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. As those crops fail, you’ll start to get hungry; you’ll start going into your stored food supplies. The historical assumption is that’s when we all go completely crazy. It’s bad. I’m sure you’ve seen the movies. There’s no good outcome there. That darkness will basically stay for around five years, until it starts to rain out of the atmosphere and then we’ll slowly but surely [get] more and more sunlight and start to rejuvenate agriculture again.
There’d be a little bit of conventional agriculture that survives—like the grow houses. For example, in Japan they have warehouses that just have racks of lettuce growing under LED lights, and that would still work, but what fraction of the population would that feed? I’m sure that the wealthy in whichever culture would still pick tomatoes and lettuce, but the vast majority of the world would not be eating those.
—Engineer Joshua Pearce, as interviewed by Yvonne Bang in Nautilus. The interview explores how we as a planet could feed 7 billion people after a global catastrophe.