Page 252 - Materials Chemistry, Second Edition

236 4 Life Cycle Impact Assessment

degradation by OH radicals (already to be considered in the inventory!). In order
to safeguard a reliable C-balance the CO assimilation by photosynthesis and the
2
emissions should be quantified in the inventory.
The main prerequisites for a contribution to the greenhouse effect are the
absorption in the atmosphere within the infrared spectral ‘window’ of about
10–15 μm and a sufficient tropospheric lifetime to allow an even distribution in the
atmosphere. Substances with a short lifetime only generate islands of measurable
concentrations near the emission sources. These properties imply a contribution
to absorption of infrared radiation emitted from the surface of the earth in the
direction of space, Tyndall’s ‘waves of heat’. The GHG have therefore an impact
comparable to the walls of glass of a greenhouse, hence the name ‘greenhouse
effect’. In real greenhouses the solar radiation can pass through windows (with the
exception of UV), the infrared heat radiation is, however, only partly emitted from
the interior.
4.5.2.2.2 Impact Indicator and Characterisation Factors 171) Impact indicator for
the impact category climate change is the enhanced radiative forcing (difference
between radiant energy received by the earth and energy radiated back to space)
−2
measured or calculated as radiation per area (W m ). This is the common and
global primary effect which can cause multiple secondary and tertiary effects. The
primary effect is related to ‘global warming’, an increase in the average temperature
near the surface of the earth (including the lower troposphere and the surface water
of the oceans). Therefore this impact category was formerly (and sometimes even
today) called global warming, which however neither designates the primary effect
correctly nor the multitude of the following effects. Radiative forcing is thus
to be used as indicator for the renamed category of climate change. This is a
typical mid-point indicator that may later be supplemented by endpoint indicators
if scientific models allow such a correlation. These would for instance include
the increase in the sea level and disastrous weather events (additional and strong
floods, hurricanes, etc.), changes in the ecosystems and an increase of heat-related
illnesses in moderate climate zones. The melting of glaciers and of Arctic ice is
already vigorously taking place.
For an impact assessment in an LCA, a measure for a relative scale of the impact
is necessary to make it possible for emissions of, for example, methane, nitrous
oxide, carbon dioxide and chlorofluorocarbon (CFC) refrigerants to be weighted
against each other and aggregated into a weighted sum. This applies to GWPs:
they indicate the mass of CO , which has the same impact as the release of 1 kg
2
of another GHG; for example,1 kg of methane corresponds to 25 kg carbon dioxide
(GWP 100 ). However, as the various GHGs have a different tropospheric life time
(methane with around 10 a is relatively short-lived) the simulations have to be
provided with a time horizon indicating the period of validity of the calculation. For
LCAs a time horizon of 100 a is usually chosen. It is nevertheless probable that for
some objectives (depending on the goal definition) shorter or longer time horizons

171) Kl¨ opffer and Meilinger (2001a).