We had a thread about this topic a few months ago.
Here's a link for you. The bit on the potential effects of even limited, regional nuclear war are particularly interesting. Obviously, Wikipedia is to be used with care, but I've followed the source links and it seems to check out.
http://en.wikipedia.org/wiki/Nuclear_winter#cite_note-6
In my T2K PbP, it's already snowing in Poland in early October of 2000.