We hear a lot about positive feedbacks when it comes to global warming. For example: ice reflects solar radiation, so as the ice melts more solar radiation is absorbed which accelerates warming; rising temperatures create more deserts; release of stored methane; water-vapour feedback; etc.
With so many positive feedbacks it would seem that once the earth has flipped out of an ice age it would take an absolutely tremendous amount of change to get us back into one. Yet the explanations given for the onset and offset of ice ages (subtle orbital variations, continental movements) do not seem that great.
So how can these subtle changes ever be enough to flip us back into an ice age?
Do we have long time span climate-models to validate our theories?