Bruce Schneier linked to an interesting 2007 journal article from Risk Management, Reduing the Risk of Human Extinction, which looks at both the risks of existential threats and what the cost/benefit analysis looks like in where we should put our money to prevent them.
One interesting issue that Jason Matheny brings up right away is just how long humanity is likely to exist — what is the “life expectancy” of our species as a whole,
Farthest out in time are astronomical risks. In one billion years, the sun will begin its red giant stage, increasing terrestrial temperatures above 1,000 degrees, boiling off our atmosphere, and eventually forming a planetary nebula, making Earth inhospitable to life (Sackmann, Boothroyd, & Kraemer, 1993; Ward & Brownlee, 2002). If we colonize other solar systems, we could survive longer than our sun, perhaps another 100 trillion years, when all stars begin burning out (Adams & Laughlin, 1997). We might survive even longer if we exploit nonstellar energy sources. But it is hard to imagine how humanity will survive beyond the decay of nuclear matter expected in 10^32 to 10^41 years (Adams & Laughlin, 1997).3 Physics seems to support Kafka’s remark that “[t]here is infinite hope, but not for us.”
While it may be physically possible for humanity or its descendents to flourish for 1041 years, it seems unlikely that humanity will live so long. Homo sapiens have existed for 200,000 years. Our closest relative, homo erectus, existed for around 1.8 million years (Anton, 2003). The median duration of mammalian species is around 2.2 million years (Avise et al., 1998).
A controversial approach to estimating humanity’s life expectancy is to use observation selection theory. The number of homo sapiens who have ever lived is around 100 billion (Haub, 2002). Suppose the number of people who have ever or will ever live is 10 trillion. If I think of myself as a random sample drawn from the set of all human beings who have ever or will ever live, then the probability of my being among the first 100 billion of 10 trillion lives is only 1%. It is more probable that I am randomly drawn from a smaller number of lives. For instance, if only 200 billion people have ever or will ever live, the probability of my being among the first 100 billion lives is 50%. The reasoning behind this line of argument is controversial but has survived a number of theoretical challenges (Leslie, 1996). Using observation selection theory, Gott (1993) estimated that humanity would survive an additional 5,000 to 8 million years, with 95% confidence.
Matheny then goes through estimating the probability of different extinction events (including the strong possibility of potential extinction events we’re completely unaware of as, say, some new technological breakthrough), and how we should properly discount the value of future generations.
Obviously there are a lot of unknowns in there, but Matheny concludes with the suggestion that just as we treat certain non-human animals as endangered and take extraordinary actions to preserve them, we might want to consider doing the same thing for our own species,
Human extinction in the next few centuries could reduce the number of future generations by thousands or more. We take extraordinary measures to protect some endangered species from extinction. It might be reasonable to take extraordinary measures to protect humanity from the same. To decide whether this is so requires more discussion of the methodological problems mentioned here, as well as research on the extinction risks we face and the costs of mitigating them.