# Is there really a 1 in 6 chance of human extinction this century?

In 2020, Oxford-based philosopher Toby Ord published a book called The Precipice about the risk of human extinction. He put the chances of “existential catastrophe” for our species during the next century at one in six.

It’s quite a specific number, and an alarming one. The claim drew headlines at the time, and has been influential since – most recently brought up by Australian politician Andrew Leigh in a speech in Melbourne.

It’s hard to disagree with the idea we face troubling prospects over the coming decades, from climate change, nuclear weapons and bio-engineered pathogens (all big issues in my view), to rogue AI and large asteroids (which I would see as less concerning).

But what about that number? Where does it come from? And what does it really mean?

## Coin flips and weather forecasts

To answer those questions, we have to answer another first: what is probability?

The most traditional view of probability is called frequentism, and derives its name from its heritage in games of dice and cards. On this view, we know there is a one in six chance a fair die will come up with a three (for example) by observing the frequency of threes in a large number of rolls.

Or consider the more complicated case of weather forecasts. What does it mean when a weatherperson tells us there is a one in six (or 17%) chance of rain tomorrow?

It’s hard to believe the weatherperson means us to imagine a large collection of “tomorrows”, of which some proportion will experience precipitation. Instead, we need to look at a large number of such predictions and see what happened after them.

If the forecaster is good at their job, we should see that when they said “one in six chance of rain tomorrow”, it did in fact rain on the following day one time in every six.

So, traditional probability depends on observations and procedure. To calculate it, we need to have a collection of repeated events on which to base our estimate.

## Can we learn from the Moon?

So what does this mean for the probability of human extinction? Well, such an event would be a one-off: after it happened, there would be no room for repeats.

Instead, we might find some parallel events to learn from. Indeed, in Ord’s book, he discusses a number of potential extinction events, some of which can potentially be examined in light of a history.