If S, M, or L is any small, medium, or large catastrophe and X is human extinction, then the probability of human extinction is
Pr(X)=Pr(S)Pr(M∣S)Pr(L∣S,M)Pr(X∣S,M,L).
So halving the probability of all small disasters, the probability of any small disaster becoming a medium-sized disaster, etc. would halve the probability of human extinction.
An idea I liked from Owen Cotton-Barratt’s new interview on the 80K podcast: Defense in depth
If S, M, or L is any small, medium, or large catastrophe and X is human extinction, then the probability of human extinction is
Pr(X)=Pr(S)Pr(M∣S)Pr(L∣S,M)Pr(X∣S,M,L).
So halving the probability of all small disasters, the probability of any small disaster becoming a medium-sized disaster, etc. would halve the probability of human extinction.