3.2.4. The Time Dimension in Risk Assessment Ecosystems exhibit a tendency to return to equilibrium after modest disturbances. Owing to this resilience, many environmental impacts will self-repair after a period of a few years or decades; but more intense impacts, that exceed the natural capacity for recovery, will take much longer to dissipate. This raises a question of how to weigh the expected time to recovery for different consequences considered in a risk assessment. In U.S. economic analyses it is conventional to weigh differently the costs that are exacted at different times. The weighting is accomplished by a discount rate which depreciates a cost by a factor that is exponential with the time difference between the present and the time when that cost is incurred. Philosophically, this is sometimes justified in terms of the uncertainty about the future. Very practically, it may be justified in terms of the opportunity cost of changing the time of an expenditure, in light of the interest that would be charged in financing that expenditure. In public decision making where a cost-benefit analysis may be mandated, the discount rate that applies is also generally mandated. There is some debate over what is the correct value of discount rate to use for public decision making. By analogy, a discounting procedure may be applied in risk assessment for converting to a common basis consequences that occur at different future times, or consequences that unfold over a period of time. Discounting will treat as more serious a consequence that occurs soon, compared to the same consequence that occurs later.
