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

One argument for this methodological choice of a 100-year cut-off is the assumption that in the future
humanity will be better positioned to cope with climate change through e.g. technological
improvements (see Sections 3.5 and 3.6 and [5] ).
1.3 Tonne-year approaches
The first presentation, given by the organisers of the workshop, introduced the problematic of
temporary carbon storage, and presented the aim of the workshop, which was to review available
options and to discuss the most appropriate method for accounting for the potential benefits of
temporary carbon storage in LCA and CF. The Moura-Costa [6] and the Lashof [7] methods, both
tonne-year approaches developed for that purpose and presented in the IPCC special report on land
use, land-use change and forestry (LULUCF) [8], were presented.
The Moura-Costa and the Lashof approaches were proposed ten years ago to account for temporary
carbon storage. Some participants explained that these authors were involved in the development of
the IPCC special report on LULUCF published in 2000 [8], and that the subject was closed in 2003,
when the IPCC published the good practice guidelines adopting the mass balance stock change
approach. Some years after that, timing issues were again receiving attention with the development of
the British standard PAS 2050 for carbon footprinting, where benefits are given to temporary carbon
storage and delayed emissions. Other standards still in development, such as the GHG Protocol and
ISO14067, are also investigating temporal issues.
These tonne-year methods aim at calculating a credit in kg-eq CO 2 for keeping carbon out of the
atmosphere for a given number of years. This credit can then be subtracted from a GHG inventory, as
it is assumed to compensate for the impact of an equivalent GHG emission.
The baseline for these methods is the cumulative radiative forcing, integrated over a given time
horizon (usually 100 years), caused by a one-tonne pulse-emission of CO 2 . For a time horizon of 100
years, the integral of the CO 2 decay curve is approximately 48 tonne-years.
The Moura-Costa approach (see Figure 1) is based on a fixed duration over which impacts occur after
an emission. This reflects typical practice in LCA and CF, as the time when an emission occurs is not
considered. According to this approach, 48 tonne-years of CO 2 is equivalent to 1 tonne of CO 2 -eq.
Consequently, storing one tonne of CO 2 for 48 years is equivalent to avoiding the impact of a onetonne
CO 2 emission, which also means that storing one tonne of CO 2 for one year can fully
compensate for the impact of an emission of 0.02 tonne (1/48) of CO 2 .
Alternatively, the Lashof approach (see Figure 2) considers that storing carbon for a given number of
years is equivalent to delaying a CO 2 emission until the end of the storage period. The decay curve is
3