SM5002 Chap.3 Impact Assessment with ReCiPe Midpoints
Impact Assessment with ReCiPe Midpoints
What the impact assessment phase is for
A life cycle inventory is a ledger of flows: the electricity, water, steel and fuel a product system draws in, and the carbon dioxide, nitrous oxide, fine particulates and waste it sends out.
On its own that ledger cannot tell a hospital whether one gown is better for the environment than another, because the flows are measured in incompatible units and act on different parts of the world.
Life cycle impact assessment, the third phase of an LCA under ISO 14040, translates the ledger into a manageable set of environmental effects so that alternatives can be compared on like terms.
The standard separates the phase into elements that every study must carry out and elements that are optional.
Every study selects impact categories, category indicators and characterisation models; assigns inventory results to those categories, which is called classification; and calculates indicator results, which is called characterisation.
Normalisation and weighting may be added, but they bring value judgements into the analysis and are handled with care.
The vocabulary that keeps the phase precise
Five terms from ISO 14044 do most of the work. An impact category is the environmental issue to which inventory results are assigned, such as climate change or freshwater eutrophication.
A category indicator is the quantity that represents the category numerically. A characterisation model describes how inventory results connect to that indicator; the IPCC model behind the hundred-year global warming potential is the familiar case. A category end-point is the aspect of human health, the natural environment or resources that is ultimately harmed.
The environmental mechanism is the full physical, chemical and biological chain linking inventory, indicator and end-point.
Classification is mostly assignment, but one emission can contribute to several categories, so the chosen method determines where it lands. Students rarely build these links by hand.
An LCIA method such as ReCiPe, CML or TRACI arrives as a package of categories, indicators and factors, which is why picking the method belongs in the scope of the study and should be justified there.
Changing the method can change the ranking of alternatives, so it is also a natural candidate for a later sensitivity test.
Characterisation is multiplication, done carefully
At its simplest, characterisation multiplies the quantity of each inventory item by a characterisation factor and sums the products within a category.
For global warming over one hundred years, a kilogram of methane is counted as 29.8 kilograms of carbon dioxide equivalent, so a release of one hundred grams contributes just under three kilograms.
When the inventory item is an activity rather than an emission, such as a distance driven or a tonne of fuel burned, the factor is usually called an emission factor, and registries such as Singapore’s Emission Factors Registry, the UK government conversion factors and the IPCC database publish them.
The arithmetic is trivial; the units are not.
A factor expressed per tonne applied to a quantity in kilograms gives an answer a thousand times too large, and a per-kilometre factor applied to miles is wrong by about sixty percent.
Software multiplies whatever it is given, so the discipline of writing units beside every number is the main protection against a result that looks authoritative and is simply wrong.
ReCiPe at midpoint, under the Hierarchist perspective
ReCiPe, developed in the Netherlands as a successor to CML and Eco-indicator 99, offers eighteen midpoint categories and three end-point areas of protection: human health, ecosystems and resources.
Each midpoint has its own reference unit, including kilograms of carbon dioxide equivalent for climate change, kilograms of phosphorus equivalent for freshwater eutrophication, kilograms of oil equivalent for fossil resource scarcity and kilograms of 1,4-dichlorobenzene equivalent for the toxicity categories.
TRACI and CML work with about ten midpoints and no end-point level.
Midpoint indicators measure an effect before any damage occurs. They sit close to the emissions, rest on widely accepted models and therefore carry less uncertainty. End-point scores describe damage directly and are easier to communicate to non-specialists, but they depend on models that are still being developed.
The course works at midpoint, which matches the written assignment’s instruction to present ReCiPe 2016 Hierarchist results.
ReCiPe also asks you to choose a cultural perspective. The Individualist view takes a short time frame, includes only undisputed impact types and assumes technology will help people adapt. The Hierarchist view follows the most common policy principles on time frame and related issues.
The Egalitarian view is the most precautionary, using the longest time frame and admitting impact types for which there is some but not yet full evidence. Naming the perspective in a report is part of making the result reproducible.
Normalisation, weighting and their limits
Once eighteen indicators are on the table, readers ask which ones are large.
Normalisation answers that by dividing each result by a reference value, such as the contribution of an average person over a year, or of a region or a representative product. A published study of surgical scissors expressed its results relative to the global average person over one hour, per use, which let every category share an axis.
Weighting goes a step further, converting categories to a common scale through monetary valuation, distance-to-target methods or expert panels, sometimes summed into a single score. ISO 14044 states that such weights reflect value choices rather than science, and the GHG Protocol does not permit weighting of inventory results.
In practice, report category results first, use normalisation to show scale, and treat any single score as an opinion that must be attributed.
What this chapter covers
- 01
Mandatory and optional elements of life cycle impact assessment
- 02
Impact categories, indicators, characterisation models and end-points
- 03
Classification of inventory results into categories
- 04
Characterisation factors and emission factors with unit checks
- 05
The eighteen ReCiPe midpoints and three areas of protection
- 06
Individualist, Hierarchist and Egalitarian perspectives
- 07
Why normalisation is useful and weighting is contested
Worked example · free
A footprint from three registry factors
- 2Convert each inventory quantity into the unit of its factor: 0.3 kg steel, 0.5 tonne gas, and 3 people times about 8.05 km, giving 24.14 passenger-km.
- 2Multiply each quantity by its factor: 0.3 times 1.73 is 0.519 kg; 0.5 times 2,695.416 is 1,347.708 kg; 24.14 times 0.17 is 4.10 kg CO2e.
- 2Sum the lines and express the dominant share: 1,352.33 kg CO2e in total, with gas combustion at about 99.7 percent.
Key terms
- Impact Category
- An environmental issue, such as climate change or land use, to which inventory results are assigned.
- Characterisation Factor
- A number from a characterisation model that converts an inventory result into the common unit of its category indicator.
- Midpoint Indicator
- A measure of environmental effect taken before damage occurs, close to the emission and comparatively certain.
- Area Of Protection
- One of the three end-point groupings in ReCiPe: human health, ecosystems and resources.
- Hierarchist Perspective
- The ReCiPe value setting based on common policy principles for time frame, used for results in this course.
- Normalisation
- Expressing indicator results relative to a reference such as one person over a year, to show relative magnitude.
- Weighting
- Ranking or combining impact categories with value-based factors; optional and not scientifically determined under ISO 14044.
Impact Assessment with ReCiPe Midpoints FAQ
Why does this course report midpoints rather than damage scores?
Midpoint results sit nearer to the emissions and use accepted models such as the IPCC global warming potential, so they carry less uncertainty. Damage-level scores are easier for lay audiences but stack further modelled assumptions on top.
Is a characterisation factor the same thing as an emission factor?
They do the same job of converting a quantity into a common unit. People usually say emission factor when the item multiplied is an activity, such as kilometres driven or fuel burned, and characterisation factor when it is a released substance.
Can I combine all the ReCiPe categories into one overall score?
Only through weighting, which relies on value judgements by individuals, organisations or panels. ISO advises that different parties will reach different single scores from identical data, so report each category separately and attribute any weights you use.
What changes if I pick the Egalitarian perspective instead?
Results are computed over the longest time horizons and include impact types with incomplete evidence, so some categories typically grow. Your assignment fixes the Hierarchist setting, which keeps everyone’s numbers comparable and reproducible.
Assessment move
Rework the registry calculation by hand until you convert units before multiplying without thinking about it. Then open a ReCiPe midpoint result in openLCA and, for each of the eighteen categories, say aloud what the reference unit is and which inventory flows are likely to drive it.
