[marcosdumay]

There is way more carbon in the ground as rocks than as oil. If you have plenty of energy, the difference is quite manageable.

Besides, as somebody already pointed out, there is that CO2 on the air that we actually want to get rid of. It's nothing compared to the rocks, and a little harder to get, but getting it first would improve things a lot.

[ok_computer]

The carbon isn’t valuable elementally as much as it is structurally and molecularly. I mean that as aromatic rings and other ready made building blocks that conveniently can be fractionally separated with pressure and temperature conditions in a column as a gross generalization. All of this is energy intensive but much less so than building up from three atom molecules with strong bonds. And much much less energy intensive than separating a trace % molecule from the atmosphere at low atmospheric pressure and translating that to complex molecules.

There needs to be more appreciation for the laws of thermodynamics when discussing technology. Everything is not a 1-dimensional reduced abstraction.

[pfdietz]

The density of carbon in seawater is also higher much than it is in air. The relative concentration of bicarbonate in seawater is a few times lower than in air (as % by mass), but because water is nearly 1000x the density of air the true amount of bicarbonate there per volume is much higher.

[tesseract]

> there is that CO2 on the air that we actually want to get rid of

For this reason I have long been slightly baffled that development of compostable/biodegradable bio-based plastics is such a priority in materials research. Sure, it's interesting in the very long run, but for the foreseeable future, converting atmospheric CO2 (via plants as an initial step) into a long lived, inert material that can just be buried after an initial use seems like a benefit.

[sophacles]

Biodegradable is only one type of degradation. Some of those compounds break down over time, or with exposure to uv or random stuff in the ground, into nasty compounds that you certainly don't want entering the water cycle or food cycle. An additional attribute of biodegradable therefore is: keeps (non CO2 pollution down).

In addition, things that biodegrade don't immediately just turn into CO2. Things like biomass (that is everything alive and dead that isn't decomposed) use a lot of that carbon. A significant fraction of the carbon in rotting stuff doesn't end up in the atmosphere for decades or longer. The carbon cycle isn't just "CO2 becomes plants which become CO2"... there's a lot more steps in between (for example, next time you eat... you are a direct next step!). Some of those steps take a very, very long time.