Our unfriendly universe: Risks of human extinction by nature

Nature harbours many existential risks. Here we describe several of the most likely natural risks and establish rough estimates for their likelihood. For each type of event, we will use a single probability to represent the odds that such an event will result in the complete extinction of humanity during a given period of time.

All of the natural threats described below have two things in common:

• Each category includes events ranging from benign to devastating. Only the extremely destructive events within each category could cause human extinction.
• Within each category, the more devastating an event would be, the more unlikely it is. Extinction events are much rarer than milder disasters. Global calamities which do not have a significant chance of causing human extinction are global catastrophic risks rather than existential risks.

Asteroids big enough to threaten humanity are rare. We’ve been cataloguing them and devising ways to deflect them.¹ Within the next century we should be much safer. Estimate of extinction risk: 1 in 7,000,000 per year (or 1 in 70,000 per century).²

The eruption of a supervolcano could endanger global civilization and even be an existential risk. Extremely powerful volcanic eruptions, which can create a “volcanic winter” appear to happen roughly once every 50,000 years.³ Humanity has already survived several of these, so it would take an extremely severe eruption to cause our extinction today. Estimate of extinction risk: 1 in 10,000,000 per year (or 1 in 100,000 per century).

Cosmic threats are a very broad category, including such things as extreme solar activity, hypernovas, dark nebulas, black holes, and gamma ray bursts. Taken as a whole, these threats only become likely on the scale of millions to billions of years.^{4 5} Estimate of extinction risk: 1 in 100,000,000 per year (or 1 in 1,000,000 per century).

Extremely severe climate change could cause our extinction if the climate changed dramatically, very quickly, and on a global scale. However, unless forced by a devastating event like those discussed above, natural climate change is extremely slow. Today, most of the observed effects of climate change are due to human action, not nature.⁶ Year by year, our impact on the global environment continues to grow. If climate change is going to be a problem for humanity, it is far more likely to be anthropogenic climate change. Estimate of extinction risk from natural climate change: 1 in 100,000,000 per year (or 1 in 1,000,000 per century).

Pandemics will likely occur in the future, but their danger is very hard to estimate. On one hand, pandemics can spread very quickly today thanks to modern transportation. On the other hand, enormous effort is being expended to prevent, detect, and contain them. We spent some time looking at the extinction risk of a global pandemic and concluded that pandemics deserve significant attention. Human-created pandemics appear to be much more likely and dangerous, but natural disease is still a risk we can’t ignore. Rolling many predictions together, we arrived at this estimate of extinction risk for natural pandemics: 1 in 100,000 chance per year (or 1 in 1000 per century). This estimate places natural pandemics as the single greatest natural existential risk in our list.

Many other natural threats are known, but are extremely unlikely.⁷ Probably there are also risks that we have not yet discovered. But thanks to our survival so far, we know that unknown existential risks can’t be dramatically more likely than the risks we’re aware of.⁵ Combining all of our estimates together, we can roughly gauge the total risk from nature. We arrive at the conclusion that natural risks have about a 1 in 1000 chance of rendering us extinct during the next century.

1. International Asteroid Warning Network. [↩]
2. Impact: Earth. Space Science Institute. Available at: http://www.killerasteroids.org/impact.php [↩]
3. Rampino, M. R. (2002). Supereruptions as a threat to civilizations on Earth-like planets. Icarus, 156(2), 562-569. [↩]
4. Bostrom, N. (2002). Existential risks. Journal of Evolution and Technology,9(1), 1-31. [↩]
5. Tegmark, M., & Bostrom, N. (2005). Astrophysics: Is a doomsday catastrophe likely?. Nature, 438(7069), 754-754. Preprint available at arxiv.org [↩] [↩]
6. Stocker, T. F. (Ed.). (2014). Climate change 2013: the physical science basis: Working Group I contribution to the Fifth assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press. [↩]
7. Bostrom, Nick (March 2002). “Existential Risks: Analyzing Human Extinction Scenarios and Related Hazards”. Journal of Evolution and Technology 9 (1). [↩]