It's not the amount so much, it's more the rate of increase. People are stuck on 1 axis. I can eat 5 hot dogs I'll be fine, sleep well. But if you make me eat them all in all in 30 seconds, well that's another story.

The CO2 itself would probably be somewhat beneficial. The problem is, humans artificially doubling an important molecule will most certainly have an impact on other systems. It's basically uncontrolled terraforming without knowing what the end result will be.