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Some cataclysmic event has transpired on planet Earth in the not-too-distant-future, and the outside air is now thick with CO2, causing all of the problems that come with it. Human beings can no longer breathe the air directly, global temperatures have spiked, and the flora and fauna of the world have been affected drastically. A few pockets of civilization have survived in highly advanced compounds, and they want to start working to undo the CO2. I'd like this process to have a decently long time period before it is sufficient for humans to survive on the surface again, at least a century.

Assuming that all scientific knowledge has survived, and assuming that my survivors can get creative with how they acquire resources, what are some methods to begin to slowly change the planet's atmosphere back?

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    $\begingroup$ This is already being proposed by various interests in the real world. It's called geoengineering, and we have a few ideas about it. The most likely to work involve seeding the oceans with various iron compounds, which should cause massive phytoplankton blooms (the limiting factor to growth is often that mineral). They'll use atmospheric carbon to grow, then die, sinking down to the bottom of the ocean. $\endgroup$ – John O Jan 9 '20 at 17:37
  • $\begingroup$ @JohnO the trick is making sure your phytoplankton blooms don't accidentally wipe out all the NATURAL sea life at the same time. It's a non-trivial problem. $\endgroup$ – Morris The Cat Jan 9 '20 at 17:57
  • $\begingroup$ @MorrisTheCat at least in my world, it's not such a concern. People are desperate to reclaim the planet; they're happy to alter it as necessary in order to survive, regardless of what has to die. $\endgroup$ – NegativeFriction Jan 9 '20 at 17:59
  • $\begingroup$ @MorrisTheCat There's no way to argue this where it doesn't devolve into politics. But the technology exists, and the cost is well within the budget of 1st world nations especially if it's the catastrophe it's made out to be. $\endgroup$ – John O Jan 9 '20 at 18:09
  • $\begingroup$ @NegativeFriction isn't "We're desperate; let's do this regardless of the consequences" rather what got them into the situation in the first place? $\endgroup$ – user535733 Jan 9 '20 at 20:21
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Fusion powered CO2 splitters.

With limitless energy (they have that, right?) one can use a variety of catalytic methods to split the CO2 molecule. Most of these reactions yield carbon monoxide and oxygen but it is possible to produce carbon (soot) and oxygen, and more promisingly with the use of some hydrogens from water, methanol and oxygen.

Your earth remediators take it one step further, splitting an oxygen off CO2 to form carbon monoxide, hydrogenating the CO to form methanol, then adding an addition CO and driving off the oxygen to form ethanol. The CO2 surplus is therefore converted to large quantities of ethanol, which is stored in flavorful charred wooden casks for later use.

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  • $\begingroup$ This is easily the best answer I've received. It has the bonus points of providing a secondary benefit to the humans-- the production of much-needed oxygen that they can separate for more inhabitable space for human beings. $\endgroup$ – NegativeFriction Jan 10 '20 at 17:21
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The problem is that nothing small groups of humans are going to be able to do is going to have much of an impact on the atmosphere. It took literally billions of us a couple hundred years to put all that CO2 into the atmosphere. Removing it would take absolutely MASSIVE infrastructure.

Now, that having been said, eliminating most of the human beings and their CO2 emissions would do a lot towards slowly bringing things back into balance. Over the course of several decades without human interference, plant life would take over massive tracts of land. The more plants you have, the more CO2 those plants are converting back into Oxygen. It'd take centuries obviously, but it's a natural process that would take place all by itself.

Now, something that your plucky survivors COULD potentially do is help accelerate that process. For example, seeding arid lands with genetically engineered seeds that can spread rapidly in areas that humanity has abandoned could do a lot over time.

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  • $\begingroup$ One of the ideas I've been considering has been genetically engineered plant life and plankton kept in their own special reverse-greenhouses. They would keep conditions cool enough for the plants to survive, and they'd be bio-engineered as carbon sinks.Most of humanity's efforts would be dedicated to creating additional reverse-greenhouses in order to speed up the rate of carbon capture. $\endgroup$ – NegativeFriction Jan 9 '20 at 17:52
  • $\begingroup$ @NegativeFriction it can't be greenhouses, your humans couldn't possibly build enough greenhouses to sink enough carbon. You'd have to seed your GMO plants over continental-scale areas to start making an impact on global CO2 concentrations. $\endgroup$ – Morris The Cat Jan 9 '20 at 17:56
  • $\begingroup$ Plants only sequester CO2. They do not remove it permanently. Once they die and decay, the CO2 is released back into the atmosphere. If you want to remove CO2 with plants, you have to basically store the dead tissue indefinitely after they die, you cannot eat it, you cannot burn it, you cannot let it decay. $\endgroup$ – Richard Smith Jan 10 '20 at 16:53
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    $\begingroup$ @RichardSmith this depends on where they die, plants in swamps and the open ocean tend to get buried before they can release the carbon. $\endgroup$ – John Feb 3 at 14:24
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With 7+ billion people and all our technology we are still far from being able to modify the atmosphere of a planet. It took more than one century of industrial development to slightly increase the content of CO2 in our atmosphere.

With less humans available we won't be able to make a significant dent into the content of the atmosphere with any tech mean. The only way would be to plant trees and plants and let them suck it out the CO2 with their growth. But, again, being few they won't probably do any better than natural propagation of plants.

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  • $\begingroup$ Our technology isn't far at all from being able to modify it. However, there are various political interests who oppose experimenting, let alone attempting such. $\endgroup$ – John O Jan 9 '20 at 17:38
  • $\begingroup$ Don't tree also exhale CO2 during the night, netting something very low in term of CO2 purification ? $\endgroup$ – MakorDal Jan 10 '20 at 15:50
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One possible way for the embattled humans would be to bioengineer a more effective organism for carbon capture. Many of the existing plants and algae can do that - but in doomsday scenario they have to do better than that.

The new organism should be:

  1. Prolific. It should grow fast and multiply quickly;

  2. Sturdy and tolerant. It should be able to occupy most areas of the overheated Earth;

  3. Don't quickly release carbon after its death. This is the most tricky part. Many organisms are capturing carbon all right - but after the death this carbon usually returns back to the atmosphere via rotting or other processes. So, the new organism either has to be build of a completely new organic compound which existing bacteria does not know how to digest (like cellulose was back in carboniferous period), or wrap itself in some kind of sturdy mineral-based shell so its organic content would be trapped inside.

The new organisms would quickly populate the Earth and begin a long process of trapping the excess carbon dioxide.

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