Here’s NETL’s carbon storage atlas:
I think those estimates are about a decade old though. Most of the prospective storage is in saline formations, not oil and gas reservoirs. They estimate 2.6-22 trillion metric tons of capacity. By comparison, net annual emissions in the US are ~6 billion tons.
CO2 cannot be converted to carbon without using more energy (this energy will make more CO2 in turn).
Bottomline - CO2 will not be converted to Carbon in an Industrial process. Plants and microbes can do it though.
There is a process showing a lot of promise, that converts natural gas (methane) into carbon black and hydrogen. So there are no CO2 emissions !!
The produced hydrogen can be used from energy/power while the solid carbon can be sequestered or used as a material like fillers for tires.
I think this gets at the scale of the problem. For the past 200 years we’ve been pulling fossil CO2 out of the ground and adding it to the atmosphere. Even if we stopped doing that entirely, it would be a while before levels got under control just letting natural processes do their thing (but, at the same time we’ve also chopped down most of the world’s forests). Any slate of solutions is going to have to take out more CO2 from the atmosphere than we are currently putting in to it, in order to make any progress. It boggles the mind how big a problem this is.
My suggestion upthread was to put it back where it came from, in coal mines. Whether open cut or deep ones.
The carbon sat there for millions of years so I suspect would be as secure as anything else we could come up with.
Here is the phase diagram for CO2 :
At ambient conditions, you’ll need to cool it down to -110F (-57 C) for it to become solid. I know no coal mines where such low temperatures are present. If you want to store at ambient temperatures 70F, then the mines will need to withstand 1000s of psi, which they again cannot.
This is the reason, CO2 is stored in geological formation of rocks or salts which can withstand high pressures.
Ah, the intermediate step is converting the CO2 to C via plantations of trees that you turn into charcoal, then bury it.
Of course it only works if the volume of CO2 removed and stored greatly exceeds that generated by the planting, harvesting and charcoaling of the trees, which I don’t have the knowledge to compute!
The USA emitted about 5 Billion metric tons of CO2 in 2020 (U.S. Energy Information Administration - EIA - Independent Statistics and Analysis)
One acre of US forest sequesters 0.23 metric tons of CO2 per year (Greenhouse Gases Equivalencies Calculator - Calculations and References | US EPA)
Therefore you will need 5/0.23 = 22 Billion Acres of forests
The total area of US including Alaska is about 2 Billion Acres !! (The U.S. Has Nearly 1.9 Billion Acres Of Land. Here's How It Is Used : NPR)
So even if we ripped out every house, school, road, sports stadium etc, in the USA and replaced every square inch of the US by forests - you will need another 10 United States sized forests to take care of the CO2 emissions of just the US in a year
Sorry - didn’t mean to bold the entire paragraph, just wanted to bold the 10 United States part.
This was my concern - the carbon fit nicely into the ground as pure carbon or hydrocarbon chains, but stuffing that genie into the bottle once it’s CO2 will be tricky. Plus, yes, one way or another turning CO2 into a solid C-something will like require feeding it about the same energy as was released when it burned.
I do recall discussions about possible solid methane deposits off the continental shelf and the risk they destabilize and bubble up… Probably we’re not in a hurry to turn the oceans into seltzer water by sequestering CO2 deep down.
If we first stop further emissions, by changing all of our energy systems over to green sources, then we could start growing fast-growing trees on the land that we have, harvest the wood, and sequester it in old coal mines or ocean rifts or whatever. But there’s no way that could keep up if we didn’t first stop the future emissions. And it also won’t work to just plant forests and then leave them alone, without the big pit.
There are a lot of communities in the US that are hurting because paper mills have closed. These places are distributed all over the US. Here is a map :
Energy produced from wood (biomass) is considered green if the resulting CO2 is captured and sequestered. There are many projects through the final stages of approval that are doing exactly that; with cheap wood from overgrown forests originally grown for paper. Communities where paper mills have shut down have welcomed these projects and they have government backing too.
Right, it’s something, but it’s not enough.
I would think that CO2 uptake by the oceans, and the algae and plankton processes, would have the most impact on the absorption of CO2.
Wouldn’t that kill the oceans?
This diagram includes links to fact sheets about different carbon management strategies, funding opportunities, etc.
The oceans are being seriously damaged by the excess CO2 (as well as the temp increase). Most anything that will decrease the CO2 in the oceans via enhancing natural methods will be a net positive.
As md-2000 notes and I was getting at in my first post the oceans are the best way to approach the absorbtion problem. Carbon capture equipment, spreading crushed rocks around, etc. just cannot possibly have the economy of scale and energy efficiency of giving the oceans a nudge.
If we get to a net-zero greenhouse gas situation and the climate hasn’t reached actual crisis levels (as opposed to the hyperbole about today’s climate), then we won’t need to plant the forests. Shortly after CO2 levels peak, temperature rises will reach a plateau. This is because the greenhouse affect of the CO2 and other greenhouse gases already in the atmosphere will be offset by natural processes, mainly the ocean, that will be removing greenhouse gases from the atmosphere, leading to a temperature equilibrium.
Explainer: Will global warming ‘stop’ as soon as net-zero emissions are reached? - Carbon Brief.
The best available evidence shows that, on the contrary, warming is likely to more or less stop once carbon dioxide (CO2) emissions reach zero, meaning humans have the power to choose their climate future.
Of course, if the future generation Earth occupants want to reduce CO2 levels in order to induce global cooling, then they’ll want to increase the global carbon sink, through efforts such as reforestation, recreation of peat bogs, reestablishment of mangrove groves, etc. However, it won’t matter if, after the initial growth, an equilibrium state is reached. Once they pull a sufficient amount of CO2 from the atmosphere, the greenhouse effect will have been reduced and the former equilibrium would then be changed into a cooling trend. It will eventually level out, but at a cooler equilibrium.
As an aside, these future forests, bogs, groves, etc. won’t actually be at an equilibrium after maturity, especially the bogs. Decomposition of dead plants doesn’t mean a 100% conversion to base chemicals. Most of the bits of the plants that are underground will stay underground and eventually become organic sediment. Less so with underwater portions of plants, but still a significant proportion of the dead plan matter will end up on the water body’s bed and turn into sediment. And even the above ground dead vegetation that ends up on the forest floor isn’t going to all rot into the air. Much of it will be covered up by other dead vegetation, with a portion cut off from air and becoming mulch, which will eventually be compacted, become part of the ground, and again be turned into organic sediment.
One further note that wasn’t discussed on the web page I linked to, and that I don’t recall seeing in this thread. There probably won’t be any net-zero solutions that don’t include the existing carbon sink. There are lots of industrial processes, especially concrete, steel, and fertilizer that release CO2. It may not be practical to manufacture green diesel. And any complete net-zero solution will have to consider carbon imports. So the consideration of the amount of CO2 being absorbed by the replanted environments after they’ve reached maturity will be important.
This is an extremely misleading diagram because it makes no judgement on the maturity of technologies. The Oil Industry (and many countries infact) use diagrams like this to pick a technology that effects them the least, and then invest a token amount in that technology , while continuing to emit CO2 
IPCC has moved away from this practice, and now marks every option with probability of success/confidence. Similarly DOE should rank each technology on a scale of 10 to 1 saying which technology is mature. This is important because we do not have time on our hands. We need a 50% effective idea to go on stream today than wait 10 years for the 80% effective idea.
Let me give an example :
One of such Greenwashing idea is the Japan-Australia partnership to make Synthetic Natural Gas. Basically the idea is that Japan will keep its current infrastructure of natural gas power but capture the CO2 made by these plants. This CO2 will then be shipped to Australia on container ships. Australia will make Hydrogen from Solar Power - then convert the shipped Japanese CO2 back to methane using this Hydrogen.
And Tada - Japan does very little change to their infrastructure.
This idea looks great on paper and goes a long way “greenwashing” and “showing government’s commitment to decarbonization”, but this is doomed from the start because of many reasons one of which is that large scale methanation is unproven technology.
Even if this is true, the plateau could mean continued ice cap melt with its concurrent sea level rise. Granted it wouldn’t be accompanied by sea level rise caused by water expansion in this hypothetical scenario, but if ice cap melt turns out to be larger than expected, coastal cities and low lying nations would still be at risk in the medium term (i.e. within the lives of some people now living.)
I leave the details to those in the know, but… More ambient CO2 means a higher level dissolved in the oceans, for the algae to use, etc. Higher temperatures have a trade-off; warmer means easier for algae to grow, but warmer means lower concentrations of dissolved gases.
The next question would be where that algae goes - eaten by plankton which are eaten by bigger lifeforms, etc. Some maybe sinks to the ocean floor, removed from the carbon cycle.
But meanwhile, more CO2 is also making the oceans more acidic and more inhospitable to life, IIRC.
I fail to see why it’s necessary to capture CO2 in Japan. Instead of capturing it and shipping it thousands of miles, would it probably be cheaper to suck it out of the ambient air in the Australian desert (and, process powered by solar power also). It’s coming from the same atmosphere, just not as concentrated a source.
As I understand, ships are the least green method of transport unless we’re back to the era of clipper ships. Trains and pipes can be electrified, ships not as easily. How much CO2 could we save by building high capacity freight rail links from China to Europe and possibly across the Bering Strait?