Consequence for Percentage of Oxygen in Atmosphere of Burning all Biomass and Fossil Fuels

On the Earth, there is 610 GT in the biosphere and 5000 GT locked up in the form of fossil fuels. If we were to burn all carbon in the biosphere, and in form of fossil fuels, how would the level of oxygen change? This question is important to us because we have evolved to live in an atmosphere which is about 21% oxygen. We breathe in air that is about 21% oxygen air breathe out air that is between 13.6 % and 16% oxygen. If the % of oxygen in the atmosphere falls significantly, we may find it hard to breathe.
Suppose all the carbon is burnt via the equation:
\[C_2 H_4 +3O_2 \rightarrow 2CO_2 +2H_2O\]

Three times as many oxygen atoms are consumed in this equations as oxygen atoms.
There are a total of 5610 GT of carbon available to be burnt. This is  
\[\frac{5610 \times 10^9 \times 10^3}{0.012} = 4.675 \times 10^{17} \: mol\]

We will require three times as many oxygen atoms as carbon atoms. Since oxygen molecules occur as  
, this means 1.5 times as many moles of oxygen, so we require  
\[1.5 \times 4.675 \times 10^{17} = 7.01 \times 10^{17} \: mol\]
  of oxygen.
The mass of 1 mol of oxygen is 32 g, so we need  
\[7.01 \times 10^{17} \times 0.032 = 2.243 \times 10^{16} \: kg\]
The mass of oxygen in the atmosphere is about  
\[1.176 \times 10^{18} \: kg\]

This is the total mass of oxygen burn up by burning all the carbon in the biosphere and fossil fuels. It is a fraction  
\[\frac{2.243 \times 10^{16}}{1.176 \times 10^{18}}= 0.019\]
  of the oxygen in the atmosphere.
There are substantial masses of carbon in the soil and oceans (1600 GT and 40,000 GT) respectively. If this is released into the atmosphere as a result of global warming, the calculations above could be substantially different.

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