I challenge this assumption. Nuclear alone could end humanity and megafauna for millions of years until something new evolves from extremophiles. Edit: ~ 12,000 nukes worldwide. Most by US and Russia with 5k each. I'm not up on my math since the cold war, but that still seems excessive for purpose. MAD is like "I will scorth the entire earth before I let you win." China's #3 count at 600 seems positively peaceful by comparison, though is still horrific overpower. Solving climate change with nuclear winter is not what I was hoping for.