What is the likelihood that civilizational collapse would directly lead to human extinction (within decades)?

by Luisa_Rodriguez49 min read24th Dec 202028 comments

195

Civilizational Collapse & RecoveryNuclear WeaponsNatural risksForethought FoundationExistential riskBiosecurity
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Epistemic transparency: Confidence in conclusions varies throughout. I give rough indicators of my confidence at the section level by indicating the amount of time I spent researching/thinking about each particular subtopic, plus a qualitative description of the types of sources I rely on. In general, I consider it a first step toward understanding this threat from civilizational collapse — not a final or decisive one.

Acknowledgements

This research was funded by the Forethought Foundation. It was written by Luisa Rodriguez under the supervision of Arden Koehler and Lewis Dartnell. Thanks to Arden Koehler, Max Daniel, Michael Aird, Matthew van der Merwe, Rob Wiblin, Howie Lempel, and Kit Harris who provided valuable comments. Thanks also to William MacAskill for providing guidance and feedback on the larger project.

Summary

In this post, I explore the probability that if various kinds of catastrophe caused civilizational collapse, this collapse would fairly directly lead to human extinction. I don’t assess the probability of those catastrophes occurring in the first place, the probability they’d lead to indefinite technological stagnation, or the probability that they’d lead to non-extinction existential catastrophes (e.g., unrecoverable dystopias). I hope to address the latter two outcomes in separate posts (forthcoming).

My analysis is organized into case studies: I take three possible catastrophes, defined in terms of the direct damage they would cause, and assess the probability that they would lead to extinction within a generation. There is a lot more someone could do to systematically assess the probability that a catastrophe of some kind would lead to human extinction, and what I’ve written up is certainly not conclusive. But I hope my discussion here can serve as a starting point as well as lay out some of the main considerations and preliminary results.

Note: Throughout this document, I’ll use the following language to express my best guess at the likelihood of the outcomes discussed:

TABLE1

Case 1: I think it’s exceedingly unlikely that humanity would go extinct (within ~a generation) as a direct result of a catastrophe that causes the deaths of 50% of the world’s population, but causes no major infrastructure damage (e.g. damaged roads, destroyed bridges, collapsed buildings, damaged power lines, etc.) or extreme changes in the climate (e.g. cooling). The main reasons for this are:

  • Although civilization’s critical infrastructure systems (e.g. food, water, power) might collapse, I expect that several billions of people would survive without critical systems (e.g. industrial food, water, and energy systems) by relying on goods already in grocery stores, food stocks, and fresh water sources.
  • After a period of hoarding and violent conflict over those supplies and other resources, I expect those basic goods would keep a smaller number of remaining survivors alive for somewhere between a year and a decade (which I call the grace period, following Lewis Dartnell’s The Knowledge).
  • After those supplies ran out, I expect several tens of millions of people to survive indefinitely by hunting, gathering, and practicing subsistence agriculture (having learned during the grace period any necessary skills they didn’t possess already).

Case 2: I think it’s very unlikely that humanity would go extinct as a direct result of a catastrophe that caused the deaths of 90% of the world’s population (leaving 800 million survivors), major infrastructure damage, and severe climate change (e.g. nuclear winter/asteroid impact).

  • While I expect that millions would starve to death in the wake of something like a global nuclear winter, my best guess is that hundreds of thousands to hundreds of millions of the survivors of the initial catastrophe would be able to subsist using leftover food stocks and supplies, before eventually working out how to feed themselves through traditional agriculture (in regions where this was still possible) and fishing and/or modified agriculture (using methods that don’t rely on climate factors like warm temperatures and regular precipitation).
  • For catastrophes that cause temporary climate effects, survivors would only need to subsist using non-traditional agriculture for a decade at most. After that, survivors could revert to traditional agriculture, which would be much easier.
  • Survivors might also face threats from radioactive fallout from nuclear detonations and power plant meltdowns. While this would increase the death toll in the years following the collapse, I expect it wouldn’t be widespread enough to be immediately fatal to everyone, nor would it cause fertility rates or life expectancy to decrease enough to threaten extinction.

Case 3: I think it’s fairly unlikely that humanity would go extinct as a direct result of a catastrophe that caused the deaths of 99.99% of people (leaving 800 thousand survivors), extensive infrastructure damage, and temporary climate change (e.g. a more severe nuclear winter/asteroid impact, plus the use of biological weapons).

  • I expect hundreds of thousands of the survivors of the initial catastrophe would be able to subsist using leftover food stocks and supplies, before eventually working out how to feed themselves through traditional agriculture and fishing and/or modified agriculture (similar to the case of a nuclear war-only catastrophe).
  • A naive back of the envelope calculation (BOTEC) makes me think that surviving populations would either stay above the minimal viable population (the minimum number of individuals necessary to maintain the level of genetic diversity to repopulate Earth), or find each other (by crossing distances as large as England or Wales). My confidence in this conclusion is higher for scenarios where survivors have long-distance communication, long-distance transportation, or are confined to a smaller geographic region (e.g. the Southern Hemisphere).
  • My main uncertainties are around 1) whether I’m underestimating the frequency and severity of shocks the survivors might face in the post-collapse environment, and 2) whether surviving groups might deliberately avoid each other — perhaps because of fears about pathogen spread — allowing their populations to dip/stay below the minimum viable population. While it seems likely to me that natural shocks would reduce the number of people that would survive in the decades following the initial catastrophe — likely to be somewhere in the tens of thousands — I think it’s fairly unlikely that either of these would actually lead to extinction. But I’m unsure.

My best guess is that the turning point at which extinction goes from under 50% to over 50% is between 99.999% population death (80,000) and 99.9999% (8,000) population death (even before considering additional starting conditions like infrastructure damage or climate change). When the surviving population is in the tens of thousands and lower, it starts to seem more likely that the obstacles the survivors would face — difficulties growing food, random natural shocks — would be enough to wipe out the remaining groups. The probability of human extinction at these levels of population death would of course be even higher if the initial catastrophe caused additional environmental obstacles (like climate change, radiation).

General arguments and considerations that kept coming up and had a big impact on my conclusions:

  • Historical survival and resilience. In fact, we know that humans have survived for thousands of years "without technology," and in a very large variety of climates and environments.
  • The grace period. The period after the initial catastrophe — during which survivors would be able to access leftover supplies, goods, and some infrastructure — would be enormously helpful in ensuring humanity’s survival before the survivors have learned basic survival skills.
  • With population loss comes “decorrelation” of survivors. A population reduced to hundreds of thousands of survivors would likely be spread out in relatively small bands or groups of hundreds or thousands of people. Given this kind of distribution of survivors, many, many failures would need to happen — at roughly the same time, and all over the world — for humanity to go extinct. But the types of failures the survivors are likely to face would only be weakly or not at all “correlated” between groups of survivors or across time (e.g. multiple generations). In other words, even if some failure mode is very likely per group and time period (a drought may be the nail in the coffin for groups of survivors in one area), this still doesn't suffice for human extinction, as many other groups would be unaffected.

Historically, the fact that different groups of humans were relatively independent (spread out, affected by different social and environmental pressures) seems to have made humanity relatively robust. We can point to lots of societies that collapsed and didn’t recover, but humanity as a whole has been very resilient — in part because it’s been made up of lots of heterogeneous groups spread out all over the planet.

  • Non-uniformity of the initial catastrophe’s impacts. One of the ways that different survivor groups will become decorrelated stems from the fact that groups are likely to end up in regions that have been differentially affected by the initial catastrophe. For example, some regions might be completely destroyed, while others will have more moderate infrastructure damage.

In fact, it’s extremely difficult to come up with a scenario in which a catastrophe would have uniform effects all over the world — where the entire world has the same degree of infrastructure damage, the same amount of goods and supplies left around, the same severity of any changes to the climate, the same rates of pathogen spread and mortality. So while it’s easy to imagine that a catastrophe that caused civilizational collapse might produce unlivable conditions in some regions, other regions wouldn’t be as badly affected (another example: a nuclear winter might make the Northern Hemisphere unsuitable for agriculture, but not all of the Southern Hemisphere). That geographic variation means that even if we can imagine a scenario that looks unsurvivable for some regions, people who find themselves in (or make their way to) less affected areas should have a much better chance of survival.

  • The population loss would have to be incredibly extreme to lead to extinction. The minimum viable population (100–1,000 people, assuming a sufficient balance of the sexes) is really, really small compared to the current population: Just 0.00001% of the people alive today would need to survive for repopulation to be possible. In a sense, there's a really “high bar” for extinction. To get there from a catastrophe that caused 90%, 99%, or even 99.9% population death seems to actually require one to hold exceptionally pessimistic views about how survivors would fare in the event of civilizational collapse. Consider the following toy calculation:

Note: In the following calculation, I assume each group’s fate is independent of the fates of other groups. See footnotes and discussion in the body of the document for comments on the strength of this assumption.[1] The higher the true correlation between survivor groups, the more my toy calculations will cause me to underestimate the probability that all of the survivor groups would be wiped out.

TABLE2

With 8 million survivors, even extremely pessimistic views of the likelihood that any one group gets wiped out don’t necessarily lead to high probabilities for extinction in most scenarios. For example:

  • Even if you thought any one group of 100 or 1,000 survivors had a 99% chance of being wiped out (by failing to feed itself, to find water, to survive natural shocks), it would still be virtually guaranteed that at least one group of survivors would survive.
  • If you thought there was a 99% chance that each one of 800 groups of 10,000 people would be wiped out, there would still only be a 1 in 3,000 chance of extinction.
  • The probability of extinction would be higher (45%) if you believed that larger groups of 100,000 would also have a 99% chance of being wiped out. But to hold that view, you’d have to think that out of a group of 100,000 people, there’s a 99% chance basically none of them would overcome the obstacles of the post-collapse environment. I grant that there would be many obstacles: Survivors would have to work out how to grow their own food (potentially in the face of extreme global cooling), how to find and purify water, how to build shelter, potentially how to fish. They would have to survive disease outbreaks, hurricanes, tsunamis, conflict with other desperate survivors, possibly radiation contamination from nuclear war or the meltdown of nuclear reactors. But even so, my view is that the probability that a given population of 100,000 people would be completely wiped out — even facing all of those obstacles — wouldn’t be as high as 99%.
  • Similarly, the probability of extinction is very high (92%) if you think that any given group of 1 million survivors has a 99% chance of being wiped out. But again, to believe extinction risk was that high, you’d have to think that there would be a 99% chance that basically none of those 1 million people would survive and reproduce. I don’t find that plausible.

What is the likelihood that civilizational collapse would cause human extinction?

Introduction

Even if a catastrophe were not so destructive as to cause immediate extinction, it could be destructive enough to cause civilizational collapse. By this I mean something like the total failure of the world's systems of governance and critical resource systems (e.g., industrial food, water, and power systems). Some have argued that these conditions could drive humanity to extinction by making it impossible for the survivors to maintain numbers above the minimum viable population — especially if the catastrophe had other longer-lasting effects (e.g., severe warming or cooling). Below, I discuss some of the approaches I’ve taken to estimating the likelihood that civilizational collapse would cause human populations to fall below the minimum viable population.

Importantly, different catastrophes could cause civilizational collapse, and would leave survivors in dramatically different conditions. The dimensions along which these conditions vary that I see as most important are: levels of population loss, infrastructure damage (e.g. damaged roads, destroyed bridges, collapsed buildings, damaged power lines, etc.), the severity and persistence of any climate impacts, and the severity and persistence of worldwide conflict. In what follows, I’ll use three case studies to discuss how I think each of these factors would affect the likelihood that civilizational collapse would cause human extinction.

Before diving in, a few important notes:

First, when I say “critical systems” in this piece I mean industrial agriculture (but not non-industrial “traditional” agriculture), water transport and purification, power production and distribution, transportation, telecommunications, and effective governance, unless otherwise noted. And when I discuss the systems having “collapsed,” I mean they are producing little to none of their usual outputs. A civilizational collapse, as I will use the term, is an event in which approximately all of the entire world's critical systems are collapsed simultaneously for over six months.

Second, this piece focuses on whether civilizational collapse would lead more or less directly to human extinction without additional catalyzing conditions, like a second catastrophe (either soon or long after the first) or centuries of economic stagnation that eventually end in human extinction. I hope to explore both of these other scenarios in future posts.

Third, even if the population dropped below the minimum viable population, human extinction would not be inevitable. Rather, the likelihood of recessive genes that lead to lower fitness would go up, but these wouldn’t necessarily lead to extinction (though they would increase the risk of it). To illustrate, consider that the 10 million Ashkenazi Jews living today descended from a population of just 350 individuals who lived between 600 and 800 years ago (Commun, 2014). While Ashkenazi Jews suffer from higher rates of breast cancer, ovarian cancer, congenital deafness, cystic fibrosis, gaucher disease, Tay-Sachs, Parkison’s, and other diseases, their population grew rapidly over the last 500 years nonetheless. Given this, my bottom-line extinction probability estimates are therefore an upper bound on the likelihood of the probability of extinction.

Case 1: 50% population loss, no infrastructure damage, no climate change (e.g. a limited pandemic)

I’ll start by discussing what might happen in the wake of a relatively moderate catastrophe — one that causes the deaths of about half the population (4 billion survivors), but which doesn’t arise from global conflict, and which doesn't cause any major infrastructure damage or climate change.

What is the likelihood that civilization would collapse under these conditions?

Before getting into the likelihood that society would recover from civilizational collapse under these starting conditions, I’ll briefly discuss whether we should expect human civilization to actually collapse in my sense in this scenario.

Expert judgment: One expert told me that some resilience experts hypothesize that the economy would collapse (i.e. currency might lose its value, international trade might cease, etc.) in a case where 40% or more people had died or were not working[2] (though I haven’t been able to find a written source corroborating this). Even if that's true, it’s not obvious that civilization would completely collapse if the world economy did. But it certainly seems possible: The goods and services required for most industries to run are not produced locally, but globally — they are accessed through international trade that presumably wouldn’t exist if the global economy collapsed.

There are people who think civilizational collapse is likely even at (or below) 50% population loss. (This includes Lewis Dartnell, author of The Knowledge, and several of the academics in the PIIRS Global Systemic Risk Research Community at Princeton University.) They often appeal to the fragile and interconnected nature of the modern globalized economy: Many critical systems have features that make them more efficient in the short run but less robust to large shocks (examples in footnote).[3] People who are pessimistic about civilizational robustness often appeal to this kind of fragility.

I’m more optimistic. Despite our increasingly interconnected systems, I believe that even in a catastrophe of this scale, our systems would be resilient enough to stay functioning — at least in some places. At current population levels, this catastrophe would still leave four billion survivors. I find it hard to imagine that four billion people working together wouldn’t be sufficiently adaptive, creative, and knowledgeable enough to keep critical systems running in some places, especially if certain regions are less affected than others. Without infrastructure damage, climate change, or major ongoing conflict, I strongly suspect that at least some regions would keep their critical systems online.

Indeed, while a few disagreed, most experts I spoke with shared the view that, while it seemed plausible that critical systems might go down temporarily at the regional level, it seemed exceedingly unlikely that all critical systems would fail all over the world in Case 1.

I explore the possibility of civilizational collapse from this sort of catastrophe in any case, because 1) I could be wrong, and 2) I think it’s a useful baseline with which to compare more severe catastrophes.

In a scenario in which a catastrophe of this scale does cause civilizational collapse, whether humanity goes extinct as a direct(ish) result of that collapse would depend on whether humanity could survive without critical systems, and for how long, and how quickly humans would be able to bring critical systems back online (if at all). I discuss this next.

What is the likelihood that humanity would survive without key systems (food, water, power) in the years immediately after a catastrophe?

Time spent on this section: 5-10 hours

Types of sources: Popular non-fiction books, op-eds in well-regarded newspapers, and expert interviews

If the survivors of the catastrophes aren't able to get critical systems back online very quickly after the initial catastrophe, would they be able to survive? I took a mix of approaches to answering this question:

Historical track record: The genus homo survived for thousands of years without complex support systems, practicing hunting and gathering for hundreds of thousands of years, and Homo sapiens practiced settled agriculture for over ten thousand years. They did this in the face of changing climates and predators, with only rudimentary tools. This is at least some evidence that the survivors of a catastrophe could do this again — plausibly for thousands of years (remembering that the scenario we're considering assumes the catastrophe didn’t cause any meaningful changes to the climate or natural environment).

On the other hand, most humans living today wouldn’t know how to survive without the support of our modern food and water systems. This might appear to suggest that the historical track record doesn’t tell us as much about the likelihood of survival among humans who face the collapse of civilization today.

Case study: To illustrate how humans might be unable to survive without critical systems, we can take a look at a few case studies. There are many examples of humans failing to survive in new environments, and/or after losing access to support systems on which they were reliant. One set of cases I looked into were those of stranded or lost European explorers who died in large numbers when forced to try to survive in the wilderness.

In the Secret to Our Success, Joseph Henrich describes several cases where groups of explorers — although physically fit, highly trained, and equipped with extensive supplies — died of disease and starvation because they couldn't exercise basic survival skills in the environments in which they found themselves. At the same time, local populations were thriving in the environments in which the explorers perished, because of their large body of highly specific, non-obvious, detailed cultural knowledge. For example, the Franklin Expedition, a group of over 100 explorers who became stranded in the Arctic, slowly died because they could not practice the hunting techniques practiced by the local Inuit for millenia. Even lost expeditions in much more forgiving and food-rich environments — the Burke and Wills expedition in Australia and Narvaez Expedition in Florida, for example — saw entire expeditions of explorers perish due to ignorance of their environment.[4]

Disanalogies from those case studies: All that said, many humans likely would know how to survive without modern food and water systems. For example, I expect that most subsistence farmers — of which there are probably between 1 and 2 billion[5] — would at the very least have the knowledge and skills to survive by growing their own food. While flow-through effects of the catastrophe might mean that some proportion of these subsistence farmers wouldn't be able to access key inputs for subsistence agriculture (or would be displaced from where they know how to survive), I expect at least 10% of them, at a minimum, would — leaving very likely more than 50 million people who would likely have the knowledge, skills, and resources to survive even without global infrastructure and centralized systems.

In addition to these people, there are apparently as many as 3.7 million Americans who identify as “preppers”. Given that most people who identify as preppers aren’t actually prepared to survive for years without critical systems (they mainy stockpile canned food and even build bunkers without necessarily learning how to acquire and prepare food and water in the natural environment), it seems plausible that as few as 1% of preppers (37,000 individuals) would survive. But even then, that 1% alone would keep us well above the minimum viable population of on the order of 100-1,000 survivors (assuming an even enough sex mix).

Even for people who don’t already know key survival skills — how to hunt, gather, and/or grow food — there would be opportunities to learn them. For example, there would likely be information sharing between those who did and those who didn’t, plus a great deal of written information on the topic in libraries and in homes. And this information would cover all of the world’s natural environments — so post-collapse humans wouldn’t be without key knowledge in new environments the way the European explorers were. To demonstrate, I searched the SAS Survival Handbook, Amazon’s best-selling wilderness survival guide, for the key pieces of information Henrich notes would have been critical for the explorers’ survival in the stories above. I found almost all of the relevant bits of information and techniques easily (and for bits that were missing, usually because they were incredibly specific, I found close substitutes).

Certainly, these survival skills would be hard to learn. Some survival skills, like starting a fire with only things found in the natural environment, are notoriously difficult to master. But many survivors wouldn’t have to learn the hardest skills: lighting fires with only sticks or flint, building shelters with only natural materials, making receptacles to hold water from natural resources alone, processing dead animals (making clothes from tanned hides, making fuel from their fat, carving bones into knives, needles, or buttons). They’d have access to knives, lighters, matches, tarps and plastic bags (for shelter) wool socks, plastic water bottles (to purify and carry water), salt (to preserve foods), alcohol (to treat wounds), thread/rope, and cooking oil (to burn). Some survivors would find iodine tablets, fishing rods, and compasses. Indeed, most survival skills look much less difficult with modern technologies.

The grace period: What’s more, at least some of these survivors would have time to learn survival skills during a period in the immediate aftermath of the catastrophe when systems have failed, but there is still a considerable stock of goods available for immediate consumption: food, water, batteries, and vitamin supplements in grocery stores; clothes in malls; cars, planes, ships, and computers that still work; gasoline at the pumps that could be siphoned out even if the power grid was down; medications in pharmacies; generators; books in libraries and in houses, etc. During this period, I expect that at least some survivors (maybe 5–25%) without experience practicing subsistence farming, hunting, and gathering would be able to meet their basic needs while learning those skills.

Without this grace period, I think it’s still fairly likely that hundreds of thousands or millions of people (largely subsistence farmers) would be able to survive the collapse, but I’m less confident. So I decided it was worth doing a BOTEC to understand the likelihood that there would be such a grace period, and how that period would last.

Note: The following BOTEC relies on particularly poor sources, makes a bunch of dubious assumptions (discussed more below), and I’m not confident I’ve thought of all of the most important supplies. It should be considered very rough.

BOTEC: To estimate whether and how long this kind of grace period might last given different population sizes, I investigated the amount of various goods/supplies that would be available in the post-collapse environment. In addition to goods that would facilitate human survival, I also considered goods that might be essential (or at least very helpful) in making use of the goods that are essential to human survival (e.g., humans don’t need petrol to survive, but that might facilitate movement, which might be essential to accessing a distributed food supply).

Note some important assumptions made here, that would meaningfully affect how realistic these estimates are:

  • I’m assuming food stocks wouldn’t perish before they were eaten (or that food stocks would be deliberately consumed in order of perishability, to ensure stocks last as long as possible). If this assumption doesn’t hold, the grace period would be shorter, perhaps by a lot.
  • I’m ignoring animal agriculture and cannibalism, in part because without a functioning agriculture system, it’s not clear to me whether enough people would be able to consume living beings. If this assumption doesn’t hold, the grace period could be a fair bit longer.
  • I didn’t look into whether the food stocks include stocks intended for animal agriculture. If they don’t, it’s possible I’m missing out on a huge amount of grain stock that could be consumed by humans — lengthening the grace period considerably.[6]

TABLE3

See table notes here.[7][8][9][10][11][12][13]

I find that the goods and supplies left after an initial catastrophe would only support a population in the billions for several months to a year — not very long at all. Given this, I expect a large fraction of the survivors who survive this initial catastrophe to die during a period of violent competition following the collapse.

I expect this period of violence to last until the survivors reach some sort of equilibrium, where they perceive there to be sufficient food, water, and shelter. The reason for this view is that it seems very irrational for groups to keep fighting if there are enough resources to go around. Of course, humans aren’t always rational, so I imagine some fighting might persist. But I think it’s very likely that many survivors would correctly conclude it would be more dangerous to fight each other than to accept a relatively low risk of not being able to meet their basic needs because of resource scarcity, if resources weren’t in fact too scarce to support the population.

Carrying capacity for hunter-gatherers and subsistence farmers: Importantly, without the technologies that enable humans to produce enormous amounts of food using a fraction of the land and water as our ancestors, the Earth would only be able to sustain a fraction of the human population it does now. Just how large a population it could support depends on the means of subsistence employed by the survivors. The carrying capacity of the Earth for hunter-gatherers is thought to be around 10 million if the survivors regress to pre-paleolithic levels of technology (if they lose, for example, flakes, handaxes, controlled use of fire, and wooden spears) (Taiz, 2013). The carrying capacity is hypothesized to be in the low billions for settled subsistence farmers — possibly as high as three billion for mostly vegetarian populations with particularly advanced agricultural systems (Ellis, 2013).

Bottom line: Taken together, if a Case 1 scenario leads to collapse (but not to additional catastrophes), I expect that millions to hundreds of millionspossibly even a few billion — of the 4 billion survivors would be able to learn enough survival skills to survive indefinitely even after the grace period. My biggest uncertainty is whether I’m underestimating the difficulty of surviving in a post-collapse world with only basic knowledge of subsistence farming and/or survival skills.

What is the likelihood that humanity would survive in the event of conflict immediately following the catastrophe?

Time spent on this section: 1–2 hours

Types of sources: Academic literature, expert interviews, and speculation

Literature review: Sociologists trying to understand human behavior in the aftermath of shocks like natural and manmade disasters largely agree that violence is not the most common reaction to emergencies (Drabek and McEntire, 2003). Rather than looting, exploitation, and other antisocial behavior, academics have found that groups tend to cooperate in the aftermath of disasters.

But a catastrophe of this magnitude would be unprecedented, and meaningfully different from the disasters that researchers have been able to observe. Unlike smaller scale disasters, where even those most impacted by the disaster could reasonably expect that help was on the way, post-collapse survivors would have no reason to expect this. As I concluded above, the leftover goods and supplies wouldn’t sustain all of the initial 4 billion survivors for very long. Without the hope of support — and facing the threat of starvation and other dangers — I strongly suspect that the limited availability of leftover supplies would almost certainly cause violent conflict at both the group and individual level. Specifically, I expect that subsets of the survivors — likely groups with access to weapons — would use violence (or the threat thereof) to hoard the surviving supplies, killing their competitors or letting them starve to death.

But as discussed, I expect any violence would mostly subside once some equilibrium had been reached — at the point where ~all of the surviving population could reasonably expect to be supported by the available resources (in other words, at the point at which the population dropped to or below whatever the Earth’s carrying capacity was). It’s at that point that regular conflict and violence would actually make the environment more dangerous than coexisting non-violently and sharing the available resources. And while resources would be somewhat more depleted by an initially large group of survivors all consuming them — and by the use of some resources directly in inter-group violence — therefore shortening the grace period, I don’t expect this would change my bottom line (but I’m uncertain about this).

My best guess is that this violent competition would whittle the number of survivors down to the hundreds of millions or fewer — a population for whom the supplies might last up to a decade or more. This is based on my best guess about the population size that I expect could be robustly supported by the supplies left during the grace period and the approximate hunter-gatherer carrying capacity of the Earth.

Bottom line: I don’t expect violence to drive the surviving population so low that humanity would go extinct. Rather, I think it’s extremely likely that violence would mostly disappear once the population had gotten small enough that all of the remaining survivors could expect to survive indefinitely on the available resources — probably hundreds of millions or fewer. My key uncertainty is whether initially large numbers of survivors, many fighting each other, would shorten the grace period enough that survivors wouldn’t have enough time to learn survival skills they’d need to survive after the grace period.

Concrete example: A moderate pandemic

So what, concretely, do I think would happen in the event of a catastrophe like a “moderate” pandemic — one that killed 50% of people, but didn’t cause infrastructure damage or climate change?

My best guess is that civilization wouldn’t actually collapse everywhere. But if it did, I expect that the ~4 billion survivors would shrink to a group of 10–100 million survivors during a period of violent competition for surviving goods in grocery stores/distribution centers, food stocks, and fresh water sources. I expect these supplies would last somewhere between a year and a decade or more — on the shorter side if the period of violence were prolonged, causing a large proportion of the supplies to be used up by larger populations before the population stabilizes at a lower number. After those supplies ran out, I expect that up to 10 million of those survivors would survive if practicing hunting and gathering, or as many as hundreds of millions if practicing advanced subsistence agriculture (after learning any skills they didn’t possess already during the grace period).

It would be reasonable to be skeptical of any or even several of the individual claims I’ve made here. But a toy calculation gives at least a rough indication of how wrong they would have to be for my bottom line to change.

Note: I assume the likelihood that any one group gets wiped out is independent from the probability that another group does. I think this is somewhat reasonable, given that there are many things that affect individual groups or subsets of groups without affecting all groups. For example, a contaminated water source used by a few groups might cause disease rates in that group to go up significantly, perhaps even causing the groups to be wiped out, but groups far away wouldn’t be affected. The higher the true correlation between survivor groups, the more my toy calculations will cause me to underestimate the probability that all of the survivor groups would be wiped out.

TABLE4

With 4 billion survivors, the degree of pessimism you have to have about their ability to survive to end up believing that no groups would survive indefinitely is actually very extreme. The exact beliefs you’d have to have would depend on whether survivors were concentrated into a few big groups, or distributed across many smaller ones. Specifically:

  • Even if you thought any given group of between 10 survivors and 1 million survivors had a 99% chance of being wiped out, it would still be virtually guaranteed that at least one group would survive.
  • If you thought every single one of 400 groups of 10 million people had a 99% chance of being wiped out, you would still only think there was a 1.7% chance of extinction. And, to hold that belief, you’d have to think that despite having extremely large populations (larger than any US city), none of those people would be able to work out how to practice very basic farming or fishing, or that none of them would survive conflict within the group, or that a natural disaster would kill all of them. Personally, I find this view implausible.
  • Similarly, the probability of extinction is pretty high if you think that any of 400 groups of 100 million survivors has a 99% chance of being wiped out. Again, though, to believe extinction risk was that high, you’d have to think that there’s a 99% chance that none of the 100 million people would overcome obstacles posed by the absence of critical systems and violence. (For reference, only 14 countries have a population of 100 million or higher.)

Given this, I believe it’s exceedingly unlikely that this scenario would lead to human extinction.

Case 2: 90% population loss, infrastructure damage, and extreme climate change (e.g. nuclear war that caused nuclear winter)

In a scenario in which a catastrophe causes the deaths of 90% of the population (800 million survivors), major infrastructure damage, and climate change — for example, a severe, global nuclear war that caused a nuclear winter — I believe the question of whether humans would be able to meet their basic needs becomes more difficult.[14] The questions I consider for this scenario are:

  • What is the likelihood that survivors are able to continue to survive using traditional forms of agriculture, given a catastrophe that causes severe infrastructure damage and climate change?
  • What is the likelihood that radiation causes extinction?
  • What is the likelihood that humanity would survive in the event of conflict immediately following the catastrophe?

What is the likelihood that survivors are able to continue to survive using traditional forms of agriculture?

Time spent on this section: 2–3 hours

Types of sources: Academic literature, non-academic reports, and expert interviews

Expert judgment: Several experts, including ALLFED director David Denkenberger, have affirmed this conclusion — they do not expect humanity to dip below the minimum viable population even in relatively extreme sun-blocking scenarios.

Literature review: The nature of all of the catastrophes we know of that would cause extreme global cooling (e.g. nuclear winter, asteroid impacts) would have unevenly distributed impacts — causing extreme global cooling in some parts of the world, but more moderate cooling in others. For example, in the case of a nuclear war between the US and Russia, nuclear winter models suggest that the most severe climate effects would be limited to the Northern Hemisphere, where temperatures would fall by 10–30 degrees C. But in the Southern Hemisphere, and especially at the equator, those effects would be much less severe: between 5–10 degrees Celsius. With heterogeneous impacts like this, it’s likely that agriculture would still be possible in some regions — especially in New Zealand and Australia, and possibly in South America and Central Africa.[15]

To be clear, I’m describing a very grim scenario, in which basically everyone in the Northern Hemisphere — and in many parts of the Southern Hemisphere — would be unable to grow food using standard agricultural techniques. Given this, I expect there would be mass starvation and violent competition and conflict until a new equilibrium was reached, one where the remaining survivors didn’t exceed the Earth’s carrying capacity. While I expect this would be a truly terrible period of widespread suffering, I believe this equilibrium would be reached long before the population got anywhere near the minimum viable population. My best guess is the population would fall to hundreds of thousands to tens of millions, but not much lower.

While I haven’t looked into this much, I feel fairly convinced that hundreds of thousands or millions of people could survive using traditional approaches to agriculture in parts of the world with more moderate climate effects (and basic mitigation strategies, like switching to crop types that are more resilient to temperature and precipitation fluctuations). And as with Case 1, at least some of the survivors in a Case 2 scenario would probably be able to survive the immediate aftermath of a catastrophe that caused civilizational collapse by exploiting food and other supplies in stores and larger stockpiles. This would give survivors some buffer time to learn additional skills required to survive once those supplies run out (e.g. fishing) or develop the techniques necessary to produce food using methods that don’t rely on climate factors like warm temperatures and regular precipitation.

BOTEC: The longer the buffer time, the more likely humanity would be to subsequently survive. But there are a number of different considerations (relative to Case 1) that affect the calculus of just how long such a grace period would be in the context of a catastrophic event like a nuclear war that killed 90% of people and caused a nuclear winter. So I’ve done a similar exercise to the one above where I try to account for some of those differences.

Note: As above, the following BOTEC relies on particularly poor sources, makes a bunch of dubious assumptions (discussed more below), and I’m not confident I’ve thought of all of the most important supplies. It should be considered very rough.

TABLE5

See table note here.[16]

Bottom line: I think it’s extremely likely that these supplies would last somewhere between around a year and a decade or more. I expect it would be closer to the lower end, given that competition and violence could lead to the depletion of supplies more quickly than if the population were reduced to a smaller number by the catastrophe directly.

All this in mind, I think it is very likely that the survivors would be able to learn enough during the grace period to be able to feed and shelter themselves ~indefinitely.

What is the likelihood that radiation causes extinction?

Time spent on this section: 2–3 hours

Types of sources: Academic papers, Wikipedia, and interviews with experts

Literature review: In the aftermath of a nuclear war, radioactive fallout from the nuclear detonations would have long-lasting health impacts. In the most extreme nuclear war scenarios considered by academics (a nuclear war between the US and Russia and their allies, using 10,000 megatons (MT) of nuclear bombs), approximately 30% of the geographic area in the Northern Hemisphere would have enough fallout to be lethal to any adult in the area (Ehrlich et al., 1983). The current US and Russian nuclear arsenals don’t currently have that kind of megatonnage (they currently have closer to 2,500 MT). If we naively assume that radiation scales linearly, we might expect a modern day US-Russia nuclear war to contaminate up to 7.5% of the land area of the Northern Hemisphere. This may not sound like much, but consider that 95% of the world’s population lives on just 10% of its land area — meaning that 7.5% of land area could be home to millions or even billions of people. What’s more, tens to hundreds of millions more might be exposed to enough radiation to be more susceptible to cancer for the rest of their lives.

On top of this, there are currently around 440 civilian nuclear power reactors scattered around the world, and likely tens or hundreds more military reactors. These have fail-safes and automatic shut down measures that are designed to ensure that all of the nuclear material in these reactors would be safely contained in the event of a global catastrophe that meant people stopped attending to them. Concretely, these fail safes make sure that water continues to be circulated around the nuclear fuel to ensure it doesn’t get so hot it causes a meltdown — i.e., an event where the nuclear core partially or completely melts, which might allow the nuclear fuel to breach its multiple layers of containment and leak out into the environment. If fuel did reach the environment, the radioactive fallout could spread across continents, creating exposure levels ranging from immediately fatal (in areas ranging from tens to thousands of square kilometers) to non-lethal but causing potential higher rates of cancer and infertility.

But some of these fail-safes could plausibly fail during a catastrophe that caused infrastructure damage (or afterward, if any components of the fail system degraded). For example, some nuclear reactors rely on backup generators to power the pumps that keep water circulating in the core of the reactor. If those backup generators eventually all broke down, the reactor might melt down.

I currently don’t have a good sense of how likely these failures would be. Newer nuclear reactors rely on more robust safety systems, with parts that wouldn’t break down as easily. And all nuclear reactor safety systems are designed to account for infrastructure damage caused by earthquakes and other physical shocks.

But in a large-scale nuclear war, it seems very plausible that at least some nuclear reactors would melt down. My best guess is that this wouldn’t happen at a large scale, but even if it did, some areas would likely be far enough away from reactors to be spared the radioactive contamination. For example, Australia has just one nuclear reactor. Even if that reactor were to melt down, much of Australia would likely remain uncontaminated (Australia is just under 3 million square miles, and the Chernobyl meltdown is estimated to have contaminated under 60,000 square miles; and only a much smaller fraction of that area was sufficiently contaminated as to be lethal to humans).

Bottom line: While radioactive fallout from nuclear detonations and power plant meltdowns would increase the death toll in the years following the collapse, I expect it wouldn’t be widespread enough to be immediately fatal to everyone, nor would it cause fertility rates or life expectancy to decrease enough to threaten extinction. And at the very least, some areas are sufficiently far away as to be relatively safe from radioactive fallout.

What is the likelihood that humanity would survive in the event of conflict immediately following the catastrophe?

Time spent on this section: 1–2 hours

Types of sources: Academic literature, expert interviews, and speculation

Historical base rate: In Case 2, it seems slightly more plausible to me that violence would lead to human extinction than in Case 1, but still fairly unlikely. I don’t think human extinction could be caused by a conflict fought with conventional weapons; there would just be too many survivors (~800 million) to be killed in conventional warfare (compare this to WWI and WWII, during which ~20 million and ~75 million people were killed, respectively).

Weapons of mass destruction: My best guess is that the only way violence in the wake of a Case 2 civilizational collapse could directly lead to human extinction is if one group of survivors had access to and deployed weapons of mass destruction. This seems unlikely to me, first because it seems hard to imagine a group of survivors incapable of recovering critical infrastructure — and barely capable of meeting even their basic needs — would be able to successfully deploy weapons of mass destruction (though I’m not very confident about this).

Second, it’s hard to imagine a scenario where the use of weapons of mass destruction kills millions of survivors, spread all over the world, without modern technologies like transportation. For example, with potentially many survivor groups, it seems hard to imagine how nuclear detonations would kill ~everyone despite the fact that the groups would likely be spread out all over the world, potentially in small bands that can’t each be individually targeted. Similarly, it’s hard to imagine how a pathogen could spread ~everywhere when survivors would likely have greatly reduced mobility (the latter isn’t obviously impossible, but it at least seems exceedingly difficult to me).

There’s one counterargument I find somewhat persuasive, which is that it seems possible that all of the survivors might be confined to a relatively small area (for example, if only a small fraction of the Earth’s land area is habitable), making them more vulnerable to a single, large attack. If this were the case, it’s easier for me to imagine that the use of weapons of mass destruction could kill all of the remaining survivors. This would presumably mean the aggressors would be killing themselves, which makes it seem even less likely to me. But we’ve seen humans come dangerously close to threatening their own survival before, often because human aggressors aren’t always good at predicting how cascading effects could threaten their survival as well. A random example to make this concrete: If all of the survivors of a nuclear war were confined to Australia, which might be less impacted by a nuclear winter, one group might choose to use nuclear weapons against another group, not realizing that the radioactive fallout or further climate change could make Australia uninhabitable, even for them.

Bottom line: I expect the survivors in Case 2 would not deploy weapons of mass destruction against their competitors, as it would likely pose a pretty big risk to the aggressor as well as the target. But I’m uncertain about this — humans have come close to making similarly self-destructive choices before. Thankfully, even if one group did use weapons of mass destruction against their competitors, I still think it’s very unlikely that their use would cause human extinction. This is because except in a few very specific and very strange scenarios, I expect the survivors would be too geographically distributed and disconnected to be wiped out by a single act of aggression. I therefore expect the result would be a much higher death toll, but not extinction.

Concrete example: A large nuclear war that causes a nuclear winter

So what, concretely, do I think would happen in the event of a catastrophe like a nuclear war that led to the death of 90% of the population, and caused severe infrastructure damage and significant global cooling?

I expect that, in addition to the billions of people killed in the initial catastrophe, hundreds of millions or more would likely die in the famines and violent competition that followed. But my best guess is that hundreds of thousands to hundreds of millions of the survivors of the initial catastrophe would survive this violent period. I think it’s extremely likely these survivors would be able to support themselves using leftover food stocks and supplies, before eventually working out how to feed themselves through traditional agriculture and fishing and/or modified agriculture (using methods that don’t rely on climate factors like warm temperatures and regular precipitation).

All of the catastrophes we know of that would lead to extreme cooling would only do so for 1–10 years, and agriculture would become possible again once the climate began to return to normal. At that point, it seems even more likely that the surviving humans would be able to meet their own basic needs by returning to traditional forms of agriculture.

My key uncertainties are around whether I’m putting too much weight on the idea that humans would figure out how to subsist without traditional agriculture just because it’s technically possible, and whether conflict could lead to extinction through channels I haven’t foreseen.

Another toy calculation suggests that these uncertainties probably aren’t troubling enough to change my bottom line.

Note: I again assume each group’s fate is independent of the fates of other groups. I actually think this is a pretty reasonable assumption in this case. I expect that the survivors of a catastrophe like a severe nuclear war would end up somewhat spread out (at least across the Southern Hemisphere), as doing so would create less competition for resources within a smaller area (I discuss this more later). The farther apart the surviving groups are, the less likely they are to be affected by the same shocks (natural disasters, disease outbreaks, conflict).

Additionally, in the event of a catastrophe like a nuclear war, transportation, communication, and other technologies that facilitate contact between geographically distributed groups would be enormously limited. This would further limit the extent to which each group’s fate ended up relating to another’s. There would be other sources of variation between groups that made their fates less correlated: Some groups might be made up mostly of farmers, while others will be made up of lawyers, some groups will tend toward cooperation, while others toward conflict, plus pure randomness (e.g. some groups might have a high proportion of survivors with genetic immunity to a particular disease).

But there are also factors that point in the other direction — factors that suggest the surviving groups would be at least somewhat correlated. For example, nuclear winter climate conditions, while nonuniform, would nonetheless impact all surviving groups. Similarly, more severe natural disasters might affect large regions, meaning that at least all of the survivor groups at the regional level might end up experiencing very similar challenges to survival simultaneously. Likewise, there might be things about "human nature" that would be shared amongst all survivors. For example, it’s possible that all of the survivors, having witnessed the initial catastrophe, would have similar psychological experiences — like shock, stress, and social distrust, among others — that would make it more difficult to survive and cooperate.

As above, the higher the true correlation between survivor groups, the more my toy calculations will cause me to underestimate the probability that all of the survivor groups would be wiped out.

TABLE6

With 800 million survivors, the degree of pessimism you have to have about their ability to survive to end up believing that no groups would survive indefinitely is actually kind of extreme. The exact beliefs you’d have to have would depend on whether survivors were concentrated into a few big groups, or distributed in many smaller ones. Specifically:

  • Even if you thought any given group of 100, 1,000, or 10,000 survivors had a 99% chance of being wiped out, it would still be virtually guaranteed that at least one group would survive.
  • If you thought there was a 99% chance that any one of 800 groups of 100,000 people would be wiped out, there would still only be a 1 in 3,000 chance of extinction.
  • The probability of extinction is higher (45%) if you believe that larger groups of 10 million would also have a 99% chance of being wiped out. But, again, to hold that view, you’d have to think that out of a group of 10 million people (again, bigger than the largest US city), not even a few hundred of those people would overcome the obstacles of the post-collapse environment (how to fish, how to farm despite global cooling, avoiding being killed by a hurricane or drought). I do not find this view very plausible.
  • Similarly, the probability of extinction is very high indeed if you think that any given group of 100 million survivors has a 99% chance of being wiped out. Again, to believe extinction risk was that high, you’d have to think that there would be a 99% chance that none of the 100 million people would work out how to survive (for reference, only 14 countries have a population of 100 million or higher).

Given all of this, my subjective judgment is that it’s very unlikely that this scenario would more or less directly lead to human extinction.

Case 3: 99.99% population loss, infrastructure damage, and climate change (e.g. a global war where biological weapons and nuclear weapons were used)

In a scenario in which a catastrophe caused the deaths of 99.99% of the population (800,000 survivors), in addition to major infrastructure damage and climate change (e.g. war using both nuclear and biological weapons), the survivors would face many of the same challenges faced in Case 2, such as food insecurity and the potential for ongoing conflict. Below, I’ll explore the additional obstacles faced by survivors in this scenario caused by the drastically smaller population, as well as the possibility of a persistent pathogen from a biological attack. Specifically, I’ve tried to understand:

  • Given that 99.99% population loss is obviously extremely significant, what is the likelihood that the survivors are so spread out that each individual group is below the minimum viable population size?
  • What is the likelihood that humanity would survive without key systems (food, water, power) in the years immediately after a catastrophe (during and after the grace period)?
  • What is the likelihood that a human population of 800,000 would survive natural shocks like natural disasters, climate fluctuations (drought), and disease?

My answers to each of these questions increases my credence in a Case 3 catastrophe leading to human extinction, but together they only increase it 10–20% over my credence in a Case 2 catastrophe leading to extinction.

What is the likelihood that the survivors are so spread out that each individual group is below the minimum viable population size?

Time spent on this section: 1 hour

Types of sources: Academic literature, popular nonfiction books, and expert interviews

With only 800,000 survivors, it seems much more plausible than in Case 1 and Case 2 scenarios that many of the surviving groups would be below the minimum viable population of 100–1,000 people. For example, it seems at least plausible that there could be 80,000 groups of 10 survivors — especially if there were pressures to stay in small groups or spread out geographically. Some people have argued that, having fallen below the minimum viable population, these groups would run the risk of dying out unless they were able and willing to find each other, forming larger (genetically viable) groups. So a critical question might be: how likely is it that groups of survivors would be able to find each other within a generation? This question breaks down further into:

  • Would they find each other by chance?
  • Could they coordinate to find each other if not?
  • Could they physically reach each other?
  • Would they want to find each other?

When thinking about whether 800,000 survivors in lots of small groups would stumble across each other by chance, I realized I didn't have an intuitive sense about how far away groups of survivors would be from each other. To get a very rough sense, I did a simplistic BOTEC, estimating how much land area a group of survivors of a particular size would have to cross in order to find another group of a similar size, in expectation.

BOTEC: In the following toy calculation, I explored two scenarios: one where the survivors were distributed evenly across the world, and another where they were distributed evenly across the Southern Hemisphere (which might be the case in the event of a nuclear war in the Northern Hemisphere).

The BOTEC relies on the following simplifying assumptions:

  • The survivors are mobile and actively trying to find each other, rather than avoiding it. If I’m wrong about this, my estimates will overstate the likelihood of inter-group contact.
  • Survivors are evenly distributed across different regions. If I’m wrong about this, my estimates will probably overstate the likelihood of inter-group contact, but will probably understate the size of each group.
  • I don’t account for obstacles like mountain ranges, nor do I account for the fact that some groups would be separated by oceans. Rather, the following best reflects something like: the ease of reaching groups on the same continent with no major geographic barriers. This assumption means my estimates will overstate the likelihood of inter-group contact.

TABLE7

See table note here.[17]

There were a few things I took away while making this table. First, any groups with populations of 1,000 or above should have enough genetic diversity to repopulate. And for groups of survivors made up of 10 to 100 people, the approximate land area each would have to cross to happen upon another group of survivors is pretty small. If the survivors were in small bands of around 10 people, they’d only have to travel across an area the size of Oxfordshire (in expectation) to happen upon another band. If the survivors were in groups of 100, they’d have to travel across an area the size of the Bay Area.

If the survivors had no way of coordinating, it could plausibly take years for them to happen across each other. But there are reasons to think it would happen even more quickly than that. First, there are likely to be points of disproportionately high traffic — attractor points — like water sources, cities (where survivors will go to scavenge supplies), roads/railroad tracks, etc. Additionally, in the first 5–10 years after the collapse, I think it’s fairly likely that at least some survivors would find ways to retain some motorized transportation, drastically increasing the area that could be covered over time. Unless the initial catastrophe was something that wiped out electronics, many cars would still be drivable, and petrol and diesel could be accessed with relative ease (until they went off after sitting there for five and ten years, respectively). Given this, I think it’s very likely that more than a few groups of survivors would find each other within a generation even if they had no means of communication.

But even that I think is too pessimistic, as I think it’s extremely likely that survivors would be able to employ at least some forms of long-distance communication to contact and connect with other survivors. Even with no advanced technology, survivors could create beacons by lighting fires at high ground. And with some luck, survivors might work out how to communicate using radios — either rebooting existing ones or making their own.[18]

Given this, I actually think it’s extremely likely that survivors would form one or more groups that were well above the minimum viable population of 100–1,000 people (assuming a sufficient balance of the sexes) at this level of population loss — even if survivors were spread out across the whole world and limited to basic forms of transportation (foot, non-motorized wheeled vehicles).

Importantly, though, if these groups were trying to avoid each other — perhaps because of fears of conflict or disease — it becomes a bit more plausible to think that small groups of survivors might stay below the minimum viable population, potentially leading to human extinction. But while I think it’s actually pretty likely that there would be at least some inter-group avoidance and conflict for at least a period of months or years after the initial catastrophe, I think it’s extremely unlikely that this aversion would exist and persist among thousands of different groups of survivors for a full generation. This is because I’d expect the benefits of larger populations (specialization, division of labor, etc.) to eventually outweigh the risks (within a period of years or decades at most) — for example as a human-borne pathogen died out after being unable to spread, or after populations were small enough that resource competition — and by extension, violence — had decreased substantially.

Bottom line: Taking into account both the logistics of coordinating and connecting with other groups of survivors, as well as the potential for decades-long intergroup avoidance, I think it’s extremely unlikely that there wouldn’t be any groups that ended up above the minimum viable population in the event of a catastrophe that left 800,000 survivors.

What is the likelihood that humanity would survive without key systems (food, water, power) in the years after a catastrophe?

Time spent on this section: <1 hour on top of the time spent above

Types of sources: Popular non-fiction books, random unvetted websites, government reports, well-regarded news websites, and expert interviews

As above, I did a BOTEC to understand how long the 800,000 survivors would be able to rely on surviving goods and supplies to meet their basic needs. I found that even the scarcest supplies would last at least 50 years (in theory), with more abundant supplies like grain stocks lasting hundreds of years (again assuming that 55% of stocks are lost during the initial catastrophe and ignoring perishability). I don’t present the full results here, because the limiting factor on how long these resources would keep survivors alive isn’t the number of survivors, but rather the perishability of the supplies. Given this, I think it’s at least safe to conclude that the surviving supplies and goods would likely keep survivors alive for well over a decade, maybe more.

During that period, as above, I expect survivors would learn to subsist without critical systems by hunting, gathering, fishing, and practicing modified and traditional agriculture and animal husbandry.

Bottom line: I expect these supplies would last somewhere on the order of several years or decades, giving survivors plenty of time to learn the survival skills they’d need to meet their basic needs after they eventually ran out/went off. Given this, I think it’s extremely likely that humanity would be able to feed itself basically indefinitely after a catastrophe killing 99.99% of people.

What is the likelihood that humanity would survive natural shocks like natural disasters, climate fluctuations (drought), and disease in the decades/centuries after the collapse?

Time spent on this section: ~1 hour

Types of sources: Our World in Data and expert interviews

While the surviving population in Case 3 is very small (for the decades or centuries until it grows again), one might worry that it could be driven all the way down to extinction as a result of natural shocks. I think it’s very likely that many groups of survivors would be seriously threatened — and some wiped out — by shocks like drought, hurricanes, disease outbreaks, or pests. But in the aftermath even of a Case 3 civilizational collapse, I expect survivors will be somewhat distributed around the world (or hemisphere) for several reasons.

First, it seems likely that the survivors would start off in different regions of the world just by chance. Second, it seems likely that travel would be limited, meaning that at least some groups would be completely disconnected from others. And third, because natural resources and surviving supplies would likely be distributed all over the world, a strategy of spreading out — in order to draw on all of the resources available and to avoid competition — would likely maximize the possible number of survivors. Given these pressures toward geographic distribution, any single shock might affect some or even many groups of survivors in a single region, but it’s hard to imagine any shock that would come close to affecting an entire hemisphere or even continent all at once. We can therefore expect that regional shocks like droughts and disease outbreaks wouldn’t be enough to wipe out groups spread out all over.

Historical base rates: Historically, natural disasters like wildfires, earthquakes, and droughts have been responsible for a pretty small share of global deaths. There aren’t great estimates for the death tolls caused by natural disasters before the mid-nineteenth century, but the most deadly year in the last century was 1931, when over 3.5 million people died as a result of natural disasters — just under 0.2% of the world population at the time. My impression is that, before the 20th century, regional shocks sometimes decimated local populations (and occasionally played a significant role in the collapse of a single civilization), but that the geographic distribution of human societies across the world meant those shocks never threatened the entire species.

There are a few reasons to think that shocks might be more frequent after a collapse. For example, anthropogenic global warming will continue to get worse (even without added emissions), leading to more climate volatility, including more frequent and severe hurricanes. And if shocks became more frequent, it becomes more plausible to think that all of the survivors might be affected by various natural shocks all at once. To my knowledge, they would not be so frequent as to mean we should expect human survivors would fare much worse than our ancestors did. They’d have to be either much more severe or much more frequent — probably both — but this is an area of uncertainty for me.

One scenario that seems like it could more plausibly lead to human extinction is one where, as described above, the survivors of the initial catastrophe are confined to a small area (like a region of Australia). Were this to be the case, it seems more plausible to me that a single shock could threaten human existence.

Bottom line: If groups of survivors were geographically distributed, as I imagine they would be, it’s hard to imagine how even a very severe natural disaster would lead to the deaths of all of the remaining survivors. If ~all of the survivors were in a small area — if there were basically no groups of hundreds or thousands of survivors anywhere else in the world — it seems plausible that all of the survivors could be wiped out directly or indirectly by a natural shock. But as I said above, I think this degree of geographic concentration seems extremely unlikely. So I think it’s extremely likely that humanity would survive natural shocks indefinitely.

Concrete example: A large nuclear war and the use of biological weapons, resulting in the death of 99.99% of the population, and causing infrastructure damage and climate change

So what do I think would happen in the event of a catastrophe like a global war involving both nuclear and biological weapons that led to the death of 99.99% of the population, caused severe infrastructure damage, climate change?

I expect hundreds of thousands of the survivors of the initial catastrophe would be able to subsist using leftover food stocks and supplies, before eventually working out how to feed themselves through traditional agriculture and fishing and/or modified agriculture (using methods that don’t rely on climate factors like warm temperatures and regular precipitation) (similar to the case of a nuclear war-only catastrophe).

Like before, for catastrophes that cause temporary climate effects, survivors would only need to subsist using non-traditional agriculture for a decade at most. After that, survivors could revert to traditional agriculture, which would be much easier.

At this level of population loss, a naive BOTEC makes me think that surviving populations would either stay above the minimum viable population, or find each other (by crossing distances as large as England or Wales). My confidence in this conclusion is further strengthened by the fact that there would be scenarios where survivors would retain or recreate long-distance communication, long-distance transportation, or are confined to a smaller geographic region (e.g. the Southern Hemisphere), making contact even easier.

While it seems likely to me that natural shocks would reduce the number of people that would survive in the decades following the initial catastrophe — likely to somewhere in the tens of thousands — I think it’s fairly unlikely that either of these would actually lead to extinction. This is because neither pathogens nor natural disasters have ever come close to causing human extinction, and have only relatively rarely even caused the collapse of a single human civilization. This seems largely due to the geographic spread of humans across the planet, which makes it hard for any given disaster to affect all groups of humans. I expect the post-collapse conditions to lead to a similar or greater level of geographic distribution.

Given this, I think it’s fairly likely, though far from guaranteed, that a catastrophe that caused 99.99% population loss, infrastructure damage, and climate change (e.g. a megacatastrohe, like a global war where biological weapons and nuclear weapons were used) would more or less directly cause human extinction.

My main uncertainties in Case 3 are around 1) whether I’m underestimating the frequency and severity of shocks the survivors might face in the post-collapse environment, and 2) whether surviving groups might deliberately avoid each other — perhaps because of fears about pathogen spread — allowing their populations to dip/stay below the minimum viable population.

Another toy calculation gives an indication of how wrong I would have to be for this scenario to lead to human extinction.

Note: As above, I assume each group’s fate is independent of the fates of other groups. In Case 3, I think this is a pretty robust assumption. I would expect survivors to be in relatively small, scattered, isolated groups for the reasons discussed above.

As discussed, the higher the true correlation between survivor groups, the more my toy calculations will cause me to underestimate the probability that all of the survivor groups would be wiped out.

TABLE8

With 800,000 survivors, the most plausible scenarios still seem unlikely to lead to extinction, even under very pessimistic assumptions about the vulnerability of each group of survivors.

  • If you thought any given group of 10 or 100 survivors had a 99% chance of being wiped out, it would still be virtually guaranteed that at least one group would survive.
  • If you thought there was a 99% chance that any one of 800 groups of 1,000 people would be wiped out, there would still only be a 1 in 3,000 chance of extinction.
  • If the survivors were in groups of 10 thousand people (80 groups total), and each of those groups had a 99% chance of being wiped out, there would be a 45% chance that humanity would go extinct. I think a 99% failure rate among groups of 10 thousand survivors is too pessimistic. I can imagine scenarios in which a single event or shock like a drought, nuclear meltdown, or natural disaster might have a chance at causing an entire population of 10,000 people to die. But I don’t think these are likely or frequent enough to put the overall probability of the loss of everyone in a group of 10,000 dying at 99%. Personally, I would put the probability that any of these groups is wiped out at 90%, at the highest. At this failure rate, the chance of extinction is about 1 in 5,000. But if you think 99% is reasonable, then it would be reasonable to think there is a 45% chance that this catastrophe would cause human extinction.
  • If the survivors were in groups of 100,000, and you put the probability that any one of those groups got wiped out at 90–99%, you could conclude that human extinction was pretty likely. Again, I think a 99% chance that a population of 100,000 is wiped out is too high, but reasonable people might disagree with me. I would put the probability that any group of 100,000 survivors might be wiped out in a post-collapse environment at 80% at the highest, which yields a probability of human extinction just below 17%.

Given all of this, my subjective judgment is that it’s fairly unlikely that this scenario would more or less directly lead to human extinction.

Other research directions

A few notes on other approaches to answering some of the questions I discussed that I considered but didn’t pursue, plus a rough guess at how long each approach would take and whether it would require specialized knowledge or skills:

  • I’d love to see someone host something like a wargame, where people (ideally actual subject-matter experts) adopted particular roles — a power plant operator, the leaders of influential countries, journalists, military generals, business executives, etc. — and participated in simulating a catastrophe. If modeled after a traditional war game, participants would have a scenario described to them, then would take turns making decisions (“moves”), each with the aim of achieving their respective interests/acting on their values. I think such a game might teach us a lot about how individuals and institutions would behave in the aftermath of such a catastrophe. (50+ hours / Special skills required)
  • Looking at lessons from science fiction books and films. I’m not sure how much there is to gain here, but I think it’s possible that works of sci-fi offer useful insights into what types of conditions survivors would face after a catastrophe of this type. A list of some of the best civilizational collapse science fiction is here. (5–10 hours / No special skills required)
  • I think there's literature on society-level factors that inform how people respond to disasters. For example, I think I’ve heard somewhere that places with higher levels of social trust have lower levels of looting, hoarding, and other antisocial disaster behavior. This might be worth looking into more. (2–5 hours / No special skills required)
  • In my discussion of Case 2, I consider one type of catastrophe that could lead to 90% population death, infrastructure damage, and climate change, where the climate change I’m referring to is nuclear winter (decreased temperature and rainfall) and the infrastructure damage I’m referring to isn’t worldwide. But there are other cases one could choose with different considerations worth exploring. For example, in a scenario with an increase in the frequency of natural disasters (like extreme anthropogenic climate change), each group of survivors would be at a higher risk of extinction from things like hurricanes and droughts. This could have important implications on the bottom line for this case that make it meaningfully different from this one. It could be useful for someone to break down the starting conditions beyond what I’ve done (e.g. rather than thinking about climate change as one parameter, one could think about temperature, precipitation, and natural disasters separately; or, instead of thinking of infrastructure damage as one parameter, one could think about electronics, transportation, etc., separately). (10–30 hours / No special skills required)
  • I’m not sure how informative this would be, but I wonder if it could be interesting to see if anyone could try to take a survival guide and some basic camping supplies and try out various survival skills like building a fire and gathering an edible meal (without advanced preparation, the internet, or help from others). (3–30 hours / Maybe some adventurousness required)
  • How much more frequent/severe would natural shocks have to be to threaten surviving populations of different sizes? (2–10hours / Some math required)

  1. I think this is a pretty reasonable assumption in the event of 99.9% population death. I expect that the survivors of a catastrophe this extreme would end up somewhat spread out (because this will mean less competition for the resources within a smaller area). The farther apart surviving groups are, the less likely they are to be affected by the same shocks (natural disasters, outbreaks, conflict). Additionally, in the event of civilizational collapse, transportation, communication, and other technologies that facilitate contact between geographically distributed groups would be enormously limited. This would further limit the extent to which each group’s fate ended up relating to another’s. There would be other sources of variation between groups: Some groups might be made up mostly of farmers, while others will be made up of lawyers; some groups will tend toward cooperation, while others toward conflict, plus pure randomness (e.g. some groups might have a high proportion of survivors with genetic immunity to a particular disease).

    On the other hand, there are also factors that point in the other direction — factors that suggest the surviving groups would be at least somewhat correlated. For example, nuclear winter climate conditions, while nonuniform, would impact all surviving groups. Similarly, larger natural disasters might affect large regions, meaning that at least all of the survivor groups at the regional level might end up experiencing very similar challenges to survival simultaneously. ↩︎

  2. This last bit suggests that we could see economic collapse even if we weren’t seeing massive population losses, but rather just an unwillingness or inability of enough people to work, as might happen in a pandemic with a disease enough people were unwilling to risk contracting (or which made people sick enough not to work, even if it didn't kill them). ↩︎

  3. Resource-pooling aims to improve the efficiency and cost effectiveness of systems that need to meet unpredictable demand. For example, demand for firefighters is both unpredictable and high-variance — there are zero or few fires most of the time, but sometimes there are several. To be prepared to respond to several fires that might all happen at once, a precinct would have to employ a lot of firefighters. But this means a lot of firefighters who are idle most of the time. To reduce inefficiency, most precincts pool resources, employing fewer firefighters while building in agreements between precincts that allow one to call on reinforcements from neighboring precincts.

    Resource pooling is widespread: For example, banks pool capital reserves, and power plants have agreements allowing them to shift power demands to another power plant if demands are unusually high at one plant. While this makes these systems much more efficient — and also more effective in the short term — it makes them more fragile when viewed over long time frames, during which there are more opportunities for rare but extreme events to cause particularly high demand. Arguably, this kind of resource pooling is at least partly responsible for the severe ventilator shortages during the COVID-19 pandemic.

    Because the components in a system with pooled resources are “coupled” (the performance of one is linked to the performance of others), resource pooling means that if 50% of the population were killed, and then even just a few components of a system with pooled resources becomes compromised, the whole system could fail. In other words, the reason systems with pooled resources are more fragile is because it means the nodes in a system become coupled — the performance of one is dependent on the performance of another. This means that if one fails, it’s much more likely that many others fail. To make this more concrete: imagine that a set of government hospital pools medical supplies, sharing between them when demand is particularly high at one hospital. Most of the time, this saves hospitals money, by allowing them to work well at lower safety margins (lower levels of surplus supplies). But when demand is high at all of those hospitals, they’ll all fail to meet demand for that supply. When the failure of one node causes others to fail, this is called a cascading failure.

    Hyper-specialization is when systems require several subsystems to all work in order for them to have any output at all, because each ingredient of the output is produced in a separate subsystem. For example, a microchip manufacturer — rather than having several factories with the capacity to build microchips from start to finish — might build each individual subcomponent in different factories, such that failure of one such specialized factory will destroy the company’s entire manufacturing capacity. If a system is hyper-specialized in this way, then losing even a small percentage of its component subsystems (which seems like it could plausibly happen if 50% of the population were killed) would mean it would fail altogether.

    In experimental literature that explores the effects of these approaches and their propensity for cascading failure, it’s often found that tightly coupled systems perform poorly over periods long enough to observe rare but extreme shocks (e.g. Elga, n.d.). For example, one study assigned participants to the role of the CEO of a power company (and tasked with the goal of maximizing profits) that supplied the power to ten cities. Some of the participants were allowed to use resource pooling — shifting surplus power supply in one city to meet unexpectedly high demand in another city. The participants were then allowed to choose what safety margins to adopt (the amount of power produced to meet an imperfectly predictable demand). Computer simulations were then run to see how often the participants’ imaginary power supply companies would be able to meet the power demands of the city. Participants whose systems were coupled were 35 times more likely to experience a total system failure, failing to meet the power needs in all ten cities (Elga, n.d.).

    If the shock is as extreme as 50% of the inputs disappearing, it seems plausible that the system could fail altogether.

    There seems to be a large and fascinating literature on the interconnectedness of systems, and how particular connections between systems make them particularly vulnerable (some examples: Buldyrev et al., 2009, Barigozzi et al., 2010). Unfortunately, a lot of this literature was inaccessible to me (it was highly quantitative, used a lot of network theory I don’t understand, etc.). But I think it could be worth looking into more. ↩︎

  4. Henrich describes in detail the highly specific knowledge that allowed local peoples to thrive in these environments. In the Arctic, this included the ability to identify the breathing holes that seals make in the ice, and to identify older ice which was low enough in saline to drink when melted, to name just a few. In Australia, it was a complex, non-obvious, multi-step process for processing one of the only available plants in the desert — the nardoo plant, which will slowly deplete the human body of vitamin B1 if not processed correctly. Henrich emphasizes that it was that collective knowledge, discovered and then passed down over generations, that allowed local populations to survive in these environments. Given that most people have not been taught this kind of knowledge, it seems plausible that we’d not have enough knowledge of the environment and agriculture to survive post-collapse. ↩︎

  5. 26.7% of the world population derives its income through agriculture (p.46); this FAO report claims that “About two-thirds of the developing world’s 3 billion rural people live in about 475 million small farm households, working on land plots smaller than 2 hectares.” (p. 1). ↩︎

  6. I expect that survivors would be able to work out how to consume grain stocks, despite the fact that they may be unprocessed, meaning that they may not be immediately ready for consumption. I believe this would be an overcomable obstacle, but if I’m wrong, it could limit the grace period to the shorter end of the range above (a few months). ↩︎

  7. Details of how I thought about food rations: I assume rations imposed wouldn’t reduce caloric intake below 2,100 per day — which is the number of calories required to be offered to refugees in refugee food programs, like that offered by the World Food Programme. If all countries that currently consume over 2,100 calories imposed food rationing that limited intake to 2,100 calories, there would be a ~20% caloric surplus on average. Calculations here. ↩︎

  8. Assumes that 30 days worth of water bottles, consumed at the rate of 1 million per minute under normal circumstances, would be stored and accessible. Also assumes that people drink ~3 water bottles per day. ↩︎

  9. Assumes that survivors could survive on just one bottle of water per day. ↩︎

  10. See dataset titled “World Grain Consumption and Stocks, 1960-2012.” ↩︎

  11. Here, food stocks is defined in the source paper as a group of “92 products” used “to reconstruct 50 years of aggregated food reserves, expressed in caloric equivalent (kcal), at the regional and global scales.” (Laio et al. 2016) ↩︎

  12. Note that many people don’t actually rely on any medications, so these are only critical to some. Additionally, perishability depends on the specific medication, but medications generally last 1–10 years before expiring. ↩︎

  13. In some places, power outages would occur within 4–24 hours. In regions where power is supplied by particularly robust power plants (e.g. the Hoover Dam), it’s theoretically possible that the power supply could go on without human support for several years. In either case, generators could be scavenged from roadwork/construction sites once centralized power supplies stopped working. ↩︎

  14. Note: Extreme climate change can mean lots of things, including increases or decreases in temperature, precipitation, and frequency of natural disasters like hurricanes, among others. The considerations I discuss under Case 2 won’t apply to all of these types of climate changes. See the section on other research directions for possible research projects that could fill this gap. ↩︎

  15. And while it’d be important for survivors to make sure they could meet their nutritional requirements, this might not be as hard as it seems. For example, consider that one could survive indefinitely on a diet of white potatoes, sweet potatoes, and butter (Nunn, 2009). ↩︎

  16. Given that there would be major infrastructure damage, it seems likely that a substantial portion of food stocks and other supplies would be destroyed during the initial catastrophe. It’s hard to say how much. For just a very rough sense, I assume 55% of these resources would be destroyed, based on the fact that about 55% of people live in countries with nuclear weapons or in nuclear weapons alliances (so, perhaps we can assume that the supplies destroyed would be proportionate to the population of countries likely to be affected by a global nuclear war). ↩︎

  17. In the Southern Hemisphere, I include all of the land area in Africa, Australia, and South America (so this is a crude estimate). ↩︎

  18. Making a makeshift radio might be a surprisingly accessible technology, Lewis Dartnell explains in The Knowledge:

    “During the Second World War, soldiers holed up at POW camps built their own makeshift radio receivers for music or news of the war effort. These ingenious constructions reveal the sheer variety of scavenged materials that can be jury-rigged to create a working radio. Aerial wires were slung over trees, or disguised as clotheslines, and sometimes even barbed wire fences were appropriated for the task. A good grounding was achieved by connecting to cold-water pipes in the POW barracks. Inductors were constructed by winding coils around cardboard toilet rolls, the scavenged bare wire insulated by candle wax, or in Japanese POW camps by applying a paste of palm oil and flour. Capacitors for the tuning circuit were improvised out of layers of tinfoil or cigarette-pack lining, alternating with newspaper sheets for insulation; the wide, flat device was then curled like a jelly roll to make a more compact component. The earphone is a trickier component to improvise and so was often salvaged from wrecked vehicles… Perhaps the most ingenious improvisation of all, however, was in creating the all-important rectifier, needed to demodulate the audio signal from the carrier wave. Mineral crystals like iron pyrite or galena were unobtainable on the battlefield, but rusty razor blades and corroded copper pennies were discovered to serve just as well. The blade or coin was fixed to a scrap piece of wood alongside a safety pin bent upright. A sharpened graphite pencil was firmly attached to the point of the safety pin… allowing fine readjustment of the pencil graphite across the metal oxide surface until a working rectifying junction was found.” ↩︎

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Thanks for this detailed post on an underdiscussed topic!  I agree with the broad conclusion that extinction via partial population collapse and infrastructure loss, rather than by the mechanism of catastrophe being potent  enough to leave no or almost no survivors (or indirectly enabling some  later extinction level event) has very low probability.  Some comments:

  • Regarding case 1, with a pandemic leaving 50% of the population dead but no major infrastructure damage, I think you can make much stronger claims about there not being 'civilization collapse' meaning near-total failure of industrial food, water, and power systems. Indeed, collapse so defined from that stimulus seems nonsensical to me for rich quantitative reasons.
    • There is no WMD war here, otherwise there would be major infrastructure damage.
    • If half of people are dead, that cuts the need for food and water by half (doubling per capita stockpiles), while already planted calorie-rich crops can easily be harvested with a half-size workforce.
    • Today agriculture makes up closer to 5% than 10% of the world economy, and most of that effort is expended on luxuries such as animal agriculture, expensive fruits, avoidable food waste, and other things that aren't efficient ways to produce nutrition.  Adding all energy (again, most of which is not needed for basic survival as opposed to luxuries) brings the total to ~15%, and perhaps 5% on necessities (2.5% for half production for half population).  That leaves a vast surplus workforce.
    • The catastrophe doubles resources of easily accessible fossil fuels and high quality agricultural land per surviving person, so just continuing to run the best 50% of farmland and the best 50% of oil wells means an increase in food and fossil fuels per person.
    • Likewise, there is a surplus of agricultural equipment, power plants, water treatment plants, and operating the better half of them with the surviving half of the population could improve per capita availability.  These plants are parallel and independent enough that running half of them would not collapse productivity, which we can confirm by looking back to when there were half as many, etc.
    • Average hours worked per capita is already at historical lows, leaving plenty of room for trained survivors to work longer shifts while people switch over from other fields and retrain
    • Historical plagues such as the Black Death or smallpox in the Americas did not cause a breakdown of food production per capita for the survivors.
    • Historical wartime production changes show enormous and adequate flexibility in production.
  • Re the likelihood of survival without industrial agriculture systems, the benchmark should be something closer to preindustrial European agriculture, not hunter-gatherers. You discuss this but it would be helpful to put more specific credences on those alternatives.
    • The productivity of organic agriculture is still enormously high relative to hunting and gathering.
    • Basic knowledge about crop rotation, access to improved and global crop varieties such as potatoes, ploughs, etc permitted very high population density before industrial agriculture, with very localized supply chains.  One can see this in colonial agricultural communities which could be largely self-sustaining (mines for metal tools being one of the worst supply constraints, but fine in a world where so much metal has already been mined and is just sitting around for reuse).
    • By the same token, talking about 'at least 10%' of 1-2 billion subsistence farmers continuing agriculture is a very low figure.  I assume it is a fairly extreme lower bound, but it would be helpful to put credences on lower bounds and to help distinguish them from more likely possibilities.
  • Re food stockpiles:
    • "I’m ignoring animal agriculture and cannibalism, in part because without a functioning agriculture system, it’s not clear to me whether enough people would be able to consume living beings."
      • Existing herds of farmed animals would likely be killed and eaten/preserved.
        • If transport networks are crippled, then this could be for local consumption, but that would increase food inequality and likelihood of survival in dire situations
      • There are about 1 billion cattle alone, with several hundred kg of edible mass each, plus about a billion sheep,  ~700 million pigs, and 450 million goats.
      • In combination these could account for hundreds of billions of human-days of nutritional requirements (I think these make up a large share of 'global food stocks' in your table of supplies)
    • Already planted crops ready to harvest constitute a huge stockpile for the scenarios without infrastructure damage.
    • Particularly for severe population declines, fishing is limited by fish supplies, and existing fishing boats capture and kill vast quantities of fishes in days when short  fishing seasons open.  If the oceans are not damaged, this provides immense food resources to any survivors with modern fishing knowledge and some surviving fishing equipment.
  • "But if it did, I expect that the ~4 billion survivors would shrink to a group of 10–100 million survivors during a period of violent competition for surviving goods in grocery stores/distribution centers, food stocks, and fresh water sources."
  • "So what, concretely, do I think would happen in the event of a catastrophe like a “moderate” pandemic — one that killed 50% of people, but didn’t cause infrastructure damage or climate change? My best guess is that civilization wouldn’t actually collapse everywhere. But if it did, I expect that the ~4 billion survivors would shrink to a group of 10–100 million survivors during a period of violent competition for surviving goods in grocery stores/distribution centers, food stocks, and fresh water sources."
    • For the reasons discussed above I strongly disagree with the claim after "I expect."
  • "All this in mind, I think it is very likely that the survivors would be able to learn enough during the grace period to be able to feed and shelter themselves ~indefinitely."
    • I would say the probability should be higher here.
  • Regarding radioactive fallout, an additional factor not discussed is the decline of fallout danger over time: lethal areas are quite different over the first week vs the first year, etc.
  • Re Scenario 2: "Given all of this, my subjective judgment is that it’s very unlikely that this scenario would more or less directly lead to human extinction" I would again say this is even less likely.
  • In general I think extinction probability from WMD war is going to be concentrated in the plausible future case of greatly increased/deadlier arsenals: millions of nuclear weapons rather than thousands, enormous and varied bioweapons arsenals, and billions of anti-population hunter-killer robotic drones slaughtering survivors including those in bunkers, all released in the same conflict.
  • "Given this, I think it’s fairly likely, though far from guaranteed, that a catastrophe that caused 99.99% population loss, infrastructure damage, and climate change (e.g. a megacatastrohe, like a global war where biological weapons and nuclear weapons were used) would more or less directly cause human extinction."
    • This seems like a sign error, differing from your earlier and later conclusions?
    • "I think it’s fairly unlikely that humanity would go extinct as a direct result of a catastrophe that caused the deaths of 99.99% of people (leaving 800 thousand survivors), extensive infrastructure damage, and temporary climate change (e.g. a more severe nuclear winter/asteroid impact, plus the use of biological weapons)."



 

Regarding case 1, with a pandemic leaving 50% of the population dead but no major infrastructure damage, I think you can make much stronger claims about there not being 'civilization collapse' meaning near-total failure of industrial food, water, and power systems. Indeed, collapse so defined from that stimulus seems nonsensical to me for rich quantitative reasons.

 

If there were a pandemic heading toward 50% population fatality, I think that it is likely that workers would not show up to critical industries and there would be a collapse of industrial civilization. I looked into whether the military could replace those workers, and it did not look feasible. Whether there would be further collapse of large-scale cooperation is less certain. If that cooperation is maintained, I agree it would be possible to have agricultural productivity similar to preindustrial Europe. However, it would mean a very rapid scale up of hand/animal farming equipment, and hand powered wells, carts that could be drawn by animals, etc (which ALLFED is planning on investigating). Some people say that modern crop varieties would actually do worse than traditional crop varieties if there were no artificial fertilizers and pesticides. If that were true or if scaling of tools were difficult, then we could have much worse agricultural productivity than preindustrial Europe.

Loss of rapid communication would likely imply fragmentation of large countries, if it is true that empires can only be maintained with I think a ~14 day communication radius (I thought the reference was here, but maybe it was something cited there). Furthermore, it is possible that cooperation outside of 100% groups is lost, particularly because of fear of the disease. In this case, I think it is likely with current preparation to only be able to do hunting and gathering. In addition, the hunting and gathering population density could be much less than historic, because the overshoot in population density could mean that plants and animals that are good to eat could be driven to extinction by desperate humans. 

Though it is possible that current food storage could be protected well, it is not clear to me that there would be a strong defense advantage. The desperate attackers would have weapons as well. If we go significantly above the carrying capacity and food is distributed fairly equally, then everyone would starve.

I would like to see these sorts of bio-catastrophes discussed in more detail. On my naive understanding, the threat of engineered pandemics seems likely to  usher in an age of disruption and surveillance and to completely undermine current liberal democratic norms

You don't think regulating the sale and use of the technologies necessary to engineer diseases would be enough?

My thought  would be that getting the level of international coordination required would be extremely hard. (I am speaking from a position of ignorance here)

Maybe there could be groups of countries that agree to coordinate on this, and isolate themselves physically from countries outside their own groups?

I guess there might be ways to deliver disease through the air or wild (wild animals), or just sneaking into a country. The solution to that is domes. :P

A quick thought on the probability terminology. I think it would be better to just use numbers for the probabilities rather than to assign numbers to technical terms. I found myself going back to the glossary a lot to remind myself what each term means. Moreover, people are prone to forget the definitions given, and so information can get lost, especially if people only engage with specific sections. The IPCC does a similar thing with its probability terminology, and I think this has led to a lot of confusion. 

I agree with this.

I am also excited to see work on such an important, neglected topic.

While I haven’t looked into this much, I feel fairly convinced that hundreds of thousands or millions of people could survive using traditional approaches to agriculture in parts of the world with more moderate climate effects (and basic mitigation strategies, like switching to crop types that are more resilient to temperature and precipitation fluctuations).

ALLFED has indeed found a number of cool tolerant crops that could likely grow in nuclear winter conditions in the tropics. However, they are generally planted far away from the tropics, so if there were not long distance cooperation, the situation would be bad. Even without long distance cooperation, artifacts have moved thousands of kilometers, but I think it takes thousands of years. One possibility would be relocating crops from nearby mountains, but that would only work in specific circumstances.

On the other hand, there could be long distance movement of people, perhaps with remaining above ground fossil fuel and current ships. But then places where agriculture is easier in nuclear winter such as Oceania could be overwhelmed with migrants.

The carrying capacity of the Earth for hunter-gatherers is thought to be around 10 million if the survivors regress to pre-paleolithic levels of technology (if they lose, for example, flakes, handaxes, controlled use of fire, and wooden spears) (Taiz, 2013). 

It appears that this is not the correct reference for that quote. Taiz says that the global population was 10 million in 8,000 BC and another one of your references said that by then the hunter gatherers had covered the globe and had 10 million population (some say only 1 million) and they would generally have had those pre-paleolithic technologies. Ellis says 100 million hunter gatherers would be possible with prehistoric technology, which is much higher than the actual population in 8,000 BC (though it would be consistent with your statement).

Several experts, including ALLFED director David Denkenberger, have affirmed this conclusion — they do not expect humanity to dip below the minimum viable population even in relatively extreme sun-blocking scenarios.

To be clear, I don’t expect it, but I think extinction is a non-negligible probability.

Before getting into the likelihood that society would recover from civilizational collapse under these starting conditions, I’ll briefly discuss whether we should expect human civilization to actually collapse in my sense in this scenario.

Doesn’t appear to be public?

Thanks for this Luisa, I found it very interesting and appreciated the level of detail in the different cases. One thought and related questions that came up when reading the toy calculations at the end of each case:

For a fixed number of survivors, there is a trade-off between groups of different sizes. The larger the groups, the more likely each group is to survive, but the fewer groups need to be wiped out in order for humanity to go extinct. 

  • What might this trade-off look like and is there some optimal group size to minimise the risk of extinction?
  • What are the game theoretic considerations of individuals forming groups of varying sizes and how do these vary depending on the extent to which people care about their own individual survival and human extinction?
  • What group sizes might we expect in practice and is there anything we could do to influence group sizes in the event of a catastrophe?

Given the low likelihood of extinction you suggest, I think these are relatively low priority questions but could be potentially interesting for someone to look at in more detail.

Thanks for this post! It has definitely left me with a more fleshed out picture of some key considerations for this (in my opinion) important and neglected topic, and of how likely various things might be. 

  • Overall, the post made me a bit less concerned about global catastrophes / civilizational collapse scenarios that don't immediately involve/represent existential catastrophes.
    • This is because some portion of my tentative concern about such events came from the following sort of reasoning:
      • "Well, an event like that would be massive and unprecedented, and seems intuitively like the sort of thing that might have permanent and awful outcomes.
      • And I don't really have a fleshed-out picture of how an event like that would play out, nor have I seen fleshed-out reasons to believe it wouldn't have such outcomes.
      • So I shouldn't be super confident it wouldn't have such outcomes, even if I didn't have in mind specific reasons why it would."
      • And this post provided what seem like strong reasons to believe an event like that wouldn't relatively directly lead to extinction, which is one of the major pathways by which it could've theoretically led to existential catastrophe.
  • That said, I think that, personally, my main reasons for concern about such events was in any case not that they might fairly directly lead to extinction
    • Rather, it was that such events might:
      • Trigger other bad events (e.g., further conflict, development and/or deployment of dangerous technologies) that ultimately lead to extinction
      • Lead to "unrecoverable collapse" / "permanent stagnation"
      • Lead to "unrecoverable dystopia"
        • Meaning any scenario in which humanity survives and regains industrial civilization, but with substantially less good outcomes than could've been achieved. One of many ways this could occur is negative changes in values.
      • (I think my views on this are pretty similar to those Beckstead expresses here)
    • I think this post has updated me towards somewhat less concern about such events causing extinction by triggering other bad events
      • This is partly because you provide some arguments that conflict in the aftermath wouldn't be extreme or would be survived
      • (That said, I'm not sure how convincing I found those particular parts - I might expand on this in another comment - and I'm a bit confused about why WMDs were mentioned in Case 2 but not Case 1 or Case 3.)
    • But it hasn't caused a major update regarding the other two of those pathways
      • Which is fair enough - one post can't cover everything, and you explicitly noted that you're setting those matters aside for followup posts
        • Relatedly, I'm excited to read those followup posts!
  • I'd be really interested to hear whether, overall, doing this research has  updated you personally towards thinking we should prioritise interventions related to risks like nuclear war and climate change less, while prioritising AI risk (and maybe some other things?) more?
    • And if so, by how much?
    • And has it led to any other key changes in your bottom-line beliefs about what we should do?
    • (Btw, I think it's also totally ok for many pieces of research to help us progress towards updates to our bottom-line beliefs about what we should do without themselves causing major updates yet. So fair enough if your answers are "Not really" or "Ask again in a couple months"!)

I'll leave some other thoughts in separate comments. (Some - like the above - will partially repeat things we discussed already, but most will be on parts you added since the version of this doc I read.)

FYI, broken link here:

I think my views on this are pretty similar to those Beckstead expresses here

Oh, good catch, thanks! I accidentally linked to the title rather than the url. Now fixed.

Thanks a lot for this. I'm not really sure why the risk of meltdown of nuclear power plants is mentioned here. This intuitively seem like a marginal concern, and would want to see more arguments for meltdown being a risk worth considering. 

  1. In the scenario you are considering, the main source of death is from something like nuclear winter in which people would die over the course of a few months. I take it in that case, people would have enough time to just turn the nuclear power plants off before they had the change to melt down. If the catastrophe killed everyone in the surrounding area immediately, there would be incentives for others to go and turn the plant off. 
  2. In the massive Chernobyl meltdown, "As of mid-2005, however, fewer than 50 deaths had been directly attributed to radiation from the disaster, almost all being highly exposed rescue workers, many who died within months of the accident but others who died as late as 2004." Up to 4000 deaths could be attributed to the disaster in the longer term - so a series of large meltdowns only seem likely to mildly reduce life expectancy over the course of a few decades, which doesn't seem like a meaningful contribution to the catastrophe. The death toll from the Fukushima meltdown is similarly small and long-term. 

Regarding conflict and/or WMDs

this piece focuses on whether civilizational collapse would lead more or less directly to human extinction without additional catalyzing conditions, like a second catastrophe (either soon or long after the first) or centuries of economic stagnation that eventually end in human extinction. 

But elsewhere, you seem to imply that by "fairly directly" you just mean "within a generation". And you do discuss (rather than setting aside) the possibility that collapse would trigger conflict, and in Case 2 you discuss WMDs.

So I feel unsure whether: 

  1. This post's conclusions are meant to be interpreted as conditional on there being no major conflict and/or WMD usage after the collapse
  2. This post's conclusions are meant to hold even if we account for the possibility of there being major conflict and/or WMD usage after the collapse (perhaps because you think such conflict or WMD usage are quite unlikely or quite survivable)

If (1) is the case, then I think I'd roughly share this post's bottom-line conclusions. (But then the discussion of conflict and WMDs leaves me confused about precisely what is being set aside - e.g., conflicts above what threshold?)

If (2) is the case, then I think: 

  • I'd be much more uncertain than this post seems to be
    • And that uncertainty would effectively make me more pessimistic (though still not necessarily seeing extinction as likely, even in relatively extreme collapse scenarios)
  • I'd feel confused about why WMDs aren't mentioned in Case 1 or Case 3
  • I'd feel that more detailed discussion of the odds and consequences of various types of conflict is warranted

---

[One group of survivors having access to and deploying WMDs] seems unlikely to me, first because it seems hard to imagine a group of survivors incapable of recovering critical infrastructure — and barely capable of meeting even their basic needs — would be able to successfully deploy weapons of mass destruction (though I’m not very confident about this). 

I think I'm (even) less confident about that than you are, for two main reasons:

  1. Groups currently lacking critical systems doesn't mean they are incapable of recovering them. Groups will likely later recover some critical systems. (If they don't, that means this was already an existential catastrophe.)
    • So critical systems initially being down probably only provides a few months, years, or decades of protection from WMD usage (assuming that that does offer protection from that at all).
    • It seems pretty plausible that many groups would recover whatever systems are required to access and deploy WMDs before people recover things like a balance of power in the international system, international norms and treaties, established command hierarchies, or local law and order. And that scenario seems concerning.
      • I also think it's plausible that the latter sort of things will never be recovered even if things like population levels and technological progress are recovered - but that topic is probably better discussed in relation to your planned followup posts on things technological stagnation or unrecoverable dystopias.
  2. You defined a civilizational collapse as "an event in which approximately all of the entire world's critical systems are collapsed simultaneously for over six months." But defining it that way was just a choice you (understandably) made to help narrow the scope.
    • I think it's probably much more likely that there'd be a somewhat less extreme collapse - in which some systems remain online, systems remain online in some places, and/or the collapse isn't totally simultaneous across systems or regions. And in that sort of a scenario, it seems easier to imagine that some groups at some times would be able to access and deploy WMDs.
      • Which (as an aside) suggests that a "less extreme" collapse might actually be more risky?

---

I expect this period of violence to last until the survivors reach some sort of equilibrium, where they perceive there to be sufficient food, water, and shelter. The reason for this view is that it seems very irrational for groups to keep fighting if there are enough resources to go around. Of course, humans aren’t always rational, so I imagine some fighting might persist. But I think it’s very likely that many survivors would correctly conclude it would be more dangerous to fight each other than to accept a relatively low risk of not being able to meet their basic needs because of resource scarcity, if resources weren’t in fact too scarce to support the population.

[...] But as discussed, I expect any violence would mostly subside once some equilibrium had been reached — at the point where ~all of the surviving population could reasonably expect to be supported by the available resources (in other words, at the point at which the population dropped to or below whatever the Earth’s carrying capacity was).

I'd guess conflict is generally driven less by a desire for basic subsistence, more by leaders' desires to gain lots of resources and power, and more by non-material concerns (e.g., vengeance) than you seem to think is the case. (Though my guess isn't based on much focused research, and I might be misinterpreting your stance anyway.) 

Here, two things seem particularly worth noting:

  • People could "correctly conclude it would be more dangerous to fight each other than to accept a relatively low risk of not being able to meet their basic needs because of resource scarcity", and yet still fight.
    • So the above passage seems to implicitly contain the strong assumption that the sole reason groups fight is because the people in the groups believe doing so will increase their safety in expectation.
    • (I imagine you didn't actually intend to imply that, though, and maybe this is just a nit-pick.)
  • Both history and the present seem to contain a lot of conflicts driven at least partly by desires for revenge or retribution.
    • And you’re envisioning a scenario in which over 4 billion people have been killed in the initial catastrophe (which might have been an act of aggression, e.g. a nuclear war), and then an additional 3 billion people have been killed by further violence, by being forced to starve to death, or by starving to death without being assisted.
      • (This is based on the fact that you write “My best guess is that this violent competition would whittle the number of survivors down to the hundreds of millions or fewer”.)
      • So I'd guess there'd be a lot of conflict driven by desires for revenge or retribution in such a scenario.

(All that said, I do still agree that further conflict that doesn't involve WMDs seems unlikely to directly drive the population to 0 or to below the minimum viable population.)

Thanks for this interesting piece.

> To illustrate, consider that the 10 million Ashkenazi Jews living today descended from a population of just 350 individuals who lived between 600 and 800 million years ago (Commun, 2014).

Should this be "600 to 800 years ago"?

Oh dear — yes it should! Edited, thanks for flagging!

Some of the probability estimate seem a fair bit too high to me. 

In case 2, you say the risk of extinction is between 1% and 10%. In that world, there would be 800 million survivors - about the same as the world population in 1750. On the one hand, agriculture would be harder due to a nuclear/asteroid winter - maybe this leads to a >10-fold reduction in agricultural land? But on the other hand, we have massively more  scientific and technical knowledge today which has led to a >600% increase in yield since 1750 for some major food crops. I suspect on balance, this would make food supply harder, but not enough to produce a 1% risk of extinction (barring another catastrophe).

A general comment - I think it would be good to make an explicit model which explains how you arrived at a probability estimate - this makes it easier to pinpoint disagreements and to understand underlying reasoning. 

Thanks for great piece! One thing which may increase the extinction risk is that after the collapse, the remaining economy will be based not on agriculture and manufacturing, but on scavenging remains of previous civilisation. The problem with such economy is that it constantly shrinking and also does not help to learn useful skills, but instead helps local warlords arise and fight over leftovers. (Example: the economy of the post Soviet Union countries declined partly because it was more profitable to sell a factory for metal than to use it for manufacturing of goods.)

This problem will be also fuelled by "dangerous leftovers": in any moment, the access to weapons will be easy than to manufacturing capabilities. If most humans will extinct, enormous amounts of powerful weapons will still be available for decades, mostly firearms. Dangerous leftovers will include pieces of what has caused the catastrophe, for example, a biological agent or radioactive contamination.  In such situation, the remaining population will die more often than rise successful children. 

An example of dangerous leftovers are... dogs. If 99 per cent people went extinct, but all dogs remains, these feral dogs will eventually start hunting of survivors. 

Here, food stocks is defined in the source paper as a group of “92 products” used “to reconstruct 50 years of aggregated food reserves, expressed in caloric equivalent (kcal), at the regional and global scales.” (Laio et al. 2016)

This is a great reference in that it does more than just look at stocks of grain. It does it for the end of each year, which is a pretty favorable case. The stocks of food would be considerably less right before harvest in the northern hemisphere, I would estimate ~3 months of food instead of 6. Also, their number is assuming 2880 kcal per day per person, which is appropriate to account for waste, but would not account for edible food fed to animals. But I agree with your rationing vegan number of approximately seven months if the catastrophe happened at the end of the year (but about half that for the worst timing scenario).

Minor comment regarding minimum viable population

the minimal viable population (the minimum number of individuals necessary to maintain the level of genetic diversity to repopulate Earth)

I think this is a narrower usage of minimum viable population (which I think is the term you mean, rather than minimal viable population) than is standard. (Though I'm not an expert here.) Wikipedia says:

Minimum viable population (MVP) is a lower bound on the population of a species, such that it can survive in the wild. [...] MVP refers to the smallest possible size at which a biological population can exist without facing extinction from natural disasters or demographic, environmental, or genetic stochasticity.

That indicates the MVP is determined not just by whether there's sufficient genetic diversity, but also whether the population can survive natural disasters and other things. (See also this comment "from the perspective of a person with a population biology/evolution background", who argues that factors other than genetic variation would be more important in determining humanity's MVP following a catastrophe/collapse.)

(This is just a comment on terminology. You do cover whether the groups would be small enough to be wiped out by natural shocks. And I think it makes sense to consider the genetic diversity and natural shock questions separately. I just think it'd be best to not use the term "MVP" when one means something like "MVP from the perspective of genetic diversity specifically".)

---

Later, you write:

The minimum viable population (100–1,000 people, assuming a sufficient balance of the sexes) is really, really small compared to the current population: Just 0.00001% of the people alive today would need to survive for repopulation to be possible.

Do you have a citation for the 100-1000 figure? I ask partly because I think the size of the MVP is plausibly an important question (and I'd thus be interested to know if there's a relatively definitive source on it), and partly because I wonder if that figure is based on considering only genetic variation or also other factors.

Also, I think the last bit of that passage is somewhat misleading: We don't just need 0.00001% of the people alive today to survive, but for there to end up being a single group of that size. As you note elsewhere, people are likely to be initially quote scattered, and will likely remain at least somewhat scattered. (But I think you address this point adequately elsewhere, and I don't think it'd really change our bottom-line conclusions, so this is just a minor thing.)

Do you have a citation for the 100-1000 figure?

Comes from here https://mason.gmu.edu/~rhanson/collapse.pdf and the papers it cites:

It seems that groups of about seventy people colonized both Polynesia and the New World (Murray-McIntosh, Scrimshaw, Hatfield, & Penny, 1998; Hey, 2005). So let us assume, as a reference point for analysis, that the survival of humanity requires that one hundred humans remain, relatively close to one another, after a disruption and its resulting social collapse. With a healthy enough environment, one hundred connected humans might successfully adopt a hunter-gatherer lifestyle. If they were in close enough contact, and had enough resources to help them through a transition period, they might maintain a sufficiently diverse gene pool, and slowly increase their capabilities until they could support farming. Once they could communicate to share innovations and grow at the rate that our farming ancestors grew, humanity should return to our population and productivity level within twenty thousand years. (Murray-McIntosh, Scrimshaw, Hatfield, & Penny, 1998; Hey, 2005)

As others have said, great piece! Well argued and evidenced and on an important and neglected topic. I broadly agree with your point estimates for the three cases. 

I think it might be worth saying a bit more (perhaps in a seperate section near the top) about why your estimates of survival are not higher. What explains the remaining 0.01-0.3 uncertainty? How could it lead 'directly' to extinction? In different sections you talk about WMD, food availability etc, but I would have found it useful to have all that together. That would allow you to address general reasons for uncertainty too. The most compelling single reason for me, for example, is the unprecedented nature of a global, post-industrial collapse.

On your suggestions for other research directions:

I'd be super interested in someone going through the old Cold War RAND reports from the 1950s+1960s looking at collapse/recovery after nuclear war, and the wider literature on civil defence. Did the Soviets produce anything similar? I don't know! Going through the 'prepper' literature might also maybe be useful? Perhaps as useful as scifi.

"For example, I think I’ve heard somewhere that places with higher levels of social trust have lower levels of looting, hoarding, and other antisocial disaster behavior." You're thinking of Aldrich, D. P. (2012). Building Resilience. University of Chicago Press. The wider field is disaster risk reduction (DRR).

I was glad to see some discussion of potential irrational behavior. As was mentioned recently on the EA forum, I think there is a tendency for rational people to assume that other people will behave more rationally than they actually will. I think we have seen a lot of irrationality in the handling of COVID. And I think it would be far worse in the case of larger catastrophes. I think I remember reading that if something like half or three quarters of one's close family and friends died, there was a high chance of one becoming schizophrenic.

I think the fact that so many thoughtful people think collapse of civilization is likely from slow climate change (e.g. 50/50 chance for 4°C temperature rise according to Mark Lynas in his 80,000 Hours podcast), which is far less extreme than 50% of people dying, should give us pause. I tend to be more optimistic, but I do recognize the possibility that stressors could be handled poorly.

This is because neither pathogens nor natural disasters have ever come close to causing human extinction, and have only relatively rarely even caused the collapse of a single human civilization.

Interesting. Do you know of a good source that makes the case for this claim (and that could also help clarify precisely what it means, e.g. what "relatively rarely" means)? 

And what about the Toba catastrophe theory? I'm aware that that theory is contested, but I don't have the impression that there's academic consensus that the theory is wrong. (I haven't looked into it much, though.) Maybe you'd argue that, even if the theory is correct, that wouldn't be a case where humanity "came close" to extinction?

(I already have the view that anthropogenic risks are a far bigger deal than natural risks. But a good source for the above claim could perhaps update me even further in that direction.)

tl;dr for this comment: 

  • I think the tables in the "Concrete example" subsections of Case 1 and Case 2, and the surrounding text, ignore that you expect the initially surviving population in a collapse scenario "would shrink to a group of 10–100 million survivors during a period of violent competition for surviving goods in grocery stores/distribution centers, food stocks, and fresh water sources"
  • This might suggest we should be more uncertain/pessimistic than you claim
    • Though I don't know if this would leave us very pessimistic, given that the corresponding table in Case 3 assumes a population of 800,000, and seems to still indicate that the (relatively direct) extinction risk would be fairly low

---

The above-mentioned tables show how likely it is that extinction would occur if the people who survive the initial catastrophe are split into groups of various sizes. Then you write things like:

If you thought every single one of 400 groups of 10 million people [for a total of 4 billion people] had a 99% chance of being wiped out, you would still only think there was a 1.7% chance of extinction. And, to hold that belief, you’d have to think that despite having extremely large populations (larger than any US city), none of those people would be able to work out how to practice very basic farming or fishing, or that none of them would survive conflict within the group, or that a natural disaster would kill all of them. Personally, I find this view implausible.

But you elsewhere indicate that you yourself think the initial population of 4b would be reduced to 10m-100m as a result of conflict for resources. This implies you think there's a ~98.75% ([4b - 50m] / 4 b) chance that any given individual would die during that conflict period. (That could mean a ~98.75% chance that any given group is wiped out, or a substantial chance that any given group will be substantially reduced in size, possibly to below the MVP.) This seems to clash with you suggesting that this view is implausible.

And the mechanism you had in mind seems to have been conflict between groups (but maybe also within?), rather than e.g. no one working out how to farm or fish.

So it seems to me like it could perhaps be more intuitive to structure the reasoning as: 

  1. To what size would conflict for resources after the catastrophe reduce the global population?
  2. How likely is it that, once the population is reduced to that size and an "equilibrium" is reached, that population would be wiped out or reduced to below the MVP by various problems (e.g., failure to learn to farm or fish, intragroup conflict, and natural shocks)?

It seems like the table is intended to be about something like 2, in which case it should perhaps use a starting population of 10m-100m (rather than 4b and 800m, for Cases 1 and 2, respectively).

This was a lot to unpack but I agree that it would be extremely difficult for the human race to go extinct. Although 99.9% of all species have gone extinct. It is our enormous numbers and ability to adapt and specialize in different ecological niches using our tools instead of our biology that makes us hard to kill. I keep coming back to this forum because I like to see the academic perspective on nuclear conflict. Although there is still a lot of misinformation on surviving a nuclear war.

In my experience most people avoid thinking about nuclear conflict and it is not something I can bring up in casual conversation. Even the people who identify as preppers fall into nuclear nihilism and say they would want to be cindered in a flash of light rather than go through a nuclear winter.

Most preppers do not build bunkers and besides bunkers are unnecessary. Most preppers have a small amount of extra supplies for a brief interruption of services. They are not planning on bushcrafting to survive or single handedly rebuilding civilization. The research you linked on preppers complains that preppers are often thought as crazies like on the TV show doomsday preppers but still studies that fringe.

Nonetheless, I think most preppers would survive a nuclear war. In fact I believe a family can guarantee their survival in a nuclear war for under $5k. That is cheap insurance against a real risk that can be spread out over several years.

I loved the book, The Knowledge, and have read it a few times. I am jealous if you got the chance to pick the brain of Lewis Dartnell. I agree with his premise that the goal should be to restore civilization as fast as possible if it should collapse. A good primer was needed even if it is missing hydraulics and electronics.

But I think in order to rebuild we first have to show that people can survive something like a nuclear war. A lot of great material on surviving a nuclear war comes from the Cold War era. At the time the governments were afraid that if they instructed people directly on how to survive a nuclear war that they would be signaling to them that they intended to start one. They didn’t want people to think it was inevitable even if statistically it is inevitable. Besides people would cut their funding if they became too afraid. This is how unofficial but official information was created like protect and survive in the UK and Nuclear War Survival Skill in the US.

In Nuclear War Survival Skills they tested their shelters against blast effects and verified a family could build a covered foxhole in 48 hours. Before the nuclear test ban treaty shelters were tested against real nuclear weapons. Most buried structures that can support the weight of earth above them can survive an airburst. We also know the UK did practice nuclear wargames to train officials to make the best possible decisions in the worst possible scenarios. They included members of the BBC in their wargames and unsurprisingly created the dystopian science fiction film Threads which has been called the most realistic depiction of nuclear war.

If a covered foxhole as described on page 123 here a few buckets of food, and electronics wrapped in aluminum foil inside a metal trashcan (to protect against an EMP), can defeat an all out nuclear attack then humanity will survive and rebuild. MAD will be remembered as the greatest waste in the history of the human race of money, time and lost potential.

I am still waiting for an analysis comparing the Brian Toon article on nuclear winter and the Los Alamos nuclear winter paper that predicts much smaller cooling effects.

I am still waiting for an analysis comparing the Brian Toon article on nuclear winter and the Los Alamos nuclear winter paper that predicts much smaller cooling effects.

I'd also be keen to see additional work on the climate, agricultural, and famine effects of nuclear war, perhaps ideally by a third team which isn't connected to either of those teams. I may soon write a (not super in-depth or groundbreaking) set of summaries/analyses of the limited work that does exist, for posting on the Forum. In the meantime, you may find the back-and-forth between the Toon team and the Reisner team interesting (if you haven't seen it already):

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JD030777

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JD031281