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It is well known that in respiration, we produce carbon dioxide, while plants, via photosynthesis, absorb CO2. My question is, if 50% of all people die today, will the total amount of CO2 decrease drastically or will it be too small a change to notice?

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    $\begingroup$ Welcome to the site, Quark. Here at the Worldbuilding Stack Exchange, we focus on developing fictional worlds, sometimes by making changes to our real, existing one. However, this question does not ask about the ramifications of this event occurring and is otherwise purely about our real, existing world and is off-topic as a result. You can check the help for guidance. $\endgroup$ – Frostfyre Jan 2 '16 at 21:15
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    $\begingroup$ Also, we generally discourage accepting an answer within half an hour of asking a question. Other answerers may be discouraged from providing a new, better answer. $\endgroup$ – Frostfyre Jan 2 '16 at 21:16
  • $\begingroup$ @Frostfyre This question wasn't accepted at chemistry or biology SE and they suggested me to post this on Worldbuilding. $\endgroup$ – Quark Jan 2 '16 at 22:51
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    $\begingroup$ Previous version on Chemistry. $\endgroup$ – HDE 226868 Jan 2 '16 at 23:04
  • $\begingroup$ @HDE226868 I had deleted it. Don't know how did it appear back again. $\endgroup$ – Quark Jan 2 '16 at 23:06
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It's probably worth noting that the carbon dioxide that we breath out doesn't come from no-where. I imagine that the oxygen (dioxide) comes from the atmosphere and that the carbon comes from the food we eat.

Here's the kicker - even if half the earth's population died tomorrow, that food would still decompose and be released as carbon dioxide (because even bacteria respire just like us!). So no change there.

But what happens if we don't grow the food in the first place? I hear you. If the food isn't grown, then it didn't consume any carbon. This is important to realize, becauseiIf you track carbon back far enough, you'll discover that it comes from drum roll please:

All plant and soil carbon comes, in the end, from CO2 in the atmosphere.

http://carbon-sense.com/2008/05/12/where-do-plants-get-their-carbon/ Also see: https://biology.stackexchange.com/questions/37264/how-do-plants-get-carbon

So humans live - we grow food, the food takes carbon from atmosphere - we eat food and return carbon. Humans don't live, we don't grow food and the carbon doesn't get taken from the atmosphere in the first place. It's a zero sum game.

So instead of thinking of humans (or any life form) as carbon dioxide producers, it might be best to think of us as carbon dioxide borrowers - we simply borrow some from the atmosphere, live our lives, and have it returned.

The reason there is concern about human activity today is not due to our respiration, but due to:

  • Deforestation, releasing a huge quantity of 'borrowed' carbon back into the atmosphere, without creating new borrowers to consume it.
  • Highly 'greenhouse' chemicals - some are far more effective than carbon dioxide.
  • Digging up dead corpses (literally oil/gas/coal) and releasing the carbon they borrowed a long time ago.

A massive death rate among humans may cause changes in these behaviours, but our respiration alone has no impact.

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    $\begingroup$ I knew that global warming is not due to our respiration. But this is too many facts put together in the answer which make it perfect. $\endgroup$ – Quark Jan 3 '16 at 16:36
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Some basic statistics:

This means that humans annually breathe out ~2.66 trillion kilograms of CO2 per year - less than 1% of the total natural output by vertebrates. If all humans died today, there would be no effect from breathing out carbon dioxide.

However, human activities spew out over ten times as much as we breathe out each year. If we got rid of about one half of the population, we could cut that down significantly. That would make a huge difference.

Also, as Cort Ammon said infrastructure would pretty much collapse. The World Without Us, by Alan Weisman, is an interesting starting point for figuring out what would happen.

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  • $\begingroup$ A request: could you unify your units a bit? Comparing kilograms and gigatonnes is a bit tricky. In particular, there's many meanings of tonne. Obligatory xkcd reference xkcd.com/558 $\endgroup$ – Cort Ammon Jan 2 '16 at 23:25
  • $\begingroup$ @CortAmmon Sure; I should have done that before. Done. $\endgroup$ – HDE 226868 Jan 2 '16 at 23:28
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The death of 3 billion people would not cause much of a change in CO2 from breathing. The CO2 we breathe is a small part of the worldwide CO2 of all species.

However, if half of our population died, it would likely collapse the entire world's infrastructure. The decrease in use of fossil fuels would matter far more. The amount of change is equal to the amount of impact you believe human civilization's CO2 has on the total.

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  • $\begingroup$ Of course, it matters how the half of population dies. There are many means of death (world war, nuclear strikes, gray goo, huge meteor or supervolcano) or possible consequences of infrastructure collapse (major fires, chemical and nuclear disasters from unmaintained facilities) which will have a much larger effect on CO2 than just the linear reduction of business as usual. $\endgroup$ – Peteris Jan 3 '16 at 10:46
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    $\begingroup$ Would infrastructure necessarily collapse? You have half the people filling occupations, but only have half the consumers. $\endgroup$ – user2448131 Jan 3 '16 at 16:40
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    $\begingroup$ @user2448131 It would, unless you took great care in choosing which half. If you randomly removed 50% of the people from the world, human infrastructure would be overextended. Things would fall apart. Also, that 50% may include key personel who are very hard to replace. Then, of course, there is the rash of panic that's going to come from 50% of your neighbors dying around you, which will encourage you to pay attention to things besides the infrastructure. $\endgroup$ – Cort Ammon Jan 3 '16 at 16:48
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It does kinda-sort depend on which half of the human race dies off, so the question of how these people all die off is important.

If the people who die off are in the top half of fossil fuel usage, then there will be a drastic reduction in anthropogenic CO2 being put into the atmosphere, while on the other hand if the people who die off are in the bottom half, then the reduction will be far less.

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  • $\begingroup$ +1 for anthropogenic! That's a new word for me, and a pretty good one too. $\endgroup$ – Henry Taylor Jul 24 '16 at 17:47

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