As the levels of carbon dioxide in Earth’s atmosphere are increasing, a new study has shown that levels of atmospheric oxygen have been declining over the last 800,000 years.
While the decline isn’t substantial at somewhere around 0.7 per cent relative to current atmospheric-oxygen concentrations in the stipulated time, the decline has accelerated over the last century with increased use of fossil fuels around the world. The findings are the result of a study carried out by researchers at Princeton University wherein researchers compiled 30 years of data to construct the first ice core-based record of atmospheric oxygen concentrations for nearly a million years. To come up with historic levels of oxygen, researchers used measured ratios of oxygen-to-nitrogen found in air trapped in Antarctic ice.
Researchers say that the decline over the last 800,000 years has been a reasonable pace, but during the last 100 years things are taking a bad turn as the oxygen levels have declined by a comparatively speedy 0.1 per cent and this is because of burning of fossil fuels that decreases oxygen levels and increases carbon dioxide levels.
Multiple studies have tried to predict levels of oxygen over the years and to determine whether they were increasing or declining or steady; however, there have been no consensus over the findings.
Researchers point out that the decline in oxygen levels in the atmosphere hasn’t been accompanied by any significant increase in the average amount of carbon dioxide in the atmosphere. This is because nature has multiple ways of controlling the concentrations of carbon dioxide with one process being “silicate weathering” wherein CO2 reacts with exposed rock to produce, eventually, calcium carbonate minerals, which trap carbon dioxide in a solid form.
As temperatures rise due to higher carbon dioxide in the atmosphere, silicate-weathering rates are hypothesised to increase and remove carbon dioxide from the atmosphere faster. Researchers suggest that the extra carbon dioxide emitted due to declining oxygen concentrations in the atmosphere stimulated silicate weathering, which stabilised carbon dioxide but allowed oxygen to continue to decline.