Human beings can do incredible things when sufficiently motivated. I am convinced that if things get so bad the threat is imminent, and there can no longer be any possible question about it, humanity is perfectly capable of coming up with last-ditch technological innovations to turn things around, whether those are mirrors in space, or tubes in the ocean, or something else. The problem with this approach, and the reason it would be better not to have to rely on it, is that much damage would have to occur first, and the expense would be enormous. Far better to avoid the problem in the first place.