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...
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.
Ecosystem resilience is based in the so called natural buffers. They are a product of the interactions between life beings. Imagine the life bar in a videogame. A natural buffer is like a 4-D life bar, where each character actualize their life bars at the same time. You lose some life and your friends will change their energy automatically to fix your health bar, Is paid with energy but also needs some "time currency".
With healthy buffers, even big impacts in an ecosystem are absorbed and the system returns to the initial situation again and again. But it has a coast in the buffer. The life bar gets shorter.
When the buffer is spent, the system is not 'elastic' anymore. At this moment the danger is extreme, but the warnings are subtle or none. Just further impacts, even tiny impacts, on the ecosystem cause a total, devastating and irrevocable collapse that may extend over several years and can't be stopped.
Is unclear if we had crossed this line yet.
The life has recovered from huge collapses at a planetary level five times yet, but it needed millions of years for that.
Just because we had luck before, does not mean that we will have luck forever.