If there’s a nuclear war followed by nuclear winter, and the sun is blocked out for years, most of us are going to starve, right? Well, currently, probably we would, because humanity hasn’t done much to prevent it. But it turns out that an ounce of forethought might be enough for most people to get the calories they need to survive, even in a future as grim as that one.

Today’s guest is engineering professor Dave Denkenberger, who co-founded the Alliance to Feed the Earth in Disasters (ALLFED), which has the goal of finding ways humanity might be able to feed itself for years without relying on the sun. Over the last seven years, Dave and his team have turned up options from the mundane, like mushrooms grown on rotting wood, to the bizarre, like bacteria that can eat natural gas or electricity itself.

One option stands out as potentially able to feed billions: finding a way to eat wood ourselves. Even after a disaster, a huge amount of calories will be lying around, stored in wood and other plant cellulose. The trouble is that, even though cellulose is basically a lot of sugar molecules stuck together, humans can’t eat wood.

But we do know how to turn wood into something people can eat. We can grind wood up in already existing paper mills, then mix the pulp with enzymes that break the cellulose into sugar and the hemicellulose into other sugars.

Dave estimates that “…if hypothetically you were to feed one person all of their calories this way, it’s only about a dollar a day from cellulosic sugar. … It’s particularly cheap because we have these factories that have most of the components already. … Because we’re trying to feed everyone no matter what, we want to look at those resilient foods that are inexpensive.”

Another option that shows a lot of promise is seaweed. Buffered by the water around them, ocean life wouldn’t be as affected by the lower temperatures resulting from the sun being obscured. Sea plants are also already used to growing in low light, because the water above them already shades them to some extent.

Dave points out that “there are several species of seaweed that can still grow 10% per day, even with the lower light levels in nuclear winter and lower temperatures. … Not surprisingly, with that 10% growth per day, assuming we can scale up, we could actually get up to 160% of human calories in less than a year.”

But to get that sort of growth, humanity would need vast numbers of places for seaweed to attach, and to hang the strands close to the surface of the sea, where they can get the greatest amount of light. The solution is to attach it to ropes and suspend them from buoys that are anchored to the ocean floor but float on the top.

Dave’s team has estimated that “the main constraint here is twisting fibers into ropes that we’re going to attach the seaweed to. We found that right now, we don’t produce that much rope — we would actually have to increase our rope-twisting capability by 300 times, which sounds kind of crazy. But it’s actually a really simple process, and people have done it in their garage with a drill, basically twisting these fibers.”

Of course it will be easier to scale up seaweed production if it’s already a reasonably sized industry. At the end of the interview, we’re joined by Sahil Shah, who is trying to expand seaweed production in the UK with his business Sustainable Seaweed.

While a diet of seaweed and trees turned into sugar might not seem that appealing, the team at ALLFED also thinks several perfectly normal crops could also make a big contribution to feeding the world, even in a truly catastrophic scenario. Those crops include potatoes, canola, and sugar beets, which are currently grown in cool low-light environments.

ALLFED even thinks humanity could throw together huge numbers of low-tech greenhouses, which would stay 5–10°C warmer than the surrounding area and allow agriculture to continue similar to before. Cost is always the issue, but Dave expects the price of basic greenhouses wouldn’t be prohibitive: “…if we look at the cost of rice, we might add another dollar a day, so you might be up to $2 a day or something like that.”

Many of these ideas could turn out to be misguided or impractical in real-world conditions, which is why Dave and ALLFED are raising money to test them out on the ground. They think it’s essential to show these techniques can work so that should the worst happen, people turn their attention to producing more food rather than fighting one another over the small amount of food humanity has stockpiled.

In this conversation, Rob, Dave, and Sahil discuss the above, as well as:

  • How much one can trust the sort of economic modelling ALLFED does
  • Bacteria that turn natural gas or electricity into protein
  • How to feed astronauts in space with nuclear power
  • Jobs at ALLFED and what they’d do with more money
  • What, if anything, individuals can do to prepare themselves for global catastrophes
  • Whether we should worry about humanity running out of natural resources
  • How David helped save $10 billion worth of electricity through energy efficiency standards
  • And much more

Get this episode by subscribing to our podcast on the world’s most pressing problems and how to solve them: type 80,000 Hours into your podcasting app. Or read the transcript below.

Producer: Keiran Harris
Audio mastering: Ben Cordell
Transcriptions: Katy Moore


Turning fiber or wood or cellulose into sugar

Dave Denkenberger: So we looked at constructing factories to produce this sugar very quickly, but what looks to be more promising is taking an existing factory that has a lot of the components we need and repurposing that to produce sugar. And one of the most promising we found was a paper factory, because it already takes wood, and it takes a lot of energy to grind it up and do that pre-processing step. And then it’s not that much more work to break the cellulose into edible sugars.

Rob Wiblin: Okay. So basically, should there be a nuclear winter or terrible volcano or something like that, we would still have a whole lot of wood and other plant matter that is just out there in nature. And there’s a lot of energy embedded in that, but the problem is humans cannot eat wood, so we need to find some way to make it digestible. And you’re saying it’s possible to turn it into the kind of sugar that we would normally eat. Like it’s possible to break it down into glucose and then we can eat that?

Dave Denkenberger: That’s right, and there are actually a couple startup companies that are trying to turn fiber into edible sugar. Now that’s great, because they’re doing some of this research, but they’re just not thinking how we would do it fast in a catastrophe. And so that’s the type of research that we’re looking at.

Rob Wiblin: So what’s the process for taking a bunch of wood and then turning it into something that people can eat?

Dave Denkenberger: Basically a lot of grinding in the beginning, break it up into pieces. We call it “lignocellulosic material,” and that comes from the cellulose, which is basically lots of sugar molecules stuck together. There’s also hemicellulose, which is similar, and then there’s lignin — which you can’t really do anything with lignin, so you need to separate those components. And then you apply an enzyme to break the cellulose into sugar and the hemicellulose into other sugars.

Rob Wiblin: Is this with a bacteria or do you use chemicals to break it down?

Dave Denkenberger: It’s typically done where you’ll purify an enzyme that is produced by an organism like a fungus or bacteria, but then it’s done without that organism in a bioreactor.

Rob Wiblin: Okay. So I guess fungus eats logs, and there’s probably bacteria that eat logs as well. And I suppose they also probably can’t directly absorb cellulose or anything like that, so they themselves have to break it down into sugar somehow, right? And if I remember from my biology class, fungus extrudes some sort of acids or other compounds that break down wood into something that then the fungus can absorb and use to get energy. Are we to some degree mimicking that process to make something that humans can also eat?

Dave Denkenberger: Exactly. As we mentioned, mushrooms are one way of turning wood into food, but it turns out they’re pretty inefficient. And so if we can just grab the enzymes from them and then turn all that cellulose into sugar, or nearly all, we get a lot more food out of it than with the natural process.

Rob Wiblin: How expensive would this be? Is this a way that we could plausibly make food at an affordable price today?

Dave Denkenberger: Yeah, amazingly inexpensive, even though we’d be doing this repurposing with 24/7 labor — we would have to pay more for that. And even considering the fact that we wouldn’t run these factories as long; we’d probably only be running them for 10 years during the catastrophe. That increases the cost, but not that much. And so if you were to feed one person all of their calories — which obviously they wouldn’t eat all their calories, but it’s a way of visualizing it — it’s only about a dollar a day from cellulosic sugar. And that’s our target here, because we’re trying to feed everyone no matter what. We want to look at those resilient foods that are inexpensive.

Redirecting human-edible food away from animals

Dave Denkenberger: We feed a lot of food to animals. And yet many animals, as I pointed out, can eat stuff that’s not edible to humans. So in a catastrophe, we would want to redirect that human-edible food away from animals as fast as possible. And so we’re looking at scaling up having animals eat the residues from agriculture.

Rob Wiblin: So this is like the leaves on wheat plants or something like that?

Dave Denkenberger: That’s right.

Rob Wiblin: That people can’t eat. How much energy is in all of the agricultural residues?

Dave Denkenberger: It’s quite big and a lot of people are looking at it as a source of fuel, and that’s where the cellulosic biofuels are looking at.

Rob Wiblin: Yeah. I guess to some extent, we’re trying to plan for a world where there isn’t agriculture, or it’s going to be hard to grow the wheat to get the leaves and so on. But I guess there’s tons of that material just lying about already on farms that we could then potentially feed to cows or pigs or so on. And there’s also just trees out there, so you can get lots of leaf matter and try to feed them to animals that would then eat that.

Dave Denkenberger: That’s right. But interestingly, even in a pretty severe nuclear winter… The technical terminology here refers to how much soot or black carbon gets injected into the stratosphere: 150 teragrams or 150 million tons is a bad nuclear winter, with eight degree Celsius loss within a year. But still you have about 40% as much light coming through, so we might be able to actually grow something photosynthetically. Just moving on from the ruminants, even in places where you have a really short growing season, you might not be able to grow crops. But you might be able to grow grass, so there could be some grazing continuing.

Rob Wiblin: Why don’t we feed these byproducts that presumably aren’t earning much revenue right now to cows and pigs and chickens? It’s a bit odd that we’re not feeding them this thing that is kind of useless rather than feeding them food that humans could potentially eat.

Dave Denkenberger: It’s a good question. And I think part of it is that, at least in the US, there’s a subsidy for corn or maize. So that makes it more economical for the ranchers to, say… Typically you do grazing early life of the cattle and then fatten them up on maize later. And so if they didn’t have that subsidy, they probably wouldn’t be doing as much feeding them human-edible food. And I think long term, as we need to get more sustainable, we need to get more food out of the same land, I think we will move more towards utilizing these agricultural residues.

Rob Wiblin: Yeah. Interesting. Okay, so there’s possible subsidies. I suppose also just calories from corn is so cheap now that if there’s practical downsides — maybe health-related downsides, or just practical issues getting the agriculture residues — if there’s any kind of downsides, maybe the feed is so inexpensive anyway that you’ll just go with the thing that’s most straightforward and most familiar.

Dave Denkenberger: Right. The cattle would not mature as fast. And so time is money here.

Seaweed production

Dave Denkenberger: There are several species of seaweed that can still grow 10% per day, even with the lower light levels in nuclear winter and lower temperatures.

Rob Wiblin: Even up to adulthood? They just keep growing at 10% a day?

Dave Denkenberger: They just keep growing. So basically you have these long lines that are floated by these buoys, and you start with little pieces of seaweed and they grow and then you just chop off the growth and then they just keep growing.

Dave Denkenberger: It is amazing. And it’s only certain species, and right now they’re often limited by nutrients. So at least in the near term, we would still have those nutrients. I talked last time about the potential overturning of the ocean — that if you cool the upper layers of the ocean, they sink and bring up nutrients. So it turns out that is not as large as I thought it was going to be, so the actual production from fish is not going to go up as much as I thought. But with seaweed, it’s just much more efficient, growing it directly, than having algae grow and then feeding it to fish.

Rob Wiblin: What kind of inputs do we need in order to grow much more seaweed? I guess you’ve got to be around a coast, and then you were saying to grow seaweed, you attach it to ropes? It’s a bit like rope-grown mussels or something like that.

Dave Denkenberger: Yes. So it turns out we produce a lot of synthetic fiber for other reasons, like clothing. The main constraint here is twisting those fibers into ropes that we’re going to attach the seaweed to. We found that right now, we don’t produce that much rope — we would actually have to increase our rope-twisting capability by 300 times, which sounds kind of crazy. But it’s actually a really simple process, and people have done it in their garage with a drill, basically twisting these fibers. But these pieces of equipment are only like $10,000 and you can make a lot of rope, so it turns out it takes a very small percent of our manufacturing budget to make a lot of rope twisters.

Rob Wiblin: I mean, most seaweed is growing out of the ocean bed on rocks and things like that, right? It attaches to something on the bottom and then it grows upwards. But that’s no good for us? I guess all of that seaweed that can attach to rocks on the bottom of the coast, that’s already growing, so we need to have some artificial environment that’s very conducive to seaweed growing. And I suppose the closer it is to the surface, the more light it’s getting, so it might grow faster if we are attaching it to ropes near the top.

Dave Denkenberger: That’s right. So seaweed, why it can handle low light levels, is it often does grow 10 meters down in the ocean. But yeah, we want to have it near the surface in nuclear winter.

Rob Wiblin: Yeah. Okay. So the limiting factor is ropes that you’ll be hanging out of boats and things like that?

Dave Denkenberger: Yeah. So you’d be taking the rope and you’d be attaching small pieces of seaweed to it. Then you string the rope out in the ocean, and it’s held upward by buoys and then also anchored at the end.

Rob Wiblin: What fraction of food do you think we might be able to get from seaweed ultimately?

Dave Denkenberger: Not surprisingly, with that 10% growth per day, assuming we can scale up that rope twisting, we could actually get up to 160% of human calories in less than a year.

Barriers to seaweed farming in the West

Sahil Shah: We essentially see the problem as one of unit economics. So we are looking at using a high amount of mechanization — both when it comes to seeding and harvesting, developing new types of seeding techniques and purpose-built vessels — as well as trying different types of materials to grow the seaweed on, which can increase yields and improve composition. As well as different types of mooring systems, which would be different to ropes — again, which would have higher yields and higher amounts of mechanization.

Sahil Shah: The main reason why it’s not grown as much in the West as it is in China and Indonesia is to do with cost and price. Seaweed here is generally seen as a luxury good, if you want to buy sea vegetables anywhere. And effectively having something more industrial than artisan means that it would open up a wide variety of new uses that we would be able to then supply to and increase the market as a whole.

Sahil Shah: I think another aspect that I would add to that is biological. We don’t understand as much about seaweed as we do about traditional crops. The genes haven’t been sequenced in the same way. We’re not able to manipulate the genome because seaweed can kind of break free and it can move about in the ocean.

Sahil Shah: There’s a very different risk profile of introducing new genetically modified strains. So those are one of the elements where it has been done in China, and actually Chinese seaweed farmers are able to engineer and grow crops that are substantively larger and substantively cheaper. But outside of China, there are regulatory barriers — understandably so — around gene-editing modification, which you don’t have to the same extent on plant-based crops, and which really do contribute to production and cost of production.

Rob Wiblin: What sorts of regulations would you like to see changed in the UK?

Sahil Shah: I think there are quite a few. One is that the barrier of proof that there won’t be a negative impact is particularly high. So it can take years and tens of thousands, if not hundreds of thousands of pounds to actually get a seaweed farm in the water.

Sahil Shah: The second thing I would add is carbon credits, where some really interesting work has been done, and is being done predominantly by a nonprofit called Oceans 2050. But when seaweed grows, especially the large kelps, up to 50% of the biomass can actually fragment down to the ocean floor, where it effectively becomes long-term sequestered. At the moment, this is very difficult to monitor and track, and there are no carbon credits available. If that were to then change and policy were to change with it, it would suddenly make the unit economics much more attractive.

Global cooperation

Dave Denkenberger: Certainly a concern here is that our initial modeling is assuming global cooperation. The UK has a lot of potato seed, and they wouldn’t be able to grow them themselves — would they actually trust another country to grow the potatoes and then give them back more food than originally? So ideally we could actually talk about some of these agreements ahead of time so that people could be pre-committed to it.

Dave Denkenberger: But certainly we are concerned about cooperation breaking down. We’re going to be moving into more regional geographic information systems (or GIS) analysis, looking at the resources of individual countries from a resilient food perspective, and then actually working out what would happen if international trade turned off — it would be much worse. What we’re hoping to do is use that information to convince governments to actually cooperate.

Dave Denkenberger: In the past — say 2007, 2008 — the actual food production shortfall was less than 1%. But because of countries doing export bans, rice price went up three or even four times. So it’s a major risk, restriction of trade. There’s even potential of restricting more than just food trade, if we lose that trust and cooperation. And that would be just catastrophic, because then you lose energy trade and minerals and components — and the supply-chain issues we’ve seen in COVID are nothing compared to that.

Rob Wiblin: Do you have an intuition for if there was a supervolcano eruption, whether humanity would mostly pull together? I don’t know. I suppose we’re going to talk about COVID-19 in a minute, but my sense was cooperation during COVID-19 was pretty good, and it could be even better during a volcano situation.

Dave Denkenberger: Potentially. I’m very concerned that if people don’t know about resilient foods then they could conclude that most people are going to die. It could be an incentive for countries to do very bad things, like steal food from your neighboring countries. So I am very worried about that, and that’s why I want to get the message out that we could actually feed everyone if we cooperate.

Dave Denkenberger: And this country-by-country analysis: if we can know roughly what each country has in terms of resources, we could actually give them a draft plan. Now of course they’ll say it’s wrong because of this reason, and you don’t know this classified information — but we’re kind of jump starting them, and then hopefully they would actually have a viable plan to respond quickly.

Overlap with space industries' need to feed people in space

Dave Denkenberger: I think there’s potential overlap. It is at a quite different scale. I would say that one potential overlap with global catastrophic risk would be the really extreme scenarios where potentially everyone has been killed, that you can think of, “Can we make a refuge with 1,000 people that might be able to repopulate the Earth?” And Elon Musk is interested in doing this on Mars, so if we could figure out how to make an independent colony on Mars with fewer people, and with less expensive, less infrastructure-intensive food production, then I think that could have some existential risk benefits.

Rob Wiblin: Yeah. I guess another option is doing that somewhere really remote on Earth, like under the sea, or in Antarctica or so on — where you’d also have to figure out how to feed people in a worst-case scenario. It sounds like you could potentially use a nuclear reactor to grow bacteria and then eat them — or otherwise just have stores of fossil fuels, or put it somewhere where you have access to fossil fuels — and then you could in theory eat that.

Dave Denkenberger: Yeah, that’s right. So again, if you look at the plans for having an underground bunker — typically nuclear — the plans were to go through regular plants and that’s really inefficient, so we could lower the cost of this significantly.

Rob Wiblin: Yeah, so it was either you have this enormous initial cost of stockpiling all the food and making it big enough to store food to feed everyone for ages, or you’ve got this horrifically inefficient process of converting electricity into human-edible food that now we can do 10x better on.

Dave Denkenberger: And the other thing if you use the stored food is that you’re breathing out carbon dioxide and you need oxygen. Whereas if you have a system that actually grows food — either plants, which is of course inefficient, but these space-based resilient foods if you want to call them that — they can act as the life support system, because they would actually take the carbon dioxide from the astronauts to make the food and then they produce oxygen.

Dave Denkenberger: I guess another extreme scenario is a runaway climate change scenario, where it might get too hot for plants to live. So that’s another scenario where having these types of “space foods” could be a good food source.

Rob Wiblin: Interesting. God, it could be a very bizarre future, I suppose. Mostly just eating bacteria grown in electricity. Again, it sounds like absolutely bizarre sci-fi stuff. I kind of can’t believe that it actually works, but sounds like in principle it could.

Should listeners be doing anything to prepare for possible disasters?

Dave Denkenberger: Well, there’s a general recommendation of having two weeks of food. And you can justify that just based on an extended hurricane outage, so I think that’s pretty obvious. I personally don’t store more food than that because I’m more interested in keeping the whole system functioning.

Dave Denkenberger: But another thing I do think about is that I think there is a significant risk of full-scale nuclear war. And so I’m personally concerned that so many people in the effective altruism community are living in typically the city centers of NATO cities. And it’s interesting that their typical city real estate prices are such that it’s more expensive to live in the city center than in the outskirts, because in the outskirts you have a long commute.

Dave Denkenberger: But this gradient in real estate price is generally not taking into account the risk of nuclear war, because most people just ignore it. So you could potentially exploit this. And if you are concerned about the risk of nuclear war, living further away could be optimal, especially if you could have a commute where you could multitask.

Rob Wiblin: Yeah, right, right, right. Or I guess now, if you can work remotely.

Dave Denkenberger: Exactly.

Rob Wiblin: People spreading out because it’s harder for everyone to get killed that way. Yeah you’re right. And I guess a lot of listeners to this show, I can see in the analytics that a lot of people are in London, Oxford, SF, LA, New York, Sydney, Berlin. Yeah, all probable targets. Except maybe Sydney. I’m not sure about Sydney. Well, maybe once they have these nuclear submarines that they’re talking about, then they’ll target Sydney as well. Interesting idea. How far away do you have to live from a city center to not get toasted?

Dave Denkenberger: It really depends on how many nuclear weapons are used and whether it’s just the US as the target or all of NATO. But I think that just living on the outskirts of a city is quite a bit lower risk.

Rob Wiblin: Easy enough. Okay, nice.

Dave Denkenberger: And you could also live in a smaller city, but that might not always be feasible.

Articles, books, and other media discussed in the show

Dave’s work:

Sahil’s work:

  • Food Systems Handbook — an open source project by Sahil Shah, Aron Mill, Uri Katz, Edward Saperia, and others

Books and articles on resilient foods:

Organisations and companies working in this space:

Other 80,000 Hours Podcast episodes:

Everything else:

Related episodes

About the show

The 80,000 Hours Podcast features unusually in-depth conversations about the world's most pressing problems and how you can use your career to solve them. We invite guests pursuing a wide range of career paths — from academics and activists to entrepreneurs and policymakers — to analyse the case for and against working on different issues and which approaches are best for solving them.

The 80,000 Hours Podcast is produced and edited by Keiran Harris. Get in touch with feedback or guest suggestions by emailing [email protected].

What should I listen to first?

We've carefully selected 10 episodes we think it could make sense to listen to first, on a separate podcast feed:

Check out 'Effective Altruism: An Introduction'

Subscribe here, or anywhere you get podcasts:

If you're new, see the podcast homepage for ideas on where to start, or browse our full episode archive.