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I am wondering if CO2 can be removed from the atmosphere by heating it up using the same lasers they use to make plasma when researching nuclear fusion, and then fire the plasma of oxygen and carbon through a magnetic field to separate the atoms before they recombine together.

Thanks for all of the answers. So for CO2, plants are the most efficient way of doing this. I have a supplemental question. Why can't plasma lasers + magnetic separation be used to get rid of stuff that is currently buried in landfill or incinerated? For example, asbestos and biotech waste. Build a site in northern Africa with a vast array of solar panels, and use the energy to reduce this stuff to its elements.

Here is a link to another question about plasma recycling:

A question about the properties of plasma and its potential use in recycling

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    $\begingroup$ And where, pray tell, would the energy to make the plasma come from? $\endgroup$ – Gert Sep 16 '19 at 21:35
  • $\begingroup$ And, of course, the accelerating potential to actually extract the ions from the plasma, give them some velocity, and then send them through a magnetic field? $\endgroup$ – Jon Custer Sep 16 '19 at 21:49
  • $\begingroup$ Perhaps a more useful question would be: does (high-energy) radiation from the sun combined with the earth's magnetic field appreciably disassociate and remove CO2 from the atmosphere? I'm guessing no $\endgroup$ – KF Gauss Sep 16 '19 at 21:54
  • $\begingroup$ Seems like there are two separate issues here: (1) if this is physically viable; (2) if it'd be economically practical. $\endgroup$ – Nat Sep 17 '19 at 7:45
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    $\begingroup$ It would be best to ask the supplementary question as a new question I think. $\endgroup$ – tfb Sep 17 '19 at 14:36
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Regardless of the process, conservation of energy implies that it takes at least as much energy to convert $\mathrm{CO_2}$ back into carbon & oxygen as you originally got from burning the carbon, so unless you have lots of cheap carbon-neutral energy at your disposal, this is a losing proposition.

So at this stage of the game, we let plants perform the conversion, using chlorophyll and free solar energy.

There are groups investigating various ways to split $\mathrm{CO_2}$; several of these split it to oxygen and $\mathrm{CO}$, but here's one, slightly similar to your proposal, that goes all the way to oxygen and carbon: An investigation of $\mathrm{CO_2}$ splitting using nanosecond pulsed corona discharge: effect of argon addition on $\mathrm{CO_2}$ conversion and energy efficiency. But as I said above, these schemes are only useful if the energy they require is from carbon neutral sources.

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    $\begingroup$ PM2Ring, using cheap carbon neutral energy to "split" atmospheric CO2, at 400 ppm, is a losing proposition under any circumstances. If you had that energy source, you would want to use it to displace a process that is currently burning fossil fuel, and let green plants absorb the atmospheric CO2. $\endgroup$ – David White Sep 16 '19 at 23:36
  • $\begingroup$ @David I agree with you. It's rather pointless using carbon-neutral energy to split atmospheric $\mathrm{CO_2}$ if you can use that green energy to instead displace a process that's burning fossil fuel. Even using green splitting tech on concentrated $\mathrm{CO_2}$, eg directly on the exhaust of a gas-burning power station, is a dubious proposition. $\endgroup$ – PM 2Ring Sep 16 '19 at 23:51
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    $\begingroup$ PM2Ring, I agree. I have a chemical engineering background, and the one thing that I know for sure is that every chemical process operates at less than 100% efficiency. For simple, one step processes (e.g., a pump with an electric driver on it), the typical overall efficiency is in the 70% range. If you have a 2 step process, you are down to 49% efficiency, etc. This means that you want to minimize the number of energy conversion steps in order to maximize efficiency of the process. $\endgroup$ – David White Sep 17 '19 at 0:59
  • $\begingroup$ As long as we're using large $\mathrm{CO}_2$-emitters, e.g. coal-fired power plants, our best strategy to offset emissions (besides ridding ourselves of the emitters) is to attach a carbon-capture unit to them. This is energetically favorable over open-air capture since the flue gas from point-sources has a much higher concentration of $\mathrm{CO}_2$ than the atmosphere. At current, it reduces a power-plant's effective output by about $30 \% ,$ assuming that the power plant sustains its own capture unit. $\endgroup$ – Nat Sep 17 '19 at 8:06
  • $\begingroup$ @Nat Fair point, but of course carbon dioxide capture is cheaper (in terms of energy) than actually splitting the carbon dioxide to carbon & oxygen. $\endgroup$ – PM 2Ring Sep 17 '19 at 9:30

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