Assessing Temporary Carbon Storage in Life Cycle Assessment and ...

Relative impact of an emission occurring at year t
1
0
Option 1: Fixed GWP
Option 2: Moura‐Costa
Option 3a: ILCD
Option 3b: PAS2050
Option 4: Dynamic AGWP
0 50 100
Time (years)
Figure 3. Illustration of the four options discussed for the assessment of temporary carbon storage and
delayed emissions in LCA and CF
A time horizon (100 years in Figure 3) is chosen beyond which the impact is zero, because the
problem is considered no longer relevant. Option 1 reflects a constant characterization factor. The
problem with this option is that a high value is given for an emission occurring one year before the
time horizon, and then no value for an emission occurring one year after. That means that a substantial
benefit would be given for delaying an emission one year more, which does not reflect reality. For
long time horizons, the consequences of this would not be significant, but for shorter time horizons, it
is better to use a decreasing characterization factor. Option 2 is the Moura-Costa approach. The
problem with this option, as stated in Section 2.1, is that it is inconsistent with the concept of time
horizon, since the benefit of delaying a unit mass pulse-emission from a number of years equal to the
time horizon is higher than the total impact of this emission integrated over this time horizon. That is
why the impact of a delayed emission reaches zero at 48 years instead of 100 years. Option 3, as used
in the ILCD Handbook and the PAS 2050 standard, is a linear approximation of option 4, which is
dynamic AGWP, or the Lashof approach. A linear approximation has the advantage of being very
simple to use in LCA, as the yearly benefit for delaying an emission is constant. As the difference
17