Safe to breath
EB reader and scientist Shane Perryman sent this email regarding the Guardian story by Peter Tatchell: The oxygen crisis ("Could the decline of oxygen in the atmosphere undermine our health and threaten human survival?") -BA
While I do not deny that there may be localised (i.e., in big cities with bad circulation) reductions of O2, I don't know why this oxygen scare stuff keeps coming up. Partly I think it's due to a misunderstanding of photosynthesis... see below.
I've attached a pdf with some scans from the text book Earth System Science.
The first graph (11-3) shows that while it is possible to measure the decrease in O2 as we burn carbon... the decrease is on the order of parts per million (ppm) per decade. As the atmosphere is ~20% oxygen it should be clear that it will take a long time to reduce the store of oxygen already in the atmosphere by a significant amount.
[The figure is also available as on page 10 of The Global Carbon Cycle in the Earth System, the slide titled "Recent History of Atmosphere CO2 and O2 Concentration"]
The second figure (11-1) shows that an approximate amount for the size of the fossil fuel reservoir as carbon is 10,000 Peta grams, i.e., 10,000 * 10^15. This sounds a lot.. and it is. Converting to mols (the chemists unit) this is ~ 8.33 * 10^17 (i.e. 8.33 followed by 17 zeros).
I do this calculation as the final figure (16-9) shows the reservoir of oxygen (as O2) in mols. The reservoir of O2 is estimated at 3.75*10^19 .
[Figure 11-1 is "Major reservoirs and fluxes of the carbon cycle including time scales. ... After Sundquist 1993". It doesn't seem to be available on the web. ... Figure 16-9 is "The global oxygen balance".]
This suggests that there is simply not enough fossil fuels to significantly alter the O2 content of the atmosphere.
The article by Peter Tatchell is making comparisons to not just prehistoric times, but oxygen concentrations millions of years before that.
When climate change deniers compare CO2 concentrations from 250 million years ago to the present we know what to expect. I wonder about this kind of journalism... false scare to confuse the punters?
However, taken at face value, I suspect this kind of thing comes up in part because there is a persistent belief that because plants are known to use CO2 and give off O2 that the O2 thus released comes from CO2! This is incorrect. The O2 released from photosynthesis comes from H2O. Thus there will never be a shortage of material for the production of O2 so long as there is water on this planet.
I have made this point a few times in various places... in the Australian Journal, Renew several people have written concerned that burial of CO2 was not a good idea because of the oxygen buried along with it. And once, on the blog The Anthropic Network when I made this point about photosynthesis (easily checked at wiki) the rejoinder from Jason was that I therefore didn't believe in the carbon cycle! Particularly galling since nutrient cycles are my field of study. One less blog to read.
The issue of regional oceanic anoxia is somewhat separate. As the solubility of O2 in water is only of the order of 5-15 ppm/L (condition dependent: eg Higher Temp means less dissolved O2) it doesn't take much excess decaying carbon to remove oxygen. Anoxia in water is generally caused by algal blooms, such that the surface water where the bloom is actively photosynthesizing may even be supersaturated with O2 while the water just below the surface is anoxic as the sinking dead algal cells decay.
Once anoxia persists, other metabolic pathways come to the fore, and in oceans the reduction of sulphate in seawater to toxic sulphide gas (H2S) may be a problem. It obviously is in the GoM.
Enough of this mini lecture.
If you have access to the ABC(OZ) documentary from a few years back called "Crude" a lot of this was covered.
[The documentary is online: Crude]
Shane Perryman is a PhD candidate at Monash University, Australia. His research specialty is Nitrogen cycling and bacterial bio-diversity in Australian streams. He is a longtime contributor of article ideas for Energy Bulletin.