View source for Atmospheric oxygen

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[[category:Lecture 1A a changing planet]]
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[[File:Oxygen_history-atm.jpg|450px|thumbnail|Figure 1. The [[partial pressure]] of oxygen in the atmosphere vs. billions of years in the past. Measuring oxygen levels billions of years ago is tricky, so the graph gives a range of predicted values. The top curve represents the level of the highest guess at a given time while the lower curve represents the best guess of the lower limit of oxygen in the atmosphere.<ref name = "Holland" /> The boundary between region one and region two is the great oxygenation event (GOE).]]
[[File:Atmospheric composition history.png|450px|thumbnail|Figure 2. The makeup of the Earth's atmosphere as a function of time.<ref> Modified from figure 17.1 The Earth's Atmosphere p 276, Chapter 17 Atmosphere, Water & Weather by P. Bimblecombe and T. D. Davies, The Cambridge Encyclopedia of Earth Sciences, Cambridge University Press 1981, 496 pp.</ref>]]
<onlyinclude>'''Atmospheric oxygen''' refers to the abundance of [[molecular oxygen]],O<sub>2</sub>, in the atmosphere; especially in the [[layers of the atmosphere|troposphere]] that allows life to flourish.</onlyinclude> Atmospheric oxygen is the most important [[need]] humans have, even more important than [[food security|food]]). 

Figures 1 and 2 show the progression of O<sub>2</sub> over earth's geologic history. While possibly not at the peak experienced 10's of millions of years ago, the level of atmospheric oxygen levels (~21%) are high enough to allow Earth's ecological systems (and humans) to flourish.<ref name = Holland> "The oxygenation of the atmosphere and oceans" Heinrich Holland, The Royal Society Publishing Philosophical Transactions B, June 2006.</ref> Significant levels of O<sub>2</sub> are, cosmically, really quite rare!<ref name = Canfield> "Oxygen: a four billion year history", Donald Canfield, Princeton University Press, 2013.</ref> Oxygen will [[Chemical reaction|chemically react]] with almost anything, so having such a large amount of oxygen available in the atmosphere means that it must be continuously produced. The oxygen is produced by [[photosynthesis]], but if that were to ever stop [[chemical reaction]]s would make it all go away quite rapidly. If the chemical signature of molecular oxygen were found on another planet it would be a sure sign of life, since molecular oxygen wouldn't exist without life creating it.

In Earth's ancient past, photosynthesis developed [[time scale of the universe|billions of years ago]], which released O<sub>2</sub>. This oxygen didn't stick around as O<sub>2</sub>, but immediately reacted with rocks, effectively 'rusting the planet'. This rusting allowed many of the oxides in minerals to form, but after a certain point (roughly 1 billion years), the rusting stopped because the chemical reactions had all taken place. This allowed the 'great oxygen event' (GOE) to take place, see the line between region 1 and 2 in figure 1. This permitted the oxygen levels in the atmosphere to grow to significant levels. This process completely transformed the Earth and is probably the most significant impact that life has had on the [[environment]]. See figure 2 for how gases that make up the Earth's atmosphere have changed with time.

Today, atmospheric oxygen is essential for higher forms of life (living organisms with many cells) to exist. Additionally, [[fossil fuel]]s can undergo [[combustion]] because of atmospheric oxygen. The [[carbon]] in the fossil fuel, like [[coal]], reacts with the [[oxygen]] in the atmosphere to form [[carbon dioxide]]. The oxygen in the atmosphere is 'free' for anyone to use. The lack of abundant O<sub>2</sub> under water is what makes submarines hard to power (can't burn fossil fuels without O<sub>2</sub>), and lack of O<sub>2</sub> in space requires space missions to use other [[primary energy]] sources like [[radioisotope thermal generator]]s or [[solar power]].

For more information on the history of oxygen please see Donald Canfield's: "Oxygen: a four billion year history".<ref name = "Canfield" /> For a scientific paper on the subject please see: "The oxygenation of the atmosphere and oceans" by Heinrich Holland.<ref name = "Holland" /> 

==For Further Reading== 
*[[Periodic table of elements]] 
*[[Oxygen]] 
*[[Atmosphere]] 
*[[Fossil fuel]] 
*[[Natural vs anthropogenic climate change]] 
*Or explore a [[Special:Random|random page]]

==References==
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