Earlier quoted context omitted.
Yeah, arguably the war in Ukraine is teaching everyone who is listening that nobody is willing to be the first to use nukes and destroy the planet, which means that nuclear powers can actually fight wars--which is incredibly dangerous, of course.
Nukes don't "destroy the planet", they're just really powerful bombs. Nuclear munitions have been detonated 2058 times already. https://www.atomicarchive.com/almanac/test-sites/testing-chr...
And of those nuclear tests, 528 were above ground, and only 200-300 of them were probably true airbursts. And all of them were in areas like atolls, deserts, and Arctic tundra where nothing burns. Their climate impact was near zero because they caused no fires and no dust, and they were spread out over 35 years.
That is different from thousands of detonations in an afternoon causing raging fires all over the globe. Estimates are that it would generate around 150 billion kg of stratospheric soot. For comparison, yearly generation is around 7-8 billion kg of soot, generally confined to the troposphere and therefore rains out fairly quickly. A nuclear exchange would produce 20 years of soot and dump it into the stratosphere, above the rain.
The largest pulses of soot that we've ever observed are the 2019 Australian megafires and the Canadian 2023 fires. These were roughly less than or equal to 1 billion kg and produced measurable climate effects that lasted months.
A full nuclear exchange would likely produce -10C of global cooling in its immediate aftermath, which would be catastrophic to agriculture and civilization. It would stabilize on the order of decades, but there would be a lot fewer humans in the world.