https://www.navalgazing.net/Nuclear-Winter

Even using the most conservative numbers here, an all-out exchange between the US and Russia would produce a nuclear winter that would at most resemble the one that Robock and Toon predict for a regional nuclear conflict, although it would likely end much sooner given empirical data about stratospheric soot lifetimes. Some of the errors are long-running, most notably assumptions about the amount of soot that will persist in the atmosphere, while others seem to have crept in more recently, contributing to a strange stability of their soot estimates in the face of cuts to the nuclear arsenal. All of this suggests that their work is driven more by an anti-nuclear agenda than the highest standards of science. While a large nuclear war would undoubtedly have some climatic impact, all available data suggests it would be dwarfed by the direct (and very bad) impacts of the nuclear war itself.