The combined action of land and oceans on atmospheric oxygen
Photosynthesis, respiration and dissolution in seawater act simultaneously and vary the concentration of oxygen in the atmosphere. These variations were measured by Keeling and Shertz (1992) in three places on the planet: Alaska, California and Tasmania. In the first two, both located in the northern hemisphere, there is a maximum of atmospheric oxygen in summer and a minimum in winter. The summer maximum is mainly due to the photosynthesis on the land surface that characterize this hemisphere. In winter, breathing and dissolving in the colder sea water combine to give rise to a minimum. Between maximum and minimum, the difference is approximately 26 ppm (parts per million) in Alaska, and 20 ppm in California. This difference is smaller in Tasmania (18 ppm), the southern hemisphere being mainly occupied by the oceans and terrestrial photosynthesis only plays a small role. These seasonal variations are very small compared to the oxygen concentration of about 200,000 ppm. The highest, that observed in Alaska, represents only 0.013%. Even a small change in the oceans balance , or global photosynthesis (even in the case of the Amazon, the so-called "lung of the planet"), could threaten the quality of the atmosphere in the medium term.

Human disturbance

For more than a century, man has been meeting his energy needs by burning coal and hydrocarbons from photosynthesis of past geological eras. This combustion consumes oxygen. Similarly, the increase in the world's population has led to the clearing of forests and replacing them with crops. This practice also results in oxygen consumption. On one hand, the biomass that constitutes the forest, very high, is largely transformed into carbon dioxide. On the other hand, agricultural practices generate an impoverishment of the soil by degradation of the organic material they contain (humus). This degradation is caused by organisms that breathe and therefore consume oxygen. The corresponding decreases are in total at the rate of about 4 ppm / year or about 0.002% of the oxygen content of the atmosphere.
Again, there are reasons to fear a lack in the decades to come, as the emissions of carbon dioxide by the man continue to increase every year .

Oxygen tends to associate with other chemicals to oxidize (burn). Thus, stocks of carbon or organic matter that do not currently participate in the living matter cycle could "go up in smoke" by incorporating oxygen. Soil organic matter is an example as we saw above when forests are cultivated. Oxidizable carbon stocks are available in the Geneva based 5th IPCC ( Intergovernmental Panel on Climate Change )report. Knowing that an oxidized carbon atom in a carbon dioxide molecule consumes an oxygen molecule, a rapid calculation gives, in case of rapid global oxidation of these stocks, the following orders of magnitude (percentages of decrease in atmospheric oxygen):
oxidation of hydrocarbons (gas) - 0,19% oxidation of hydrocarbons (petroleum) - 0,07% oxidation of coal - 0,16% oxidation of organic matter of soils - 0,64% oxidation of permafrost - 0,55%
Oil, gas and hydrocarbons, if they pose a threat to the climate, can therefore cause, in the case of intensive mining, a decrease in the oxygen concentration of the atmosphere.

According to a study of the INRS "National institute of research and security" on Confined Spaces, the measurement in oxygen content in a confined or polluted space must be controlled: Measure Oxygen Content Together, the oxygen content of the ambient atmosphere will be monitored using a portable oxygen meter. If the oxygen content is less than 19%, penetration should only be carried out with respiratory protective equipment. It should be noted that any measured oxygen concentration below 20.5% already reflects
an anomaly in the atmosphere of the confined space (oxygen consumption or accumulation of another gas).