that the ocean is responsible for 50  % of primary production on Earth, which sustains our food system.


 While we should not worry about the future supply of oxygen for humans to breathe, we should be concerned about the increased displacement of fish from the vast ocean areas where oxygen is depleted.

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