That's a big if. The global climate is an extremely chaotic and complex system; ur ability to accurately model it is still extremely nascent. (Scientifically, understanding the global climate seems at least as hard as deciphering the architecture of the human genome, say.) If you are at all skeptical of our ability to master geoengineering in time for it to matter -- I'm talking a few decades here -- then it behooves you to opt for conservation instead. Literally the only system that we know for a fact is sustainable is the one that existed before industrialization.