In a seminal article, Weitzman (1998) claimed that the correct results [when uncertain about the discount rate] are given by using an effective discount factor for any given time t that is the probability-weighted average of the various possible values for the true discount factor R(t): Reff(t) = E[R(t)]. From this premise, it is easy to deduce, given the exponential relationship between discount rates and discount factors, that if the various possible true discount rates are constant, the effective discount rate declines over time, tending to its lowest possible value in the limit t → ∞.

This video attempts to explain this in an excel spreadsheet.

I think this is just what is known as Weitzman discounting. From Greaves’ paper Discounting for Public Policy:

This video attempts to explain this in an excel spreadsheet.

Makes sense. Thanks Jack.