ATMOSPHERIC OXYGEN IS SELDOM IN THE ARGUMENT OF GHG REDUCTION.

Atmospheric oxygen (O₂) is the most crucial element for life on earth, being essential in respiration and releasing energy from carbon-based macromolecules, primarily through photosynthesis. As O₂ and carbon dioxide (CO₂) are inextricably connected in both respiration and photosynthesis, it is quite surprising that when the global warming impact of CO₂ is considered, the impact on atmospheric O₂ levels is not simultaneously taken into account.

This association in the atmosphere is linear and highly correlated, as can be seen from the figure below [ from Sampaio et al., 2022. Carbon Capture Science & Technology 2, 100032]

The figure shows the relationship between the means of changes in CO₂ and O₂ as measured at different atmospheric monitoring stations for the years 1991 to 2020; the relationship implying that each 1.0 ppm increase in CO₂ brings a corresponding loss of 2.15 ppm of O₂ in the atmosphere. In total, O₂ in the atmosphere is currently about 20.95% and CO₂ about 0.043% (430ppm), which shows that quantitatively the loss is not that significant, but yet estimates show that as about 100 Gt of O₂ is removed per year with the current release of CO₂, the concentration of O₂ will decrease from 20.95 to 20.825% at the year 2100 and significantly more in the sea – the sea being responsible for many life functions. The cause of the loss of O₂ is primarily from the anthropogenic combustion of fossil fuel. This combustion consumption increased from 2.0 Gt per year in 1900 to 38.2 Gt per year in 2015 [Huang et al., 2018; Science Bulletin 63, 1180-1186]. This certainly is cause for concern.

The question then is why the concern with CO₂, while methane (CH₄) is also released from fossil fuel sources. The answer is twofold: (1) the concentration of CO₂ in the atmosphere is 430ppm and that of CH₄ 1940ppb (or 1.94ppm), i.e. 0.45% that of CO₂, which is finite, and (2) CH₄ stays in the atmosphere for a relatively short period and is regularly oxidised by the hydroxyl radical to CO₂ and H₂O, whereas the staying time of CO₂ is much longer and therefore accumulating: The IPCC has modelled it, using four components each with a different decay function: 21.73% never decays, 22.4% has a 394.4 year lifetime, 28.24% has a 36.54 year lifetime, and 27.63% has a 4.3 year lifetime. Thus, the lifetime of about 44% is almost indefinite, whereas 56% variably between 4.3 year and 36.5 years enters the vegetation and soil together with the CO₂ from the oxidation of CH₄ through photosynthesis.

Conclusions: Whereas much emphasis has been put on the reduction of methane which clearly is not justifiable, the focus should be on the drastic reduction of anthropogenic carbon dioxide levels as the accumulation of carbon dioxide in the atmosphere does not only have a substantial warming effect, but the accumulation also decreases atmospheric oxygen systematically and irreversibly. Thus, the time has possibly also arrived to start promoting oxygen production and reduce oxygen consumption.