Atmospheric oxygen (O₂) is the most crucial element on earth for the aerobic organisms that depend on it to release energy from carbon-based macromolecules. This is the first study to systematically analyze the global O₂ budget and its changes over the past 100 years. It is found that anthropogenic fossil fuel combustion is the largest contributor to the current O₂ deficit, which consumed 2.0 Gt/a in 1900 and has increased to 38.2 Gt/a by 2015. Under the Representative Concentration Pathways (RCPs) RCP8.5 scenario, approximately 100Gt (gigatonnes) of O₂ would be removed from the atmosphere per year until 2100, and the O₂ concentration will decrease from its current level of 20.946% to 20.825%. Human activities have caused irreversible decline of atmospheric O₂. It is time to take actions to promote O₂ production and reduce O₂ consumption.

The bar chart shows the annual averaged values from data taken between 1990 and 2005. (Fig. 4)

Scientist first calculated the budget (D_ATM) without the residual factor. However "The equation [...] omits the respiration of wild animals, weathering of organic matter and volcanic oxidation, which are insignificant compared to the processes above and are hard to quantify. Thus, the residual term is introduced to correct this bias and is calculated based on the difference between the observational D_ATM and the simulated D_ATM from 1991 to 2005."

The clouds are drawn after the figueres in the review, so there may be a small variance in the location on the globe. However, the amount of points drawn in the clouds are exact and represent the data from the review correctly.