Yes, the size of the blasts possibly, but the pacific and the siberia tests by the US and USSR were probably much bigger than anything a ballistic missile could carry today, so . . .what are we missing ?
There's a very big unknown here, which is how much soot a nuclear-induced conflagration of a city pumps into the stratosphere. We know that stratospheric soot would induce global cooling, for a time period of at least a few years, based on the observed effects of existing natural stratospheric soot pumps known as volcanoes. And we also know that without stratospheric pumping of soot, climatic effects are at best localized in both space and time (see, e.g., Kuwaiti oil well fires).
Personally, I am very unconvinced by the assertion that nuclear weapons causes severe global cooling. The most powerful volcanic eruptions in recorded history are VEI 6 or VEI 7, which means they erupted up to around 100 cubic kilometers of stuff. If you assume a city has an average height of 100 meters (for the sake of round numbers), you'd need a firestorm to completely consume 1000 square kilometers of city to produce the same volume of material. Furthermore, a volcano's eruptive column is a relatively narrow plume of ejection as opposed to a broadly dispersed firestorm base, so it seems that you'd have a far lower conversion rate of material into stratospheric soot--and correspondingly you'd need to burn even more area to achieve the same effect.
And at the end of the day, even the most severe volcanic winter in recorded history (1815) happening today would be roughly equivalent to rolling back global warming to 1850. For less than a decade.
I honestly have no idea, and perhaps our sensors were not accurate enough at the time.
To put another bound on the question, Krakatoa was estimated at 200Mt in a single blast, and that definitely did affect the global climate. Again gut feel, but my suspicion would be that many distributed explosions around the globe totalling 200Mt would be worse in terms of direct effect just because there would be more physical mass available to be thrown into the air. Plus the soot would be radioactive.
I’d imagine it’s like filling a sink with water (and the drain slightly open) and use dye to simulate nuke debris. Try one or two drops every minute for ten minutes, vs. an entire cup of dye all at once.
Obviously the actual climate models are far more complex but for ELI5, I think this is the general idea.
We quickly realized this was a terrible idea(some tests in NV had intense fallout in NY) and stopped doing atmospheric tests by 1963.
I doubt this very, very much.
Also to note: those were single blasts (as opposed to the dozens that would now cover any given region), some were underwater, none of them interacted with amounts of structures and living matter comparable to a populated city.
My layman’s understanding is that huge warheads are a less efficient usage of nuclear materiel (in terms of sheer destruction) than a larger number of mid-sized warheads.