Underground sequestration is highly problematic, given the possibilities and local consequences of leakage, but also because it is highly unlikely there will be or could be storage capability adequate to the dimensions of the requirement. I have seen one wet scrubber process that produces gypsum as an output, with wall board as the final product. But that would likely produce a glut of wall board if relied on solely to solve the problem.

I do tend to prefer processes that convert CO2 to a solid form, but we may well find that even converting it to limestone, cement and marble, given technical feasibility and regardless of cost, would be inadequate. After all, the oil I lived above in Whizbang Oklahoma was covered by thousands of feet of limestone. It is likely that that limestone was a significant part of reducing the atmospheric concentration of CO2 to the levels we have known in the 19th and early 20th century.

As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."