Any solution cheap enough that the developed world can shell out for it without worrying about the developing world not paying their fair share would get implemented in an instant.
FYI, the Royal Society says we could fix it with geo engineering for a few 10s of billions USD a year [0]. That seems pretty cheap, only about 10-100USD per person per year for the US and EU. Less if the rest of the developed work chips in. It would even cost less Net as we would save the most from not having to build flood defenses or import food or keep out refugees.
