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I'm trying to find a plausible way to reduce global warming in a world past the tipping point.

I recently read this article : https://www.sciencedaily.com/releases/2017/11/171127173225.htm. It suggests that we may be able to use carbon dioxide as a fuel.

Would that work to reduce the greenhouse effect?

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    $\begingroup$ Read farther down in your link. The membrane under discussion acts as a filter to separate carbon dioxide from other molecules mixture. It does NOT offer a way to combust carbon dioxide. $\endgroup$ – user535733 Jul 21 at 21:41
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    $\begingroup$ @AlexP please answer in answers not comments. Thanks. $\endgroup$ – Monica Cellio Jul 21 at 22:37
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    $\begingroup$ @MonicaCellio: A comment provides an opportunity for the user to edit the question. Once an answer is given, the question becomes fixed. Anyway, Mike Scott and Cadence already provided the same information as answers, so now there is nothing to be done. $\endgroup$ – AlexP Jul 21 at 22:43
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    $\begingroup$ If you burn fuel you get energy out. If you want to un-burn fuel you have to put the energy back in. Where does the energy come from? $\endgroup$ – user253751 Jul 22 at 4:27
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    $\begingroup$ Carbon capture and recycling will always require more energy than you got from burning the fuels in the first place. That's basic thermodynamics. They are ways to fix the problem after humanity managed to cover more than 100% of their energy demand using carbon neutral energy sources. If you want to stop global warming by reducing the amount of CO2 in the atmosphere, you first have to stop buring carbon-based fuels. $\endgroup$ – Philipp Jul 22 at 10:52

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It is technically possible to burn carbon dioxide, but not in a practical way. The reason burning carbon produces energy is that the total potential energy of carbon and oxygen is minimized by the CO2 configuration. Splitting them up into carbon and oxygen again requires an addition of energy. Therefore, in order to burn carbon dioxide, you need to find something that will produce even lower total potential energy by displacing the oxygen. This depends on the electronegativity of the atoms in question. Oxygen is extremely electronegative, but fluorine is even moreso, and fluorine compounds are notorious for burning things that ordinarily won't burn, including combustion products such as water and silicon dioxide (common sand).

There are two problems with the idea of using carbon dioxide and fluorine as a fuel, though. The first is that fluorine compounds are, on their own, rare and dangerous and hard to deal with. More importantly, when you take carbon dioxide and displace the oxygen with fluorine, you end up with fluorocarbons, which are worse greenhouse gases than the carbon dioxide was to begin with!

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  • $\begingroup$ There is an even worse problem, which is the unavailability of highly oxidising fluorine compounds. I don't think there's any natural process that produces them, and if there is I'm sure they don't stay unreacted for long enough to accumulate. So there are no places where you could mine them out of the ground, which means you'd have to make them. But that will always take more energy than you could get by reacting them with CO2 - the second law of thermodynamics guarantees that. $\endgroup$ – Nathaniel Jul 23 at 18:18
  • $\begingroup$ @Nathaniel A fair point. Right now an important source of fluorine production is as a byproduct of something else (fluorapatite, for instance, yields both phosphoric acid for fertilizers and hydrofluoric acid) so the fluorine is "free" - but I doubt that would hold true if you drastically increased demand. $\endgroup$ – Cadence Jul 23 at 20:02
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    $\begingroup$ damn, I really thought that your "fluorine compounds" link was going to lead to this site $\endgroup$ – Baldrickk Jul 24 at 11:09
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    $\begingroup$ @Baldrickk I'd forgotten that site! I've edited the link in, since it's a lot more evocative about the sheer danger of working with these things. (Be sure to scroll down to the comments for a few remarks about bromine trifluoride, which is similarly awful.) $\endgroup$ – Cadence Jul 24 at 21:34
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You can’t use carbon dioxide as fuel, and that’s not what the article you cite is about. You can turn carbon dioxide (plus hydrogen or water) into fuel, but the process will need more energy than you will later release by burning the fuel, so you’ll need to get that energy from somewhere.

But yes; if you get the energy without burning fossil fuels and you use atmospheric carbon dioxide (or carbon dioxide that would otherwise enter the atmosphere), it will be pretty much carbon-neutral and thus help to reduce the amount of global warming.

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    $\begingroup$ "You can turn carbon dioxide (plus hydrogen or water) into fuel, but the process will need more energy than you will later release by burning the fuel, so you’ll need to get that energy from somewhere." Right. Sunlight is that energy, the process is photosynthesis, and it turns CO₂ and H₂O into firewood. That's great for removing CO₂ from the atmosphere, but if you actually burn the fuel, guess what. $\endgroup$ – Ray Butterworth Jul 22 at 0:22
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    $\begingroup$ @RayButterworth but the other by-product of photosynthesis is oxygen which can be burned with other things than coal. Burning it with H gives H₂O which is arguably better than CO₂. Now all we need is some H :) $\endgroup$ – Simo Kivistö Jul 22 at 5:54
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    $\begingroup$ @RayButterworth But burning the wood just puts back the CO2 it took out of the atmosphere, for no net effect. If you burn oil instead, you’re putting new carbon into the atmosphere that hasn’t been there for millions of years, and the wood will eventually rot and return its own carbon to the atmosphere anyway (unless it ends up in a peat bog or similar). This isn’t about actually removing carbon from the atmosphere, it’s about reducing the amount of carbon that we add to the atmosphere. $\endgroup$ – Mike Scott Jul 22 at 6:22
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    $\begingroup$ @SimoKivistö Oxygen isn't burned; it's the thing doing the burning. $\endgroup$ – chepner Jul 22 at 18:36
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    $\begingroup$ @RayButterworth ...the process is photosynthesis, and it turns CO₂ and H₂O into firewood... add a few million years of geology and you get coal and oil. Then our problems will be over! $\endgroup$ – Grimm The Opiner Jul 23 at 11:24
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According to your link:

The process can work with any level of carbon dioxide concentration, Wu says -- they have tested it all the way from 2 percent to 99 percent -- but the higher the concentration, the more efficient the process is.

The atmospheric concentration of carbon dioxide is .0391%. That's well under 2%. This would not work well at reducing the concentration of carbon dioxide in the atmosphere.

It is designed to reduce the carbon dioxide emitted from something like a coal power plant. It is a mitigation strategy for burning fossil fuels, not a way to reduce the concentration of carbon dioxide in the atmosphere.

This will not decrease the carbon dioxide in the air. It would (assuming it works as hoped) reduce the rate of increase.

There are proposals that more directly address temperature increases or attempt to reduce the carbon dioxide concentration in the air. But this isn't that. This is simply rate of growth reduction.

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  • $\begingroup$ Of course, if you capture the carbon dioxide released from power plants, you're well on your way to reducing the carbon dioxide in air too - there's many ways in which carbon dioxide is fixed in nature, and reducing the increase might very well be enough to cause the levels to drop over time. You could imagine a near closed-cycle power plant that feeds the produced carbon dioxide to vast vats of algae that in turn produce fuel for the plant from sunlight and water. The main question is whether that's actually a good use of our resources to going e.g. solar or nuclear. $\endgroup$ – Luaan Jul 24 at 7:21
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Yes, but not from the link in the question.

According to the link in the question, Carbon Dioxide can be used to produce an energy storage medium in a rudimentary way, a bit like the chemical production of alcohol out of sugars by yeast or even sugars out of CO2 sunlight and water. In all these cases the net energy output is less than the energy put into the system.

Carbon can (theoretically) be used to power a nuclear fusion reaction, as can oxygen. This is what happens in massive stars in their old age. It requires temperatures of upwards of 500 Mega Kelvin (about 3000 times hotter than the center of the sun as modeled by NASA. Oxygen requires greater than three times the temperature. The pressures are equally enormous and beyond our current capabilities to sustain. The CNO reaction cycle can be found detailed in a straightforward way in this wiki article, and is common in stars slightly larger than the sun.

Carbon and Oxygen fusion proper need more high pressures than this. We'd need to be able to mimic the conditions supposed to exist in the center of stars at least 8 times more massive than the sun. I can't help but feel that the development of force-field technology would facilitate this. We're not there yet.

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    $\begingroup$ By way of reference, to support fusion the hydrogen in our star's core is around 13x denser than lead... $\endgroup$ – Michael Jul 23 at 19:16
  • $\begingroup$ Of course, we'd be silly to use carbon fusion even if we could; why bother with such horrible fusion fuel when we have oceans full of deuterium? :P Even in massive stars, the energy gained from carbon or oxygen fusion is relatively small - their main effect is as catalysts for the fusion of hydrogen to helium (the CNO cycle is a cycle - you get C, N and O back out at the end). $\endgroup$ – Luaan Jul 24 at 7:26
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While it is possible to create fuel from CO2 (by adding water and energy, for example), this does not stop the greenhouse effect.
That is, if you intend to burn the fuel again, which will put the CO2 back in the atmosphere. So, the net content of CO2 in the atmosphere will remain roughly the same.

But you still have a positive effect, as the net content of CO2 doen't rise. Plus, you could bury the resulting hydrocarbons. Or you can reduce the CO2 to (fairly) pure carbon, and bury that. That would actually reduce the amount of CO2 in the atmosphere, thus reducing the greenhouse effect.

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  • $\begingroup$ Also perhaps promoting the use of things like farmed wood as a construction material could help there. Growing fast growing trees and desiccating the wood to be used in construction where it will be prevented from rotting is a good way to sink carbon. Bonus points that you get to put the sequestered carbon to use in a way that doesn't release it into the atmosphere but is still able to provide beneficial utility. $\endgroup$ – MttJocy Jul 25 at 20:22
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Geoengineering, mass carbon sinks, and zero carbon emissions.

So, suppose the tipping point has been passed. CO2 emissions are triggering mass methane emissions from under-sea and tundra-locked sources. GHG levels are skyrocketing even with no more human intervention.

  1. Mass produce alternative power sources. Nuclear, for example. Now we have GHG-free scaling energy production, which we'll need in order to out-compete any remaining GHG-addicted civilizations, and for later projects.

  2. Mass natural carbon sinks. We could forest most marginal agricultural land and the like to soak up half of all carbon human civilization has emitted. Not nothing, but it is something you can do with mass constricted labour that offsets the GHG emissions of keeping humans alive.

  3. Mass industrial carbon sinks. Pulling CO2 (and methane) out of the air and producing heavier hydrocarbon compounds. These hydrocarbon compounds can be used for materials (plastics) or for fuel (gasoline is high-density low-tech fuel) or just sequestered. It may be impractical to convert all transportation (including air and military) to electrical; this is a carbon-neutral source of fuel (as you first pull the CO2 out of the air, then burn it - neutral) rather than pulling more out of the ground. (It may still be cheaper to pull hydrocarbons out of the ground in one spot, and sequester "lower quality" hydrocarbons elsewhere tho)

  4. Sulfur and other geoengineering. When a volcano erupts, there is a short-term (on years scale) global cooling caused by certain sulfur compounds in the upper atmosphere. Produce these artificially and inject them into the upper atmosphere as a kind of refrigerant. Similarly, covering parts of the Earth in white or reflective material to reduce solar heating, or building orbital shades.

This path is extremely dangerous; sulfur, for example, could cause an overshoot in the wrong direction. And it doesn't solve issues like ocean acidification caused by higher CO2 levels.

But it does give a semi-plausible situation for a post-runaway-GHG world, where it starts and the entire world is treating it like a real emergency.

Humans are drafted into tree planting efforts.

Transportation is electric and espensive; all higher-energy density transport (airplanes, tanks, etc) is highly expensive/restricted.

Countries that fail to obey rules have crippling sanctions or even war placed on them.

Huge nuclear plants running mass decarbonization engines. Most electrical generation goes to this, so electricity is also expensive.

Plastics are as common as today; plastics are a carbon sink.

Sulfur fountains and their management is a large part of the international regime.

Global trade remains, using mega-sail barges.

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  • $\begingroup$ This doesn't answer the question, which is about using CO2 as a fuel. $\endgroup$ – Mark Jul 22 at 21:43
  • $\begingroup$ @mark that is covered in point 3? $\endgroup$ – Yakk Jul 22 at 22:16
  • $\begingroup$ It mentions using CO2 as fuel, but doesn't answer "would it work?" $\endgroup$ – Mark Jul 22 at 22:21
  • $\begingroup$ The main problem with injecting those sulfur compounds into the atmosphere isn't really that it would get too cold, but rather that it would disrupt rain - with the potential of causing widespread droughts and flash floods. Don't forget that the whole water cycle depends on sunlight evaporating water. $\endgroup$ – Luaan Jul 24 at 7:38
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Where do you think fossil fuels came from? CO2 that was removed from the atmosphere by plants (converted into the carbohydrates &c that the plants were composed of), then captured underground by various geological mechanisms.

The obvious problems with using this process to address global warming are

1) The process takes tens to hundreds of millions of years to have an effect; and

2) If you then burn the plant material for fuel, you're right back where you started.

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  • $\begingroup$ @1: Actually, on a global scale the process is only a percent or so slower than the sum of manmade combustion, natural decay, and volcanic CO2 production taken together. It is that tiny deficiency that makes atmospheric CO2 rise slowly. Also, photosynthesis gets more efficient (and faster) as concentrations of CO2 rise. Unfortunately, at the present rate of CO2 production, the equilibrium is at a concentration somewhat higher than current. @2: That's true for any carbon conversion process. $\endgroup$ – Ralf B Jul 24 at 5:49
  • $\begingroup$ @RalfB Sadly, some plants seem to be vulnerable to excessive amounts of carbon dioxide in the air. This is not a problem for the main oxygen production organisms (like algae), but could pose serious danger to our food supply - wheat is one of those plants that would be in trouble, and wheat is still by far the major source of protein in the world. $\endgroup$ – Luaan Jul 24 at 7:28
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One option is to separate out the carbon as described here: Scientists turn carbon dioxide into coal at room temperature.

The process involves using a gallium-based catalyst at room temperature that generates carbon flakes from the carbon dioxide. The article refers to it as coal (but coal is only one formation of carbon), but the properties indicated in the article suggest it is closer to graphite/graphene in structure.

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    $\begingroup$ This technique has the same problem as those in all the other answers here: It requires far more energy than you get out of it, which begs the question why you aren't using that energy to power whatever process you are currently powering by carbon-based fuel combustion. $\endgroup$ – Philipp Jul 22 at 10:30
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    $\begingroup$ @Philipp actually it requires far less energy than the other proposals, generates re-usable carbon along with other compounds that can be used to generate power. In addition with hydroelectricity generation you can reverse global warming where carbon-dioxide levels are the primary factor. I strongly recommend reading the paper. $\endgroup$ – Aaron Jul 22 at 10:39
  • $\begingroup$ @Aaron, less energy than other proposals, yes, but it's still only 75% efficient (ie. 25% of the energy you put in goes into other things than producing coal). $\endgroup$ – Mark Jul 22 at 21:42
  • $\begingroup$ A catalyst is not magic. It speeds up the rate of a particular reaction--but it speeds it both ways. If you have the thermodynamic conditions to favor CO2 to carbon conversion--in other words, if you are pumping energy into the system--a catalyst will make it go faster. It will not magically create the required energy. If you do not supply the energy, the same catalyst will speed up the natural rate of carbon oxidation. $\endgroup$ – Ralf B Jul 24 at 5:42
  • $\begingroup$ @Philipp There's two points; one is for carbon storage (since coal is a lot more compact than carbon dioxide, and less prone to leaks). The other is for energy storage - it's a relatively cheap way of storing energy produced through unstable energy sources like solar (if you could actually achieve the 75% efficiency in real industrial scale, it would be better than most PSH's, and without the massive reservoirs, and with essentially unlimited scaling). $\endgroup$ – Luaan Jul 24 at 7:33
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CO2 is the product of combustion of carbon and oxygen. Similarly, H2O is the product of combustion of hydrogen and oxygen.

You can't burn something that's already been completely burnt because the burning process is the act of converting atoms from a higher energy state to a very low energy state, and CO2 is completely burnt.

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  • $\begingroup$ "Completely burnt" <-- ClF3 begs to differ. ;-) $\endgroup$ – R.. Jul 23 at 3:21
  • $\begingroup$ @R.. this is why I explicitly mentioned, "of carbon and oxygen". $\endgroup$ – RonJohn Jul 23 at 3:24
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For something to be useful as an energy source, there must be a reaction using readily available reagents that provides more energy out than it takes in.

Unfortunately, CO2 is essentially "ash," as in the waste product of an energy source.

Although plants do take in CO2, they use it as a raw material to create an energy carrier, which is different from an energy source. In the case of plants, their energy source is the Sun, which they use to split water and attach the resulting hydrogen to the CO2 to create carbohydrates (sugars). These sugars serve as the "battery" which partially stores the energy they've received from the Sun. Later, the plants will take in oxygen to burn these sugars, and release CO2 themselves in the process. However in the case of plants they are generally able to take in more CO2 than they produce, which is why CO2 is such a good fertilizer for plants, and growing plants is a good means of carbon sequestration.

Animals come along and eat the plants to steal their "batteries," (sugars) and thus they can get more energy out of the plant to power themselves than it takes them to eat the plant, and give off CO2 as a waste product (which the plants then pull in again to make more sugars). Some of those plants also die and get buried, where their sugars undergo reactions that eventually turn them into other energy carrying molecules that apes will fight over to maintain the lifeblood of their comfortable lifestyles.

CO2 is an extremely stable molecule due to its pair of double bonds, and it doesn't really like to react with a lot of things under normal conditions. So, unless your world has something that can allow you to react the CO2 and get more energy than it took to create, you won't be able to use CO2 as an energy source. Remember that ultimately, even though we use fossil fuels, our civilization is powered by the Sun. It's just that that energy has already been captured for us; we're stealing batteries that were already charged millions of years ago, and the result of releasing that energy is a waste product that is already in its lowest energy state. Above all things, the laws of thermodynamics cannot be violated. It will take more energy in to do something with the CO2 than you will ever get out of it.

This is the same problem with the "hydrogen economy," as hydrogen is an energy carrier, not an energy source, and you still need something to generate the hydrogen in the first place (and do so in a way that is economically feasible). The only way to do this efficiently without fossil fuels is likely to be nuclear, but I digress.

A better approach to mitigate the carbon would be to engineer an organism that can use it. In fact, on Earth, most forms of the chlorophyll molecule are more efficient at higher CO2 concentrations. Also, the vast majority of oxygen-producing photosynthesis on the planet comes from phytoplankton in the ocean, and phytoplankton is one organism whose chlorophyll molecules perform better at higher CO2 concentrations.

Unfortunately though, phytoplankton growth is limited by the availability of iron, so some plans for dealing with CO2 on our own planet involve fertilizing the ocean using large amounts of iron to cause massive phytoplankton blooms. These blooms would then pull in the CO2 from the air and presumably die, falling to the bottom where their bodies would entomb the CO2 from the air. It's essentially what happens naturally, but this would be supercharging it.

So for your civilization to mitigate the CO2 in their atmosphere, rather than attempt to use the CO2 as an energy source, which would be by definition impossible, it might make more sense for them to engineer phytoplankton that can efficiently pull CO2 out of the atmosphere, preferably in a way that doesn't require some kind of fertilizer (such as iron). There is a danger here though, as if the phytoplankton grows unchecked, it could pull too much CO2 from the atmosphere in a relatively short amount of time (on the order of decades or less), and cause a crash of the ecosystem. Not only would pulling too much CO2 out of the air potentially cause cooling, but it could starve plants on a wide-scale and disrupt the carbon cycle, and worse, collapse the food web.

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