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From what I know about the world of Dredd, the Earth is pretty much either irradiated wasteland or cities. There are still billions of people living in the megacities taking up all the oxygen, and there are no plants to produce more. It seems eventually the planet would simply run out or dip below sustainable levels. Was this ever addressed?

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    Do the oceans still have phytoplankton? There might be enough oxygen in the world if the oceans still produce tons of it. – Molag Bal Apr 22 '16 at 16:49
  • @anaranjada - The Black Atlantic is heavily polluted, to the point that swimming in it will dissolve flesh. – Valorum Apr 22 '16 at 18:45
  • Based on the fact that you've accepted an answer that has zero references to the Dredd universe, I'm debating whether to tempted to flag this for a transfer to Earth Science: SE or Biology:SE – Valorum Apr 23 '16 at 17:00
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    @Richard Being new here I'm not exactly sure if there are even rules regarding which answer I could accept. Your answer was good, but unfortunately I can only accept one. His was an answer I liked because it could be extended to other apocalyptic universes that would have the same issue. – DeeV Apr 24 '16 at 2:39
  • @deev - You're completely free to choose whichever answer you like as your "accepted" one. – Valorum Apr 24 '16 at 2:48
37

The out-of-universe answer is the Dredd universe is a satire of an overpopulated, over-urbanized police state and not intended to make scientific sense.

That said, just how long could billions of people keep breathing if the Earth's oxygen cycle broke down? A long time. The Earth's atmosphere is really, really, really, really big compared to puny humans (though not their machines).

A single human uses about 550 liters of pure oxygen per day. Not air, oxygen. At an atmospheric density of about 1.2 g/L that's about 660 g or 0.66 kg per person per day or 240 kg per person per year.

The atmosphere of the Earth weighs about 5.15e18 kg. 3/4 of that is within 11km of the surface, so let's use that: 3.86e18 kg. How much of that is oxygen? Oxygen is about 21% by volume with the rest being nitrogen and 1% miscellaneous that I'll ignore. But we want the percentage of oxygen by mass.

We can use the density of those gases to work out their mass ratios in air. 1.331 g/L for O2 and 1.205 g/L for air at normal temperature and pressure.

1 liter of air weighs 1.205 g. 21% of the volume is oxygen, so it contributes 0.21 * 1.331 g/L = 0.28g. Divide them to get the mass-ratio of oxygen: 0.28g / 1.205g = 23%.

23% of 3.86e18 kg is 8.9e17 kg. That's a decent approximation of how much oxygen is in the atmosphere. Humans can survive as low as 17% oxygen, 6.6e17 kg, so they can safely consume 2.3e17 kg.

How long will it last?

2.3e17 kg / 240 kg per person per year = 9.6e14 person-years

Lacking a census for the Judge Dredd universe, I'll use 10 billion people or 1e10. They will have 9.6e4 or 96,000 years of breathing what's left in the atmosphere before there is a problem.

The bigger problem will be the build up of carbon dioxide in the atmosphere, we're far more sensitive to too much CO2 then to too little O2. I'll leave that for someone else to work out.

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    Surely they have some machines consuming oxygen. – Molag Bal Apr 22 '16 at 19:38
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    @anaranjada That would require knowing something about how the Dredd universe produces power, and that would require trying to make sense of the Judge Dredd universe (good luck). I think we can safely assume they're not burning fossil fuels anymore to produce power (ie. oxidation) as the required global transportation network has broken down. – Schwern Apr 22 '16 at 19:48
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    This answer is ... puts on sunglasses ... breathtaking. YEAH! – corsiKa Apr 22 '16 at 20:14
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    @Richard Sci-fi is doing its job when reading it sparks good science questions, and the OP seems happy with the answer. This is rapidly getting meta. – Schwern Apr 23 '16 at 18:45
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    @Schwern - Except that there palpably are plants. Even the most cursory examination of the Dredd universe shows that there's plenty of plant life about, just less of it than before :-) – Valorum Apr 23 '16 at 18:52
26

Despite the damage caused by the Atomic Wars it would appear that the world of Judge Dredd isn't quite as dead as might otherwise seem. The oxygen cycle isn't broken, nor is there any evidence that people have breathing difficulties beyond that which you'd expect in a world that's heavily polluted.

The Antarctic apparently hash lush jungles.

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Pan-Africa still has areas of savannah (grasslands)

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The Pacific ocean is still alive with fish and octopi (whose food supply would require algae to survive)

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And they've even tried farming in the Cursed Earth, implying that microbial life is still working, despite the radiation.

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