> It's somewhat arrogant of us to assume that we understand enough about a system as complex as the Earth's biosphere and atmosphere to know for sure a larger percentage of CO2 will necessarily be bad long term. It's feasible that it could be better, even if only after a painful transition.

That's the problem. Maybe it's better, maybe it's worse, and there is only one way to find out. But if it's worse, finding out is very bad. And there is no way to know ahead of time.