If you look at the long term trend, and I mean over millions of years, you can see that we have evolved and flourished in a several tens of thousands of years long spell of untypically cool temperatures. Sooner or later this will change, and in the past cool periods like this have tended to be followed by periods as far above the average temperature as we currently are below it, not good, or not for us anyway. As far as I can see there doesn't appear to be any detailed consensus on what exactly was responsible for these historic temperature changes. Movement of tectonic plates, cyclic changes in the planet's orbit and axis of rotation, changes in solar activity, all must have played a part, and of course all are in a continuous state of change. But if scientists can't agree precisely what combination of factors caused these events in the past, then hard to predict when the next one might be. Could be in a hundred thousand years, or it could be we are seeing the start of it now, helped along by our own contributions.