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

A fixed time horizon can be used (e.g. 100 years), which begins at the moment the first emission
occurs, or a variable time horizon, which also begins at the moment when the first emission occurs, but
finishes on a given year (e.g. 2100). With a fixed time horizon, the impact is assessed over the 100
years following the first emission. With a variable time horizon, the impact is assessed over a time
horizon beginning when the first emission occurs and finishing in e.g. 2100. Policies are usually
looking at a given point in time on which some targets are fixed. The same can be done in LCA and a
variable time horizon can be used. However, the problem with a variable time horizon is that it will
change through time. Each decade, for example, it will probably be pushed back another 10 years, so
that it will be constantly updated. PAS 2050 and the ILCD Handbook 1 use a fixed time horizon as they
are looking at the radiative forcing occurring 100 years following the formation of the product. To be
consistent, an assessment done next year should use the same characterization factors as an assessment
done today, which means that it is better to use a fixed time horizon.
3.6 Discounting future emissions
A distinction was made between the choice of a time horizon and discounting, two different ways to
express time preferences. Choosing a time horizon consists in looking at a particular environmental
problem, which can be measured and documented, and then in making decisions on the emergency of
the situation or the relevance of future actions. Discounting is based on the assumption that future
impacts are less important because future generations will be better able to cope with the damage.
There is science behind discounting (economics, social science), but it is different from the “physical
discounting” on which time horizons are based. The general attitude in the LCIA community is to
avoid discounting and time cut-offs. The choice of time horizons or discount rates is value-laden, but
cannot be excluded from this subject because it is impossible to give a value to temporary carbon
storage without using time preferences.
3.7 Treatment of biogenic carbon
A general discussion followed on the treatment of biogenic carbon uptakes and emissions. The first
comment was about the timing of the sequestration. If you have a forest, you cut a tree at time zero to
make a wooden product, and then you plant another tree that will grow thereafter, the sequestration is
not occurring at time zero, but is also delayed. This temporal issue should also be considered in the
calculations.
The other issue treated was whether credits should be given to temporary carbon storage only and only
if a new sink is created. Some argued that credits should not be given to transfer carbon from one non-
1 This is the case provided that temporary carbon storage or delayed emissions are part of the goal of the LCA.
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