The carbon stores in earth, sky, and ocean are in chemical equilibrium, so if one attempts to deplete one reservoir, carbon will flow into that reservoir to restore the equilibrium. There are debates on how fast this will happen, and nobody seems to really know. Just how big is the ocean reservoir? Turns out it's huge, about fifty times as large as the atmospheric reservoir. In other words, only 2% of the world carbon inventory is in the air, compared to the ocean. (Dirt and vegetation are also far smaller than the ocean.) Specifically, the usual estimate for the ocean reservoir is 39,000 gigatons of carbon.
Given proposed remedies that all claim at most megatons to a few gigatons per year, it's hard to see how one can even keep up, let alone reduce to zero fast enough to matter, even if the ocean deep doesn't fight us. And most proposals lack a full, contractually-binding engineering estimate of costs.
Actually, it may be that the ocean deep will remove and sequester carbon far faster than humans ever could.
The converse is also true -- carbon levels in the atmosphere will stall despite sustained heroic carbon-capture efforts until the far larger ocean reservoir is also exhausted.
To permanently remove CO2 from the ocean deep, it is traditional to convert CO2 to chalk, which eventually converts to limestone. How much chalk does it take to equal 100 ppmv of CO2 from the atmosphere? Converting 100ppmv of CO2 to chalk yields 656 cubic kilometers of chalk or limestone. If deposited as a layer one centimeter thick, it would occupy about 65,568 square kilometers, about the size of Lithuania, which is tiny compared to the sea.
So in all cases, bring numbers, and a _very_ big calculator.
Joe Gwinn