Since 1989, when Ralph Keeling of the Scripps Institution of Oceanography began extremely high precision measurements, there has been a steady decline of molecular oxygen in our atmosphere. The decrease is small, just 0.03% in the past 20 years or an annual rate of about 2 ppm (out of 210 000ppm of atmospheric oxygen). Measurements of the bubbles of air trapped in ice cores from Greenland and Antarctica suggest that this oxygen decline started in the late eighteenth century, at the beginning of the industrial revolution, when fossil fuel burning increased dramatically.
This decrease in atmospheric oxygen is not unexpected, for the combustion of fossil fuels, while in the process of producing carbon dioxide, destroys O2. For every 100 atoms of fossil-fuel carbon burned, it has been estimated that roughly 140 molecules of O2 are consumed. Since every molecule of additional carbon dioxide locks up two oxygen atoms, the free oxygen decline is greater than the carbon dioxide increase. However, the rate of decline in molecular oxygen is only about two-thirds of that expected from fossil fuel combustion. While not fully verified, the difference may be explained by the increase in biomass known to occur as a result of the increase in CO2. Plants grow a bit faster than before, leading to a greater storage of carbon in tree wood and soil humus. For each atom of extra carbon stored in this way, roughly one molecule of extra oxygen accumulates in the atmosphere.
Although the oxygen decrease is unlikely to get to be large enough an effect to be of major concern, the oxygen decrease is expected to continue in the future as fossil burning continues and concentrations of atmospheric carbon dioxide continue to increase.
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