For terrestrial geologic storage, leakage can be directly monitored during and after the injection period, though this is more challenging for subseafloor reservoirs. If geologic storage results in a functionally stable form on a short timescale — for example, via subsurface mineralization — fugitive emissions associated with the full lifetime of storage may be estimated based on direct observations of the storage reservoir. If instead the integrity of geologic storage requires ongoing monitoring and maintenance — for example with the injection of supercritical CO₂ — the potential for future fugitive emissions must be modeled. In the latter framework, we consider the potential for future leakage as a durability uncertainty rather than a leakage uncertainty (see Storage Monitoring and Maintenance). For alternative storage systems, like mineralization in concrete, it is possible to directly measure the conversion of input CO₂ into a functionally stable form and therefore the total leakage from the storage system.