With respect to externalities, two temporal aspects have to be
addressed: (1) discounting future damage relating to current emissions
and (2) accounting for and discounting emissions distributed
over time but nevertheless related to current waste management.
Emissions occurring now (or at a specific time), for example CO2
emissions, have damage effects distributed over time, so any associated
externality costs could be discounted to a present value (or
the value at the time of the emission). Future emissions, for example
CH4 emitted over time from a tonne of landfilled waste, should
be accounted for within the LCA , but the annual damage cost (representing damage costs at the
moment of emission) should be discounted to present value (or
the value at the time of treating/disposing of the waste).