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This post is the first part of my summary of The Precipice, by Toby Ord. It is about what existential risks are, and it explores the natural sources of existential risks. Future posts will explore the danger from other sources, our place in the story of humanity, and the importance of reducing existential risk.

What is an existential risk?

An existential catastrophe is any event that would destroy humanity’s potential. This could take a few forms:[1]

  • Ordinary extinction: Every human on Earth dies, or there are too few survivors to repopulate it.
  • Permanent civilisational collapse: An enormous catastrophe collapses civilisation and severely damages the environment in a way that makes it impossible to rebuild. This would be a world without writing, cities, and law. A collapse of civilisation might or might not be an existential catastrophe; it depends on whether we can rebuild.
  • A world in chains: The entire world is locked under totalitarian rule. Advanced technology allows permanent and powerful indoctrination, surveillance, and enforcement, leaving no chance for an uprising and no internal or external pressure to change. Like civilisational collapse, this presents an existential catastrophe if the situation is permanent. 

How can we estimate the danger?

One way is to assume the risk is negligible until there is strong scientific evidence determining that it is higher. This ensures that risks are not exaggerated but does not usually reflect our current understanding of the risks and might lead to dangerous underestimation of emerging risks.[2] Instead, Toby Ord begins with an initial impression of the size of the risk, then adjusts this estimate according to the scientific evidence.

The sources of natural extinction risk

Asteroids and comets

Sixty-six million years ago an asteroid hit Earth off the coast of Mexico, burning everything within 1,000 kilometres. The worst effects were caused by a billowing cloud of dust and ash (and sulphate aerosols from the vaporised sea floor) which blocked out the sun and cooled Earth. In the end, every land vertebrate over five kilograms went extinct (Longrich, Scriberas and Wills, 2016).

Supervolcanic eruptions

The very largest volcanic eruptions don’t look like typical volcanoes. Instead of mountains spilling out molten rock, supervolcanoes collapse into a vast craterlike depression (a well-known example is the Yellowstone caldera). One of these eruptions happened 74,000 years ago in Indonesia. Glowing rocks rained down as far as 100 kilometres away, and places as far away as India were covered in a metre-thick blanket of ash. Although this was not close to being an extinction-level event, supervolcanic eruptions present a small risk of civilisational collapse.[3] Even though we could likely rebuild civilisation, most of the extinction risk here is driven by the possibility of permanent civilisational collapse.  

Stellar explosions

Sometimes large stars explode, instantly releasing the same amount of energy as our sun will over its 10-billion-year lifetime. If this happened close to Earth, it could alter the climate and erode the ozone layer, leaving us exposed to UV radiation.

Estimating natural extinction risk

There are many other potential dangers.[4] And our understanding of natural risks is recent and growing. It was only in the 1960s that we learned that Earth may have been hit by a large asteroid and we detected the first signs of the bursts of energy emitted by exploding stars. There has been no slowdown in our discovery of new risks, and we do not know what caused several historical mass-extinction events. We should expect to learn about new sources of extinction risk in the coming decades.

Luckily, we can estimate the total natural extinction risk without complete knowledge of the individual risks by examining our track record. Homo sapiens have survived for over 200,000 years. If the risk had been 1% per century, then there would have been a 99.9999998% chance that we would have gone extinct by now. Based on this, we can be extremely confident that the risk is below 0.34% per century, and our best guess is that the risk is below 0.05% per century.[5]

We might also consider that humans have spread to diverse environments all over the planet, so it’s likely that only mass-extinction events truly threaten us. There have been five of these events since complex life developed — over 540 million years ago— making the extinction risk one in a million (0.0001%) per century.

Where possible, we can supplement this track record with our scientific understanding of the risks to get estimates for individual risks that are sometimes substantially lower than our track record suggests. For instance, we have identified about 95% of the asteroids less than 10 kilometres in diameter and likely all asteroids greater than 10 kilometres across; and we know none are going to hit us this century. Astronomers have also estimated the chances of a stellar explosion close enough to destroy 30% of the ozone layer at about one in 5 million.[6]

Overall, the picture is incredibly reassuring. While it would be prudent to continue to improve our scientific understanding of these risks and monitor them, these risks are very small over the next century.

The next post will begin to explore the existential risks caused by nuclear weapons, climate change, advanced biotechnology, and artificial intelligence. 

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Image of the earth from: www.tobyord.com/earth


Nick Bostrom (2002). Existential Risks: Analyzing Human Extinction Scenarios and Related HazardsJournal of Evolution and Technology 9.

Nick Bostrom (2013). Existential Risk Prevention as Global Priority. Global Policy 4/1.

Dario Buttazzo, Giuseppe Degrassi, Pier Paolo Giardino, Gian F Giudice, Filippo Sala, Alberto Salvio & Alessandro Strumia (2013). Investigating the Near-Criticality of the Higgs Boson. Journal of High Energy Physics 2013/89.

Nicholas R Longrich, J Scriberas, and Matthew A Wills (2016). Severe Extinction and Rapid Recovery of Mammals across the Cretaceous-Palaeogene Boundary, and the Effects of Rarity on Patterns of Extinction and Recovery. Journal of Evolutionary Biology 29.

Max Tegmark and Nick Bostrom (2005). Is a Doomsday Catastrophe Likely? Nature 438.

  1. ^

    See Bostrom (2002, 2013).

  2. ^

    For instance, this method would conclude that the extinction risks from climate change are negligible because the scientific evidence does not show conclusively that even extreme climate scenarios would result in human extinction. But extreme climate scenarios have also been largely neglected by researchers, and rapidly increasing our carbon emissions could have currently unforeseen harmful effects. Until new research shows that rapid warming simply cannot drive us extinct, we cannot be confident that the risk is extremely low. Ord estimates the extinction risk from climate change to be around 1 in 1,000 this century, as we will see in the next two parts of this summary.

  3. ^

    As with asteroids, the biggest threat comes from the dark cloud of volcanic dust and sulphate aerosols that would block out the sun and cool Earth. There is a lot of uncertainty about how much previous eruptions have cooled Earth (estimates from the Toba volcano in Indonesia range from 0.8 to 18 degrees Celsius of cooling, with the best estimates around 1–2 degrees). With only six months of food reserves, a supervolcanic eruption could result in the starvation of billions of people and the collapse of civilisation.

  4. ^

    Many pose no risk of extinction— for instance, catastrophes such as hurricanes or tsunamis. Some risks are vanishingly small over the coming century. For instance, there is little chance of another ice age over the next thousand years or another star passing through our solar system in the next few billion years; and for the next billion years there is little risk from the eventual brightening of our sun. Other risks are vanishingly small in general. For instance, some physical theories suggest that space is not a true vacuum and could collapse to a true vacuum state. However, Tegmark and Bostrom (2005) argue that we can have 99.9% confidence that the risk is less than one in a billion per year. Others suggest it is much lower (Buttazzo et al., 2013) or endorse a theory of physics in which space is already a true vacuum and so this poses no risk.

  5. ^

    Other, similar ways of estimating the risks give similarly low results, with best-guess estimates always below 0.05%. It’s plausible that we should consider humans inclusively, to include Neanderthals or perhaps the entire genus Homo. If so, we will arrive at lower best-guess estimates. Alternatively, we could consider the extinction of other species in our genus to be indicative of our own chances, which would give a best-guess estimate of at most 0.05% per century. We would get lower best-guess estimates if we looked at other mammals, or indeed other species in general. These estimates are likely to be overestimates because they include noncatastrophic extinction (for instance, gradual evolution into a new species) and because humanity has spread to a variety of environments and developed technologies that could help protect it from natural risks.

  6. ^

    We face a similar risk from bursts of gamma rays, thought to be the result of a particular kind of exploding star or the collision of neutron stars. These have the same energy release as a normal exploding star but concentrated into two narrow cones. This risk is estimated to be about one in 2.5 million. Searching the skies, we see no likely candidates for such stellar explosions or collisions, but we cannot entirely rule them out, yielding a moderately reduced risk this century in particular.

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