This isn't even a question of "nuclear winter" (a real, if still scientifically debated, hypothetical risk stemming from a much larger-scale exchange). Instead, it's a question of large-scale radiological and toxic contamination as well as disruption of global weather patterns.

Essentially all locations on Earth are connected via atmospheric circulation (i.e., global wind patterns). Radiological or other contamination can spread far beyond its source region, especially if initially lofted to great heights by a large explosion or ensuing conflagration.

Additionally, soot, smoke, and other particles generated by large-scale conflagrations in fuel-rich regions (which includes cities, ports, & large petrochemical facilities) can be lofted into stratosphere, where they can linger for months to years & disrupt global climate.

While it is true that there are genuine scientific uncertainties surrounding the details and dynamics re: how hypothetical nuclear exchanges of different size and location, and subsequent mass fire/firestorm events, would actually play out, *that is very much beside the point.*

There are innumerably many reasons why any exchange of nuclear weapons is horrifying to contemplate. But I want to dispel rumors that newer science suggests that large-scale consequences would be much less severe than perceived during Cold War era. That simply isn't the case.