The way I see it, sooner or later it will come down to atmospheric engineering (i.e. managing reflective airborne molecules, like an artificial volcanic winter). The question will be wether we have this technology sorted out until the situation becomes desperate and whether the world will be politically stable enough for an entity to take responsibility over this - it's going to be like a sword of damocles over everyone's heads and once we've started with this we can't go back (because everyone will keep burning fossil fuel until it has run out).