|Adam Nieman: "Global water and air volume" (Science Photo Library)|
O2 levels have varied radically during last 600 million years:
|(from Wikipedia article "Atmosphere of Earth: Third Atmosphere")|
- as corroborated by analysis of ancient gas bubbles trapped in amber.
Writing in the Guardian newspaper in 2008, Peter Tatchell said that research by Professor Robert Berner suggested "humans breathed a much more oxygen-rich air 10,000 years ago", though I can't track down the original statement and suspect Tatchell may have misunderstood. (A paper by Berner on oxygen in the Phanerozoic era can be read here.)
Tatchell appears to be on firmer ground voicing concerns about air quality in cities, though what's in the air is more worrying than what's absent, as this article from last month's Mail Online India edition says. That said, cities that are prone to temperature inversion layers (e.g. Los Angeles, Beijing) may find that not only is smog locked in, but oxygen not replenished from the surrounding area as fast as it is being consumed.
Globally, there seems to have been a very small decline in atmospheric oxygen since 1990, according to an 18-year longitudinal study by Dr Ralph Keeling. According to a post on the Climate Emergency Institute website, the decline is even less than Keeling had expected, and it's possible that increased CO2 is stimulating the growth of vegetation.
Certainly the self-styled "Rational Optimist" Matt Ridley claims greenery is increasing, but I am inclined to take his professional bullishness with a pinch of salt. Surface spread as seen by satellite misses the third dimension: the UN FAO estimates (2012 forest report) that forest cover has dropped by around a third in the last 10,000 years, and the loss has accelerated from an average of 360,000 hectares per year since civilization began, to 5.2 million annually over the last decade.
Which brings us back to the carbon dioxide-global warming debate. CO2 is a "greenhouse gas" but there are so many other factors affecting the Earth's surface temperature that I don't think anyone can say which way the thermometer is going to move. However, if the sea continues to warm up there is a danger that the level of dissolved oxygen in the oceans will be reduced. A study reported last year in Science Daily says that 15% of the seas are "dead zones" and suggests that an increase of a couple of degrees - as has happened since the end of the last Ice Age - can have significant effects. Again, industrial pollution and waste dumping exacerbate the damage.
Further, as this 1994 paper by Duursma and Boisson says:
"Oxygen concentrations are ... the consequence of larger terrestrial and aquatic loops in which factors of temperature, light, nutrients and co2 play a role; for longer periods, elements such as sulphur and iron are involved. Hence the present level of 20.946 vol. % of atmospheric oxygen is merely temporary, and will change in the course of millions of years. The question of an optimum concentration for sustaining life on earth is equally time-dependent; but bearing in mind that these changes occur over periods of the order of millions of years, evolutionary processes are likely to keep pace with oxygen changes."Those evolutionary processes may or may not have a place for humanity in the long run, but we have plenty more pressing threats to worry about. One of which is that the health and productivity (for us) of the seas may be compromised if warming continues and subsurface oxygen is depleted.
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