Arbitrary quantity about 10-15 billion.

But as to quality; agreed, the quantity would collapse unless the quality is there to sustain it.
That is why the oceans seemd so important to me. If we're going to "invest" in something to soak up CO2, why not put it where the byproduct would be food. Maybe I'm too optimistic about the oceans ability to soak up CO2, but isn't it orders of magnitude greater than anything we could produce.
It was this thread's "CO2 to Fuel" theme that made me want to talk about CO2 to food also.

~SA

Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.