On the other hand, the scientific studies that took place at around the time of the Threads broadcast and similar indicated that it would take less than 1% of the global stockpile to generate a devastating nuclear winter. If the Russians have only 10% of the arsenal they possessed in the 80s it still wouldn't take a huge proportion to end life as we know it in the northern hemisphere.
Which is possibly the safest guarantee, because even a widescale nuclear exchange limited to Europe would also have a devastating effect on China. They might well have had a word on that front.