4.2 On the Strengths of LCA for Environmental Footprints
61
The carbon footprint assesses not only carbon emissions but also non-carbon
greenhouse gas (GHG) emissions by using substance-specific factors: namely, global
warming potentials (GWPs) that account for the relative global warming effects of a
mass unit of each GHG. The GWP is determined by sophisticated atmospheric models
and set by the equivalence principle, representing the integrated radiative forcing over
a specific time horizon (e.g., 100-yr) with a reference to CO 2 . Subsequently, these
comparable results of GHGs are aggregated into a single impact indicator expressed
in a CO 2 -equivalent mass unit (e.g., kg CO 2 -eq.). As such, likening carbon footprint
to a mass or weight would be akin to equating blood pressure with a distance because
of its unit of measure (mm Hg). This comparison overlooks the distinction between
the physical unit of a phenomenon (in this case, infrared radiative forcing) and the
accounting unit in which we happen to express something (in this case, the mass of
CO 2 that would have to be released to cause an equivalent impact).
Therefore, the success of the carbon footprint concept should not be attributed
only to the fashionable term “footprint”, borrowed from the ecological footprint
community. Rather, a far more fundamental reason is the scientific underpinning
of the characterization models, which allows the GWP to be one of the most established, consensus-based characterization factors. By contrast, the way that the ecological footprint deals with carbon emissions are much less rigorous. So-called carbon
hectares, which dominate the overall value of the ecological footprint in many studies,
are calculated by adding all energy-related carbon compounds in kilograms and
dividing the sum by a constant carbon sequestration rate (CSR). This procedure
disregards the difference of impact strength between different carbon emissions.
Carbon hectares are thus proportional to the total carbon weight of the energy carriers
(e.g., coal). Furthermore, non-carbon GHGs fall outside the scope of the ecological
footprint, even though N 2 O, for instance, is one of the most important GHGs that
contribute to climate change.
In summary, impact characterization is the key to understanding the carbon footprint concept. There is no need to translate the ecological footprint into land area, as
proven by the ecological footprint, which attempts to do so but fails to substantiate
the conversion convincingly. Hammond’s (2007) uneasiness about mistaking weight
for impact is indeed reasonable, although not for the carbon footprint, but rather for
the ecological footprint.
4.2.2 Moving from the Material Footprint to a Resource
Depletion Footprint
In view of the success of the ecological, water, and carbon footprints, it is not
surprising that an expanding list of indicators with “footprint” in their names will be
continuously introduced to the public. The recent appearance of the material footprint
represents an example (e.g., Schoer et al. 2012; Wiedmann et al. 2015). It is defined
as the total mass of materials used for economic processes. By using this mass-based
61
The carbon footprint assesses not only carbon emissions but also non-carbon
greenhouse gas (GHG) emissions by using substance-specific factors: namely, global
warming potentials (GWPs) that account for the relative global warming effects of a
mass unit of each GHG. The GWP is determined by sophisticated atmospheric models
and set by the equivalence principle, representing the integrated radiative forcing over
a specific time horizon (e.g., 100-yr) with a reference to CO 2 . Subsequently, these
comparable results of GHGs are aggregated into a single impact indicator expressed
in a CO 2 -equivalent mass unit (e.g., kg CO 2 -eq.). As such, likening carbon footprint
to a mass or weight would be akin to equating blood pressure with a distance because
of its unit of measure (mm Hg). This comparison overlooks the distinction between
the physical unit of a phenomenon (in this case, infrared radiative forcing) and the
accounting unit in which we happen to express something (in this case, the mass of
CO 2 that would have to be released to cause an equivalent impact).
Therefore, the success of the carbon footprint concept should not be attributed
only to the fashionable term “footprint”, borrowed from the ecological footprint
community. Rather, a far more fundamental reason is the scientific underpinning
of the characterization models, which allows the GWP to be one of the most established, consensus-based characterization factors. By contrast, the way that the ecological footprint deals with carbon emissions are much less rigorous. So-called carbon
hectares, which dominate the overall value of the ecological footprint in many studies,
are calculated by adding all energy-related carbon compounds in kilograms and
dividing the sum by a constant carbon sequestration rate (CSR). This procedure
disregards the difference of impact strength between different carbon emissions.
Carbon hectares are thus proportional to the total carbon weight of the energy carriers
(e.g., coal). Furthermore, non-carbon GHGs fall outside the scope of the ecological
footprint, even though N 2 O, for instance, is one of the most important GHGs that
contribute to climate change.
In summary, impact characterization is the key to understanding the carbon footprint concept. There is no need to translate the ecological footprint into land area, as
proven by the ecological footprint, which attempts to do so but fails to substantiate
the conversion convincingly. Hammond’s (2007) uneasiness about mistaking weight
for impact is indeed reasonable, although not for the carbon footprint, but rather for
the ecological footprint.
4.2.2 Moving from the Material Footprint to a Resource
Depletion Footprint
In view of the success of the ecological, water, and carbon footprints, it is not
surprising that an expanding list of indicators with “footprint” in their names will be
continuously introduced to the public. The recent appearance of the material footprint
represents an example (e.g., Schoer et al. 2012; Wiedmann et al. 2015). It is defined
as the total mass of materials used for economic processes. By using this mass-based