There's very much a debate about the assumptions that went into the original nuclear winter work. Cities today have a lot more steel and concrete, and it's not

There's very much a debate about the assumptions that went into the original nuclear winter work. Cities today have a lot more steel and concrete, and it's not clear if the heat necessary for high altitude plumes would form. AFAIK no one has definitively settled the matter, but it is something think tanks publish papers on occasionally.