An MIT scientist asked how is it that people are so prepared to accept the inaccuracy of weather forecasts even 3 or 4 days out but on the other hand seem to be willing to accept that it is possible to essentially forecast weather 100 years from now? It is puzzling since one of the more notoriously unpredictable things is weather and that is fundamentally understood by everyone but then people take forecasts of 3 to 5 degrees warming over the next 100 years as undisputed fact.
Over millennia weather patterns have been nothing but variable so past experience would also seem to caution against predicting long term trends. There is some debate over what would happen if the melting of the Arctic polar cap introduced so much fresh water into the Atlantic that the "great conveyor" effect of ocean circulation would disappear. A Woods Hole Oceanograhic Institute study says that this is what caused the glacial period 20,000 years ago.
The point is, nobody knows. It could go either way and warming could lead to significant cooling but nobody really has a clue.