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Environmental Toxicology and Chemistry, Vol. 15, No. 9, pp. 1442–1444, 1996q 1996 SETAC

Printed in the USA0730-7268/96 $6.00 1 .00

Letter to the Editor

RESOURCE DEPLETION IN LIFE-CYCLE ASSESSMENT

To the Editor:

Guinee and Heijungs [1] have suggested a definition of re-source equivalency factors for life-cycle assessment (LCA)that is described as scientific, but appears to be laced withhidden value judgments and denied social choices. The meth-od, which is very similar to the Swiss EcoPoints [2], relatesthe resource extraction listed in a life-cycle inventory to theglobal depletion of the resource in question and the abundanceof the element, fossil fuel, or species on earth. The definitionis based on two value judgments: first, in the selection ofgeologic abundance as a normalizing factor and second, in theassumption that the depletion of one resource has the sameimpacts as that of another. The definition is open to inconsistentinterpretations of what constitutes resource depletion, and itfails to consider the limits of human access to resources.

The impact assessment for LCA has been divided into threesteps: characterization (identification of impact categories),classification (quantification, and, where possible, aggregationof impacts within the given impact categories), and valuation[3]. The task of characterization and classification, two tech-nical steps, is to bring the information on environmental im-pacts connected with a specific product life cycle into a formthat the ‘‘evaluator’’ conducting the third step of impact as-sessment—the public or a member of the public—can easilyunderstand. In many impact categories the total effect scoreis the sum of emissions or other damages times a substance-specific equivalency factor [4]:

effect score 5 equivalency factorOi

3 emission or extraction (1)

Guinee and Heijungs [1] suggest a formulation of biotic andabiotic effect scores that is based on this general form ofaggregation. Biotic effect scores consider only the depletionof stock resources (e.g., deforestation and species extinction),but not the sustainable utilization of renewable resources. Abi-otic effect scores include fossil and mineral resources. Defi-nitions for the abiotic depletion potential (ADP) and the bioticdepletion potential (BDP) are as follows:

2production reserveres referenceADP 5 3 (2)1 2production reservereference res

2deaccumulation reserveres referenceBDP 5 3 (3)1 2deaccumulation reservereference res

The mining of iron ore and hunting of an elephant are givenas examples for production and deaccumulation, respectively.The definition of the ‘‘reserve’’ of an abiotic resource is itsgeologic abundance, derived from the composition of theearth’s crust, and that of a biotic resource is the stock of aresource (e.g., number of elephants).

In formulating their equivalency potentials, Guinee and Hei-jungs made at least three value judgments: to treat the depletionrates for all resources as equal, to define ‘‘reserves’’ as geo-

logic abundance, and to assign a zero value to utilization ofrenewable resources. The formulation of equivalency factorsitself is based on an implicit value judgment. It is assumedthat the same degree of depletion of one resource (e.g., oil),is equal to that of another resource (e.g., vanadium). In thesame vein, it is assumed that the same degree of ‘‘unsustainabledepletion’’ of an animal, for example, the elephant, is equalto that of a plant, for example, a particular wildflower. Sucha formulation does not agree with economic importance or ourpsychologic inclinations.

The definition of the abundance of a resource (‘‘reserve’’)is arbitrary: the proven reserve, the probable reserve, the par-amarginal resource, the total resource, and the geologic abun-dance could be chosen [5]. Proven reserves depend on theamount of prospecting that has been done for a particular re-source, and the amount of estimated total resources tells littleabout our ability to recover the resource. Resources such asaluminum may be very abundant in the earth’s crust, but re-quire much energy to recover. Each of the five possible defi-nitions for reserves here would yield a different weighting ofresources, and it requires a value judgment to decide whichto choose.

The third value judgment is to treat the sustainable use of flowresources as environmentally benign. The plantation of cotton,corn, or wood requires space and thus competes with wildernessareas or recreational space. Even the sustainable harvest of gameor fish alters ecologic balances and reduces the number of pred-ators (eagles, wolfs, bears) that can survive on a given land area.To exclude the impact of planting or hunting on nature is to saythat we value land equally whether it is a national park or a cornfield. The Sustainable Process Index [6] values natural flow re-sources in proportion to the land area required for their capture;it could serve as an alternative model for the inclusion of re-newable resources into LCA. Inconsistent interpretations of theBDP may result from the choice of boundary of analysis, thatis, the geographic scale to which the terms in the equations refer.A particular material, such as wood, may come from a sustainablesource even if deforestation occurs on the global level. Does itthus contribute to the biotic resource effect score? Or, a resourcemay be derived in an unsustainable manner from a particular plotof land, but with no net ‘‘deaccumulation’’ on the global level.According to the definition given by Guinee and Heijungs, thiswould still lead to a BDP and hence an effect score of zero.

Human access to resources is limited by the earth’s finitesurface area and by the availability of low-entropy energy andmaterials. A mineral reserve as defined by Guinee and Heijungscannot be depleted, but in reality, the dispersion of a metal inthe environment increases entropy and thus prevents our re-newed access to the metal. In a sustainable future, the utilizationof mineral resources will be based on renewable energy sources,and the energy cost and land area required will limit the avail-ability of resources. The energy required to utilize resources isan important indicator of their availability and should thereforebe included in the formulation of a resource depletion potential.

Letter to the Editor Environ. Toxicol. Chem. 15, 1996 1443

The resource equivalency factors suggested by Guinee andHeijungs present an interesting first attempt at characterizingresource depletion within the framework of life-cycle impactassessment. Hiding implicit value choices behind scientific lan-guage is inappropriate, however, and this author does not agreewith some of the value choices made. The values that underliea particular formulation of equivalency factors should be madeexplicit; the selection of any particular formulation is a matterof social choice, not scientific merit.

Edgar HertwichUniversity of California at Berkeley310 Barrows Hall No. 3050Berkeley, CA 94720, USA

REFERENCES

1. Guinee, J.B. and R. Heijungs. 1995. A proposal for the definitionof resource equivalency factors for use in product life-cycle as-sessment. Environ. Toxicol. Chem. 14:917–925.

2. Ahbe, S., A. Braunschweig and R. Mueller-Wenk. 1990. Meth-odik fur Oekobilanzen auf der Basis oekologischer Optimierung.Schriftenreihe Umwelt Nr. 133. Bundesamt fur Umwelt, Wald undLandschaft, Bern, Switzerland.

3. Consoli, F., et al. 1993. Guidelines for life-cycle assessment: Acode of practice. SETAC Special Publication. Society of Environ-mental Toxicology and Chemistry, Pensacola, FL, USA.

4. Fava, J.A., F. Consoli, R. Denision, K. Dickson, T. Mohin andB. Vigon, eds. 1993. A conceptual framework for impact assess-ment. SETAC Special Publication. Society of Environmental Tox-icology and Chemistry, Pensacola, FL, USA.

5. Tietenberg, T. 1996. Environmental and Natural Resource Eco-nomics, 4th ed. Harper Collins, New York, NY, USA, pp. 115–120.

6. Narodoslawsky, M. and C. Krotscheck. 1995. The sustainableprocess index (spi): Evaluating processes according to environ-mental compatibility. J. Hazard. Mater. 41:383–397.

The authors’ reply:

Hertwich [1] criticizes a paper in which we propose a systemfor deriving resource depletion equivalency factors for use inlife-cycle assessment [2] because the proposal contains hiddenvalue judgments. The core of his criticism is not so much thatvalue judgments are involved, but that they are implicit andhidden behind a veil of scientific claim; Hertwich gives ex-amples from our paper.

We completely agree with Hertwich that our proposal con-tains a considerable number of value judgments. Moreover,we are glad that he acknowledges that any method for deter-mining resource depletion equivalency factors necessarily con-tains value judgments and social choices. The fact that sub-jective choices must be made was one of the reasons we sub-mitted this proposal for discussion by the scientific commu-nity: ‘‘The method presented in this report represents one wayof characterizing resource extractions within the frameworkof LCA. There are other possibilities. . . . The subject of re-source characterization should thus be submitted to the sci-entific debate . . .’’ [2, p. 923]. From this perspective, we wel-come comments such as those of Hertwich on the subjectivechoices that we made in our proposal.

Hertwich also feels that we present our private choices asarguments based on scientific grounds. We do not share hisopinion that we left these value-bound choices implicit. Wehave tried to give good arguments for the choices that wemade, but we attempted to make quite clear that these argu-

ments were not intended as a solid proof. Many of our word-ings are in accordance with that: ‘‘It seems appropriate,’’ ‘‘inour opinion’’ [2, p. 918], ‘‘we prefer’’ [p. 919], ‘‘we propose’’[p. 920]. Even the title (‘‘A proposal . . .’’) expresses our pru-dence. Furthermore, we think that we were one of the first tostate explicitly the intrinsic rhetorical nature of this subject:‘‘There is no empirically correct method for aggregating ex-tractions of resources to one overall depletion score that canbe verified experimentally. In this case, it is necessary to de-sign methods on the basis of logical–theoretical reasoning. Itis often possible to test methods by detecting logical contra-dictions, but one cannot truly validate a nonempirical method’’[2, p. 922].

Let us look in more detail at the three points that Hertwichmakes.

1. We assumed that all resource depletion rates are equallysevere. Indeed, we did and gave reasons for doing so: ‘‘In ouropinion, these value-bound parameters [substitution and socialvalue] should not be part of the scientific characterization butrather be treated in social debate and through value-based val-uation’’ [2, p. 919]. So, we make an explicit and nonscientificchoice (‘‘in our opinion’’) that enables us to restrict the furtherdiscussion to scientific parameters (‘‘physical data’’).

2. We assumed that geologic abundance is a measure of thereserve. Quite right. We devote a discussion to this subject:‘‘Because of these drawbacks, we propose using another defi-nition of reserve. . . . Provisionally, then, we propose to applythe ultimate reserve concept . . . implicitly assuming the ratiobetween the ultimately extractable and ultimate reserve to beequal for all resource types’’ [2, p. 920]. We made a choice,not haphazardly, but with supporting reasons, and we are wellaware that other choices can be made.

3. We assumed that use of sustainably managed resources isfree of environmental impacts. No, we didn’t. What we actuallydid was to make an equation for depletion in which the sus-tainable use of a renewable resource does not contribute to theimpact score of depletion of that resource. For example, sus-tainably harvested wood gives a zero contribution of wood de-pletion. It does contribute, however, to other impact categories:land is used, pesticides can have impacts, some birds and otheranimals will disappear, etc. We did not forget these impacts,but assumed that they had already been accounted for by adopt-ing the life-cycle procedure: ‘‘The economic process of re-source extraction generally has a number of environmentalimpacts. . . . These are all accounted for by including the pro-cess of extraction in the product life-cycle. . . . This articlefocuses on the elaboration of equivalency factors for deple-tion’’ [2, p. 918]. Our paper was devoted to the characterizationof resource depletion, not to the characterization of resourceextraction.

We hope that Hertwich’s comments stimulate further debateon the foundations and principles of methods for the charac-terization of resource depletion. In a recent report, such acomparative discussion is undertaken [3]. More reflection isnecessary. Obviously, substantial subjective choice is in-volved. We recommend that other contributors deliver theiropinion in the form of concrete proposals for resource-deple-tion equivalency factors. In our view, the inherently subjectivenature of this topic requires and deserves a platform for sci-entific communication, even though the answers themselvescan not be qualified as scientific. The main point of Hertwich‘‘that subjective choices may never be hidden’’ remains an

1444 Environ. Toxicol. Chem. 15, 1996 E. Hertwich

important statement. We think that we have given adequateattention in that respect.

Reinout HeijungsJeroen GuineeCentre of Environmental ScienceLeiden UniversityP.O. Box 9518, NL-2300 RALeiden, The Netherlands

REFERENCES

1. Hertwich, E. 1996. Resource depletion in life-cycle assessment.Environ. Toxicol. Chem. 15:1442–1443.

2. Guinee, J.B. and R. Heijungs. 1995. A proposal for the definitionof resource equivalency factors for use in product life-cycle as-sessment. Environ. Toxicol. Chem. 14:917–925.

3. Lindfors, L.-G., K. Christiansen, L. Hoffman, Y. Virtanen, V.Juntilla, A. Leskinen, O.-J. Hanssen, A. Rønning, T. Ekvalland G. Finnveden. 1995. LCA-Nordic Technical Report. SpecialReports 1 and 2. TemaNord, Copenhagen, Denmark.