Changing the global climate is easy, trying to figure out what you're going to wind up with after is the hard part. My guess is the climate will readjust itself as it has been doing for billions of years and the earth will abide. The geological record tells us the climate is to a great extent self-regulating, at least to long-term runaway excursions. However, this does not mean it is necessarily stable in the short term. Excursions in climate may eventually create feedback to control them, but weather will oscillate around some new equilibrium which may not take our agriculture or settlement patterns into account.

If the climate starts to change, the safest policy is to do nothing, until it finds its own equilibrium. The damage to our civilization may be severe, but life, and probably humanity, will survive. Only if it is pretty clear it is in a runaway slide to disaster, and we have a much better understanding of it than we do now, should we be tempted to intervene. We would only risk making matters worse.

The problem today is that we are already "intervening", just because of our normal industrial and agricultural activities. And we haven't the slightest clue what the results will be.