If an asteroid impact or nuclear winter blocked the sun for years, our inability to grow food would result in billions dying of starvation, right? According to Dr David Denkenberger, co-author of Feeding Everyone No Matter What: no. If he's to be believed, nobody need starve at all.

Even without the sun, David sees the Earth as a bountiful food source. Mushrooms farmed on decaying wood. Bacteria fed with natural gas. Fish and mussels supported by sudden upwelling of ocean nutrients - and more.

Dr Denkenberger is an Assistant Professor at the University of Alaska Fairbanks, and he's out to spread the word that while a nuclear winter might be horrible, experts have been mistaken to assume that mass starvation is an inevitability. In fact, the only thing that would prevent us from feeding the world is insufficient preparation.

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Not content to just write a book pointing this out, David has gone on to found a growing non-profit - the Alliance to Feed the Earth in Disasters (ALLFED) - to prepare the world to feed everyone come what may. He expects that today 10% of people would find enough food to survive a massive disaster. In principle, if we did everything right, nobody need go hungry. But being more realistic about how much we're likely to invest, David thinks a plan to inform people ahead of time could save 30%, and a decent research and development scheme 80%.

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∙ A potential donor evaluates ALLFED

According to David's published cost-benefit analyses, work on this problem may be able to save lives, in expectation, for under $100 each, making it an incredible investment.

These preparations could also help make humanity more resilient to global catastrophic risks, by forestalling an ‘everyone for themselves' mentality, which then causes trade and civilization to unravel.

But some worry that David's cost-effectiveness estimates are exaggerations, so I challenge him on the practicality of his approach, and how much his non-profit's work would actually matter in a post-apocalyptic world. In our extensive conversation, we cover:

* How could the sun end up getting blocked, or agriculture otherwise be decimated?
* What are all the ways we could we eat nonetheless? What kind of life would this be?
* Can these methods be scaled up fast?
* What is his organisation, ALLFED, actually working on?
* How does he estimate the cost-effectiveness of this work, and what are the biggest weaknesses of the approach?
* How would more food affect the post-apocalyptic world? Won't people figure it out at that point anyway?
* Why not just leave guidebooks with this information in every city?
* Would these preparations make nuclear war more likely?
* What kind of people is ALLFED trying to hire?
* What would ALLFED do with more money?
* How he ended up doing this work. And his other engineering proposals for improving the world, including ideas to prevent a supervolcano explosion.

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The 80,000 Hours Podcast is produced by Keiran Harris.