That may well be the case, but it's always a mistake to assume we know everything. That's how we got into this mess in the first place. It wasn't long ago that we didn't know what germs were, didn't have electricity, didn't understand things like relativity or quantum mechanics. We don't know what we're going to learn tomorrow, or next year, or 50 years from now.

If there's an option A in which we both fail to do anything to reduce carbon in the atmosphere and also fail to do anything to cool down the planet and an option B where we for now fail to do anything to reduce the carbon in the atmosphere but manage to cool it down enough to provide a stop gap, option B might get us to a point where we can actually do something. At the very least it could give us a little more time before we fully run out of options for survival. Option A doesn't give us that breathing room and doesn't make things better in the mean time.

Saying that neither option can fully solve our problem doesn't mean it's not worth doing the thing that makes the immediate issue less severe. It's a bandaid, to be sure, but sometimes a bandaid is what you need to slow down an infection until you can get somewhere where you can actually heal.

I don't know the long term answer. I don't think anybody does. But I also don't think we can say with honesty, given the history of human knowledge and technology, that we actually know whether or not an answer will exist in the future. In the mean time, we should probably be acting to create the possibility that we can make use of an answer if we find one.