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I know it's possible to capture $\ce{CO2}$ with various chemical reactions. For example NASA's space shuttle had some kind of regenerative $\ce{CO2}$ scrubber. But how expensive is it? Could a huge number of these devices, or something like it, significantly reduce the atmosphere's $\ce{CO2}$ level?

I realize this is probably not practical, but it's an interesting thought experiment. To make my question more specific: can anyone show some rough calculations of how much $\ce{CO2}$ you could remove if you had, say, 2 trillion USD to spend? To simplify, assume that energy is at current prices, but clean. (i.e., if the device requires electricity, assume it's coming from a nuclear power plant, solar farm, etc.).

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    $\begingroup$ Humans are increasingly producing $\ce{CO2}$, and there's never a world-wide agreement for using cleaner methods. Even if we could, we'd end up cleaning in circles. $\endgroup$ – M.A.R. Jul 27 '15 at 17:11
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    $\begingroup$ How about we plant a bunch of trees? $\endgroup$ – Aura Jul 27 '15 at 20:58
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    $\begingroup$ Why not use non-toxic, solar powered, environment friendly, self sustaining and duplicating CO2 scrubbers? We just need more of them. $\endgroup$ – PTwr Jul 27 '15 at 23:05
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    $\begingroup$ Algae is a much more efficient CO2 scrubber than trees. treehugger.com/urban-design/… $\endgroup$ – Timbo Jul 27 '15 at 23:29
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    $\begingroup$ Start with reading what the IPCC has to say: 1, 2. The only scenarios seriously considered are Bioenergy with carbon dioxide capture and storage and afforestation. However, I don't think 2 trillion USD is actually a very large amount of money in this context. You need to consider indirect (e.g., socio-economic) costs. $\endgroup$ – Roland Jul 28 '15 at 14:37
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This is very hard to answer precisely, as there are many different carbon capture strategies, and economics at the scale required is quite different from our normal understanding. However, I'd love to see some attempts to at least get order of magnitude estimates, or sources with more in-depth analyses.

Here is an implementation of carbon capture and storage which is useful to consider for its simplicity rather than its real-world applicability, to give a sense of scale. One way to remove anthropogenic $\ce{CO2}$ is to do "inverse combustion", more specifically:

$$\ce{CO2(g) -> C(s) + O2(g)}\ \ \ \ \ \ \ \mathrm{\Delta H=+390\ kJ/mol}$$

Assume this process can be done with perfect efficiency, and that the only energy expense in the process is to drive the reaction forwards (that is, zero energy consumed in transportation, collection, construction, etc). According to this source, the amount of anthropogenic $\ce{CO2}$ emissions from 1750 to 2008 has totalled about $1250\times 10^9\ \mathrm{t_{CO_2}}$. Suppose you wish to remove all this carbon dioxide (only about half is in the atmosphere, the rest is trapped in the ocean or land) using the above process. This would require about $\mathrm{10^{22}\ J}$ of energy, which Wolfram Alpha suggests is about 50% more energy than can be retrieved from combustion of all global proven oil reserves in 2003. Put another way, this is about 20 times the world energy consumption in 2012, or 150 times world electrical energy production. This would put the cost of this process in the range of hundreds of trillions of US dollars, meaning two trillion USD barely makes a dent.

Is there some other process where two trillion USD is close to enough? I seriously doubt it.

Edit: Several comments and answers mention biological sequestration, which is a legitimate carbon capture strategy. I did not consider it, however, because its costs are far more complicated to calculate. My intention with this answer was to find a quick and comparatively simple way to attach an energy cost to carbon sequestration. Whether the monetary cost even makes sense at this scale (how do you define "monetary cost" when it's larger than world GDP?), I don't know.

But here's another amusing comparison, which should somewhat temper hopes that biological sequestration will be a magic bullet. The amount of anthropogenic carbon released between 1750 and 2008, ~350 billion metric tons of carbon, is comparable to a significant amount of the biomass on Earth (all eukaryotic life contains approximately 560 billion metric tons of carbon, multicellular life is a fraction of this). Thus, biological sequestration of the majority of anthropogenic carbon would be broadly equivalent to sacrificing all eukaryotic life on Earth (or ~10-30% of all living organisms by mass) in order to collect and bury carbon, then seeding the Earth back to its current biological state.

Over 250 years of constant stimulation to produce as much energy/goods from fossil fuels, we have released a lot of carbon.

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    $\begingroup$ It basically boils down to the fact that we'd have to 'unburn' all the fossil fuels we've used so far, and then some, and that energy has to come from something other than said fossil fuels. Not easy to pull off..... $\endgroup$ – whatsisname Jul 27 '15 at 20:29
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    $\begingroup$ This answer neglects the fact that trees exist and can fix CO2 for lower energy requirements, being solar powered. $\endgroup$ – March Ho Jul 27 '15 at 23:15
  • $\begingroup$ We can cut that enthalpy cost by a factor of about two (179 kJ/mol) if we sequester as limestone instead of carbon (and reuse all your simplifying assumptions). [ @MarchHo correctly observes that I conflated enthalpy of reaction with enthalpy of activation, although neither of us used these words. So the catalysis bit is retracted.] $\endgroup$ – Eric Towers Jul 27 '15 at 23:16
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    $\begingroup$ @EricTowers but is there a better way than photosynthesis to capture solar energy AND simultaneously sequester carbon? $\endgroup$ – Timbo Jul 27 '15 at 23:33
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    $\begingroup$ The energy requirement of photosynthesis are irrelevant, since they are supplied by the sun and not by humans. $\endgroup$ – March Ho Jul 27 '15 at 23:39
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It's... feasible. There are a number of technologies that are being considered. Costs will be high, some number of billions or trillions: you're talking planetary engineering, here.

The most obvious option is to plant trees. The obvious problem with that is that even planting them at the same speed as they are being removed is infeasible. Unfortunately trees csan store only a limited amount of carbon; once a forest's got as crowded as it can, it's maxed out its carbon sequestration capacity, and will never store another gram.

An option that addresses this is to heat crop residue, storing the "biochar", the charcoal from the burned crops, in landfill or even in farmland to enrich the soil. The output of biochar systems is typically around 20% char, 20% usable biogas, 60% usable bio-oil, making them both a net energy source, and a carbon sink.

This system is being considered as an additional revenue stream for the sugarcane growers of Brazil, where it would (if universally adopted) sequester about 330,000,000 tons of carbon annually.

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    $\begingroup$ Out of curiosity, I did a quick calculation of how much area would have to covered by Amazonian forest to sink the amount of $\ce{CO2}$ mentioned in my answer. Using a value of 61 petagrams of carbon per 424 million hectares for above-ground carbon density of the Amazon shown in this article, we reach an area ~40% larger than Russia, which means 16% of the land area on Earth covered in dense jungle. Doesn't sound possible. $\endgroup$ – Nicolau Saker Neto Jul 27 '15 at 23:35
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    $\begingroup$ Good point. Forestry's arguably possible in theory, but not economically sane in today's world. $\endgroup$ – Dewi Morgan Jul 28 '15 at 1:48
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    $\begingroup$ You don't need to grow them all at the same time. Each crop is (ideally chared first, then) burried, say in an old coal mine, and then the land used for another crop. A first step would be to stop burnimg fossil coal in the first place. $\endgroup$ – JDługosz Jul 28 '15 at 15:19
  • $\begingroup$ Yes: as an alternative to mere forestry, biochar has the advantage that every year's crops can become part of the carbon sequestration, which makes it, to me, a very economically sensible part of the toolset to clean up atmospheric CO2 (it'll take many tools: biochar, artificial trees, scrubbing towers, ocean seeding, etc). Char needn't all be put into landfill or used-up mines either: it can be ploughed back into the earth to help drainage, used in carbon filters, etc. It could also be used in the place of regular charcoal and burned, of course: sadly the most likely outcome, economically. $\endgroup$ – Dewi Morgan Jul 28 '15 at 22:39
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    $\begingroup$ The forests couldn't do it because they'd be CO2 starved. All plants are currently CO2 starved air flow is needed to keep growing but trying to grow a vast forest or any other plant solution would leave them stunted. $\endgroup$ – user2617804 Jul 29 '15 at 0:11
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"Give me a half a tanker of iron and I will give you another ice age"

That's the claim of people who believe in iron seeding the ocean. The link claims that "the addition of silicic acid or choosing the proper location could, at least theoretically, eliminate and exceed all man-made CO2", but no citation is given. As to cost?

"Current estimates of the amount of iron required to restore all the lost plankton and sequester 3 gigatons/year of CO 2 range widely, from approximately 2 hundred thousand tons/year to over 4 million tons/year. The latter scenario involves 16 supertanker loads of iron and a projected cost of approximately โ‚ฌ20 billion ($27 billion)."

but again, citation needed!

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    $\begingroup$ That's pretty much free compared to all the other attempts (including those we are actually trying, and including the huge costs sunk in designing more efficient devices etc.). If they actually had good evidence this would work (and of course, that it wouldn't cause bigger issues which is always a possibility), cost wouldn't be the thing that would stop this. And of course, eventually, the carbon will either return (as fish breed out of control with their new cheap source of food) or drop to the ocean floor (possibly creating more trouble in the future). $\endgroup$ – Luaan Jul 28 '15 at 7:20
  • $\begingroup$ @Luaan Experiments have been conducted several times. Iron fertilization: Experiments - and yes, there are questions of other issues and the definition of sequestration. $\endgroup$ – user2175 Jul 28 '15 at 14:20
  • $\begingroup$ My own guess is that the human population will increase so much over the next 200 years that "farming the ocean" will be necessary to prevent mass starvation. So iron seeding is going to go forward no matter what. $\endgroup$ – user14717 Jul 28 '15 at 15:47
  • $\begingroup$ @user14717 That's a common theme in human history. The starvation part anyway - it hasn't come to pass quite yet, no matter how strongly the guys believed in their pet hypothesis. So far, we've always been a few steps ahead of that technologically. The fixed thermodynamic limits are probably going to make human population infeasible long before we hit limits in food production (unless we somehow lose the food sources we already have, which is of course possible - we're getting better and better in self-contained food production, though). $\endgroup$ – Luaan Jul 28 '15 at 16:22
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the $64,000 Question is ... Is it economically feasible to use a 'synthetic' process over a naturally occurring one. In this case, No. Zooplankton are the prime movers of CO2 in our atmosphere. They basically turn gaseous CO2 into Calcium Carbonate a lab phrase for sea shells. Increasing the number of Zooplankton is key to the process. UV and oil slicks and the Sargasso seas of plastic are lowering those numbers.

so the answer to the question is what process would one employ that was the most cost effective solution to the problem, clearly A. clean up the oceans and promote natural zooplankton growth or B. mimic the metabolic pathway that results in the production of calcium carbonate, as a GM/organic solution. Either A or B is a far better and sustainable solution that does not reintroduce/produce CO2 for the energy to power an artificial non-organic/biologic method.

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  • $\begingroup$ Welcome to chem.SE! This is one borderline answer. I wouldn't know if it's VLQ or not. But it would pay if you be more elaborate. $\endgroup$ – M.A.R. Jul 28 '15 at 16:39
  • $\begingroup$ This states part of the problem (without references) but doesn't provide any solutions so it doesn't really answer the question. $\endgroup$ – bon Jul 28 '15 at 17:51

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