We've never actually created a thermonuclear winter but it was indeed a prediction. I think the problem is one of scale rather than technology ... can we do enough?
True, but I think there was a vulcanic explosion that caused several cold and miserable years? It was too far in our past, though, so again it is part speculation I think. Don't remember the specifics, I think it was during the 30 year war?
That is of course not a viable solution, it only shows that apparently man made mechanisms can both heat or cool the atmosphere.