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Old December 4 2013, 04:22 PM   #2
Robert Maxwell
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Re: Photosynthesis, hypothetical question

As always, this comes down to math.

According to this link, an average person consumes about 550 liters of oxygen per day.

According to this link, there are approximately 1 quintillion kilograms of oxygen in our atmosphere.

Based on oxygen's molecular weight, it takes 700 liters of oxygen to sum up to 1 kilogram. That means there are 700 quintillion liters of oxygen in our atmosphere. (Remember, this is all quite approximate.)

So, accounting only for humans, let's take 6.5 billion into 700 quintillion. That works out to about 107 billion days' worth of oxygen. Divide by 365 days a year, and we're talking about 295 million years to run out of oxygen.

Of course, we don't have to run out for it to be fatal. Current atmospheric concentration of oxygen is about 21%. I've seen various numbers but it looks like anything below 17% is getting into dangerous, possibly deadly, territory. That means a mere 20% in oxygen reduction in the atmosphere would doom us.

So, 20% of 295 million years is still... 59 million years. Once you start accounting for other oxygen-breathing species and other ways humans consume oxygen (combustion, etc.) you may yet shave quite a bit more off, but it looks to me like, if the carbon chain collapsed tomorrow, it'd be a very long time before lack of oxygen did us in. We'd all starve to death first, because without the carbon chain, there's no food chain.
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