broccoli or

In the past Oxygen level percentage of atmospheric [mass] pressure on Earth has [varied] [after the Great Oxygenation Event] from less than 1% to 35% [of mass] sea level pressure. [According to accepted calculation]

Physics being what physical facts are [in Newtonian terms] changes in situation will cause a change, regardless of whether it is looked at and considered or not.

Irrespective of the switch to electric motors and renewable [clean] energy production the mass of atmospheric composition is now on a trend towards more volcanic production, more annual forest fires, the subsequent carbon & Particulates production along with the associated increasing Oxygen Consumption.

Irrespective of the loss of rainforest carbon absorption capacity and the general loss of permanent land photosynthesis to stored carbon sinks, Photosynthesis will reduce globally.

The oceans that produce the vast majority of the Oxygen within the Oxygen cycle are scheduled to experience a global reduction in annual sunlight percentage reaching the surface.

This will be due to the effects of the increase in atmospheric particulates caused by the exponential increasing of annual burnings from forests and volcanoes. A secondary effect of this [in addition to the accepted potential of climate change / global warming] will be an increase in atmospheric water vapour, initially from precipitation and then added to by high atmospheric methane decomposition.

Atmosphere             fire            +        Soot        –    Luminosity 

                           Photosynthesis +  Water vap   +         fire          = O2%\

The upside for the surviving human communities will be that at lower altitudes [sea level+3000ft] the ‘soup’ will contain a higher partial [%] pressure of usable Oxygen. The second effect will be that at higher altitudes the current partial pressure of Oxygen will be higher, despite the lowered Oxygen percentage [currently 20.9%] of Oxygen in the atmosphere.

The phases are [scheduled to be] fossil carbons, methane cascade, volcanic surface balance activity, spontaneous combustion, [added to land erosion & sea level rising] Oxygen depletion detected – Water vapour increasingm atmospheric pressure increasing with the Key reduction in sunlight photosynthesis production under the added sooty particulates.

Oxygen reduction percentage is [difficult to estimate] likely to start to have an apparent effect under the primary effect in an increased atmospheric pressure, caused by the increase in water vapour, most evident at sea level[?]. The primary health negative effect will be causing breathing difficulties, ill health and deaths by the increase in partial pressure of toxic pollutants and carbon/sulphurous++ particulates at sea level+%–..

chips?

The solution to the above destiny is a reversal and a counter absorption of Carbon, faster than the usual+ end to an Eemian interglacial or the process will correct itself [if there is such a thing as correct] 

However the methane event and the subsequent sea level rise will require a big upgrade to human [Paris] intention and current technological ablity And the avoidance of [Hateful, Mad, Competitive & DISRESPECTFUL!]..

Atmospheric Oxygen @ 20.9% -