How does one curb global warming? Do trees actually help? Any techs that are a "must" have? Can it be reversed once it starts, or do the effects just halt?

Other than coal plants and Nukes, what else contributes to global warming?

you cannot stop global warming, just override your nuclear plants

http://forums.civfanatics.com/showthread.php?t=239031

When a Civ discovers fission, nuke them out of existance so they can't build nuclear plants.