Average time between two occurrences of a given severity event, inverse of the annual exceedance probability.
The return period of an event is the inverse of its annual exceedance probability: a 100-year return period event has a 1% probability of occurring in any given year. This definition assumes statistical stationarity, an assumption challenged by climate change. A 100-year return period does not mean an event can only occur once per century: over a 30-year exposure, the probability of experiencing at least one such event is approximately 26% (1−0.99³⁰). Return period is the reference metric for catastrophe pricing and extreme risk modeling.
200-year return period flood: 0.5% annual probability. For an insurer exposed over 40 years, probability of occurrence ≈ 18%.
return period, recurrence interval, temps de retour, période de récurrence, ARI