Let me take the most likely one: the nuclear winter case. Say two countries that both have access to nuclear weapons get very angry at each other, and then retaliate, destroying most of the major cities in the opposite country. The vast bulk of humanity would survive, eventually. Say maybe we lost 5 percent of the population. Ninety-five percent of us would still be alive. But then as those cities burned, you’d end up getting soot in the upper atmosphere that stays there and darkens the entire planet. And all the crops fail.
As the world went dark, you’d have a couple of the more hearty crops survive—the trees would last a little while. But our standard crops? Your wheat, your rice, your corn? That’s all dead. You don’t get that harvest, and that’s what we feed the world with. Vegetable gardens, everything’s just dead. You can’t grow in darkness…
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