In my post Hobson’s Choice, Evolution, and Civilization I considered several degrees of extinction, including extinct in the wild, functional extinction, pseudoextinction, and local extinction (also called regional extinction).
The possibility of local extinction is particularly interesting in the context of mitigating existential risk, because the local extinction of a species may be consistent with the survival of the species elsewhere. Indeed, evolution has given us numerous examples of species that go extinct throughout much of their range, which latter is then eventually repopulated from refugia. (E.g., cf. Refugia revisited: individualistic responses of species in space and time)
Existential risk mitigation, understood at its appropriate level of generality, is essentially a strategy of redundancy that would be tolerant of local extinction events.
If, for example, humanity can establish multiple self-sufficient and independent centers of human civilization, this redundancy would make humanity tolerant to the local extinction of any one of these centers, in so far as a relict human population would survive at other centers, from which it could repopulate the region of local extinction, if this locality remained habitable.
Minimally, a single surviving relict population can repopulate a species’ range of distribution, so that of multiple redundant centers of civilization only one would have to survive in order to achieve the aim of existential risk mitigation.
In the past, human refugia were regional habitats on Earth were it remained possible for human beings to survive climate change. We have had the good fortune that this was sufficient for our survival in the past, but the growth of science has made us aware that much larger events could occur that could put our species or our civilization at risk. In order to mitigate such risks, we need to expand our range. However, we have covered the entire Earth, so that the only way to expand our range is to go beyond Earth.
For the first time in our history, industrial-technological civilization has made it possible for us to travel beyond Earth, and even to construct our own refugia were a relict population could survive when the next global-scale disruption of the biosphere renders the Earth uninhabitable to human beings. We can be certain, statistically, that such events will occur in the future, although we cannot predict when or what such an event will be.
Because we know that we are subject to global disruption of the biosphere that could result in human extinction if there were no off-planet refugia, existential risk mitigation means the establishment of a human population tolerant of local extinction, where “local” means the surface of the Earth.