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Comment Re:Not a bad guess (Score 1) 158

That is even more recent than large human populations. True, it started when the population was about one billion, but global industrialization is only a product of the six or seven most recent decades in many countries. Having said that, if the global anthropogenic CO2 emissions in the last 250 years are considered to be in the range of 2000 gigatonnes, with each tonne of CO2 implying the consumption of about 360 kilograms of oxygen (mostly coal consumption), that still amounts to only about one third of the human breathing consumption I've mentioned above. But you've also made a mistake by a factor of about three, I think: 90 ml of diesel is about 3 MJ or energy, not 3 kWh.

Comment Re:Not a bad guess (Score 3, Informative) 158

The amount of oxygen is our atmosphere so massive that the contemporary human population of seven billion would have to breathe for twenty thousand years to decrease its share in the atmosphere from 21% to 20%, without replacement. Of course, a mere century ago, the population was just 1.5 billion. Another century back, 0.9 billion. A thousand years back, about 0.25 billion. It's estimated that all the humans who ever lived numbered about 100 billion, that gives you something like a grand total of 0.2 percent of the current oxygen amount in the atmosphere having been consumed by all human beings who ever lived, if each of them lived sixty years on average. Perhaps cattle could multiply it by a factor of several.

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