Page 42 - Materials Chemistry, Second Edition

A N OVERVIEW OF THE LIFE CYCLE ASSESSMENT METHOD 23

While the unit process is the central element of the inventory analysis, the
central element in impact assessment is the impact category. ISO defines it as a
"class representing environmental issues of concern to which life cycle inven-
tory analysis results may be assigned." Perhaps more helpful are some exam-
ples: climate change, toxicity, and depletion of fossil energy carriers.
As climate change (often used interchangeable with global warming) is a
well-known issue, we will illustrate the main ideas of impact assessment with
this case. The inventory table contains a number of greenhouse gases: C0 2 ,
CH 4, N 2 0, etc. These are known to contribute all to the phenomenon of cli-
mate change. Climate change involves long sequence of causal mechanisms:
emissions of greenhouse gases lead to changes in the composition of the atmo-
sphere, which lead to a change in the radiation balance, which in turn leads to
a change in the temperature distribution, which leads to changes in climate,
which leads to changes in ecosystems and human activities, etc. The further we
proceed in this causal chain, the more uncertain and speculative our knowledge
becomes. While quite some scientific evidence is available with respect to the
composition of the atmosphere, the impacts on biodiversity are debated. Many
of these later impacts are even conditional on our future activities, including
future emission scenarios and mitigating actions. To be able to quantitatively
model the emissions of different greenhouse gases into an impact indicator for
climate change, we must do several things.
First, we must choose a certain point in the causal mechanism. This can be
at the front end (change in radiation balance), at the back-end (change of bio-
diversity), or somewhere in between (change in temperature). In LCA, two
main schools have emerged:

• Those that focus on the front-end, the so-called midpoint approach;
• Those that focus on the back-end, the so-called endpoint approach.

The midpoint approach has the advantage that it includes fewer debatable
assumptions and less-established facts; the endpoint approach has the advan-
tage that it provides more intuitive metrics (like loss of life years instead of kg
C0 2 -equivalents). Regardless of the choice between midpoint and endpoint,
the indicator chosen is referred to as the impact category indicator, or category
indicator for short.
Second, a way must be found to convert the emission data into the cho-
sen impact indicator. Scientists in chemistry, meteorology, ecology, etc,
have developed model fragments to estimate the atmospheric life-times
of greenhouse gases, their effect on the radiation balance and the forma-
tion of clouds, the effects of temperature on the distribution of species, etc.
These fragments have been combined by workgroups from the UN-based
International Panel on Climate Change (IPCC) into quantitative models of
the impacts of greenhouse gas emissions. Part of this is the global warm-
ing potentials (GWPs), which are quantitative measures of the strength of
different greenhouse gases. Many midpoint LCI A methods apply GWPs