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Review
The Abiotic Depletion Potential: Background,
Updates, and Future
Lauran van Oers †, * and Jeroen Guinée †
Faculty of Science Institute of Environmental Sciences (CML)—Department of Industrial Ecology,
Leiden University, P.O. Box 9518, Leiden 2300, RA, The Netherlands
*Correspondence: oers@cml.leidenuniv.nl; Tel.: +31-(0)71-527-5640
† These authors contributed equally to this work.
Academic Editors: Damien Giurco and Mario Schmidt
Received: 17 December 2015; Accepted: 23 February 2016; Published: 2 March 2016
Abstract:
Depletion of abiotic resources is a much disputed impact category in life cycle assessment
(LCA). The reason is that the problem can be defined in different ways. Furthermore, within a
specified problem definition, many choices can still be made regarding which parameters to include
in the characterization model and which data to use. This article gives an overview of the problem
definition and the choices that have been made when defining the abiotic depletion potentials (ADPs)
for a characterization model for abiotic resource depletion in LCA. Updates of the ADPs since 2002
are also briefly discussed. Finally, some possible new developments of the impact category of abiotic
resource depletion are suggested, such as redefining the depletion problem as a dilution problem.
This means taking the reserves in the environment and the economy into account in the reserve
parameter and using leakage from the economy, instead of extraction rate, as a dilution parameter.
Keywords:
ADP; abiotic depletion potential; life cycle assessment; abiotic natural resources; elements;
minerals; resource availability; scarcity; criticality; reserves
1. Introduction
From the beginning of the life cycle assessment (LCA) approach, the depletion of abiotic resources
has been one of the impact categories taken into account in the environmental impact assessment.
Natural resources are defined as an area of protection by the SETAC WIA (Society of Environmental
Toxicology and Chemistry Working group on life cycle Impact Assessment) [
1
] and are part of the Life
Cycle Impact Midpoint-Damage Framework developed by the UNEP (United Nations Environment
Program)/SETAC life cycle initiative [2].
However, abiotic resource depletion is one of the most debated impact categories because there is
no scientifically “correct” method to derive characterization factors [
3
]. There are several reasons for
this: (1) abiotic depletion is a problem crossing the economy–environment system boundary, since
reserves of resources depend on future technologies for extracting them; (2) there are different ways
to define the depletion problem, and all can be justified from different perspectives; (3) there are
different ways of quantifying a depletion definition, and none of them can be empirically verified,
since they all depend on the assumed availability of, and demand, for resources in the future and on
future technologies.
The debate on abiotic resource depletion and how to evaluate it has recently started again. This
is partly because of the ongoing debate in the LCA community; see for example the guidelines
of the International Reference Life Cycle Data System (ILCD) and the PEF, in Europe. (The ILCD
Handbook on LCA aims to provide guidance for good practice in LCA in business and government.
The development of the ILCD was coordinated by the European Commission and has been carried out
in a broad international consultation process with experts, stakeholders, and the general public [
4
,
5
].
Resources 2016,5, 16; doi:10.3390/resources5010016 www.mdpi.com/journal/resources
Resources 2016,5, 16 2 of 12
DG Environment has worked together with the European Commission
'
s Joint Research Centre (JRC IES)
and other European Commission services towards the development of a harmonized methodology for
the calculation of the environmental footprint of products (including carbon). This methodology
has been developed building on the ILCD Handbook as well as other existing methodological
standards and guidance documents (ISO 14040-44, PAS 2050, BP X30, WRI/WBCSD GHG protocol,
Sustainability Consortium, ISO 14025, Ecological Footprint, etc.) [
6
]. In addition, the debate on the
criticality of resources has revived the debate on how to evaluate the use and depletion of resources by
society [7–13].
In the context of the ILCD handbook on LCA, different characterization models for abiotic resource
depletion have been reviewed by the LCA impact assessment community [
12
,
14
]. The characterization
factors for abiotic resource depletion defined by Oers et al. [
15
] and recommended in the Dutch LCA
Handbook [
16
], were selected as the best available operational method at present for so-called “use
to availability ratio” methods [
12
]. However, contrary to the baseline method recommended in the
Dutch LCA Handbook [
16
], the ILCD handbook and the PEF adopted a version of the abiotic depletion
potential (ADP) that is calculated using the reserve base instead of the ultimate reserve estimations.
This alternative choice was one of the reasons why the debate was resumed.
In this context it is useful to reflect on the assumptions that were made when developing the ADP
and to think about possible future developments. This article aims to briefly describe the background
considerations, options, and final choices made at the time of the original development (1995) and
the latest update (2002) of the ADP. This description largely builds on the elaborate reporting of the
original method developed by the Leiden Institute of Environmental Sciences (CML) [3,15,16].
2. Description of the Characterization Model for ADP, Considerations, Options, and Choices
2.1. Fundamentals and Choices (1995–2002)
Life cycle impact assessment (LCIA) is the phase in which the set of results of the inventory
analysis—mainly the inventory table—is further processed and interpreted in terms of environmental
impacts. Based on an evaluation, the different elementary flows contributing to a specific impact
category are aggregated into one impact score. Thus, the core issue addressed by the characterization
model for abiotic resource depletion is: how serious is the depletion of one particular natural resource
in relation to that of another, and how can this be expressed in terms of characterization factors (ADPs)
for these resources?
The development of the model requires many decisions to be made, which together frame
the problem. This paper focuses only on the depletion problem of abiotic resource deposits [
3
,
15
].
The present section describes a selection of these issues and choices in more detail.
2.1.1. Definition of the Problem
When conducting an environmental assessment, it is debatable whether or not abiotic resource
depletion should be part of the environmental impact assessment. After all, the problem mainly refers
to the depletion of functions that natural resources have for the economy. One might, therefore, argue
that resource depletion is basically an economic problem, rather than an environmental problem.
This would imply that no separate impact category should be defined for the depletion of resources.
Note that the environmental impact of the extraction process itself will, however, still be assessed
through the contribution of current extraction processes to other impact categories.
Next to this, the problem of depletion of abiotic resources can still be defined in different ways,
such as a decrease in the amount of the resource itself, a decrease in world reserves of useful
energy/exergy, or an incremental change in the environmental impact of extraction processes at
some point in the future (e.g., due to having to extract lower-grade ores or recover materials from
scrap) etc. [12,16–18].
Resources 2016,5, 16 3 of 12
In Guinée and Heijungs [
3
] and Oers et al. [
15
], resource depletion was considered an
environmental problem in its own right, while recognizing that views differ on this. The problem
was defined as the decreasing natural availability of abiotic natural resources, including fossil energy
resources, elements, and minerals.
2.1.2. Concepts for Assessing Depletion
How can the “decreasing availability” of a given resource be determined? In other words, what
are possible indicators of resource depletion? The number of indicators that have been proposed even
exceeds that of the definitions (see for an overview, for example, ILCD [
12
,
14
] and Klinglmair et al. [
8
]).
Many discussions focus on the dichotomy between price-based and physics-based indicators.
Although the price of a resource can be regarded as a measure of its scarcity and societal value, it
reflects more than just that. Prices are also influenced by the structure of particular economic markets,
national social conditions reflected in labor cost, the power of mining companies with a monopoly,
the costs of identifying new reserves, etc. For these reasons, prices of resources do not seem to be an
appropriate indicator of depletion.
A depletion indicator could also be based on the various unique functions that resources can
fulfill in materials and products. When trying to assess the availability of possible resources one
would like to take into account possibilities for substitution. Oers et al. [
15
] undertook a preliminary
exploration of taking substitution possibilities into account. However, elements and compounds may
have very different potential functions, and possible shifts in potential functions in the future are
very difficult to anticipate. Hence, it was concluded at the time that including substitution was not
feasible in a characterization model for resource depletion. An exception was made for fossil energy
carriers, as they were assumed to be fully interchangeable, particularly regarding their energy carrier
function. It was therefore suggested to define a separate impact category for fossil fuels, based on their
similar function as energy carriers [
15
]. However, this recommendation was not yet implemented in
the baseline characterization factors described in the Dutch LCA Handbook [16].
Guinée and Heijungs [
3
] decided to base the characterization model for abiotic resource depletion
on physical data on reserves and annual de-accumulation, with de-accumulation defined as the annual
production (e.g., in kg/yr) minus the annual regeneration (e.g., in kg/yr) of a resource, the latter of
which was assumed to be zero. In addition to this, Oers et al. decided that the implementation of
substitution options (which touches upon issues of scarcity and criticality) was not (or not yet) feasible
within LCA [15].
2.1.3. Definition of Availability and Natural Stocks Versus Stocks in the Economy
When assessing the availability of resources one can use the concept of availability in a narrow
or a broad sense. Availability in the narrow sense focuses on the extraction of the resource from the
stock in the environment, the primary extraction medium, whereas availability in the broad sense
focuses on the presence of resources in stocks in the environment as well as the economy (geo- and
anthropospheres).
Ideally based on the definition of the depletion of abiotic resources, the available resource should
encompass both natural stocks and stocks in the economy. The criterion for depletion of the resource is
whether the resource derived from the environment is still present and (easily) available in the stocks
of materials in the economy. After all, as long as resources are still available in the economic stock after
extraction, there is no depletion problem.
Guinée and Heijungs [
3
] and Oers et al. [
15
] decided to adopt the narrow definition of availability,
while recognizing that, eventually, a broad sense definition would be preferable, assuming that it
would be possible and practically feasible to define a proper indicator for this and that the necessary
data would be available. The Discussion section below briefly introduces a preview of a possible
new approach.
Resources 2016,5, 16 4 of 12
2.1.4. Types of Reserves and Definitions
Estimates of the amounts of resources (elements, minerals, fuels) available for future generations
depend on the definition of reserve that is used. When talking about the reserves of resources
there might be confusion about the type of reserve being considered. The LCIA and geological
community do not use the same definitions as traditionally used by leading geological institutions.
Drielsma et al.
[
18
] have compared the definitions as used by the Committee for Mineral Reserves
International Reporting Standards (CRIRSCO) with definitions of reserves as used in the ADP [
15
]
(Table 1). For better communication between both communities in the future the terminology of
resources and reserves should be harmonized. Within the geological community the institutions
are currently converging towards the CRIRSCO definitions. It seems logical that within the LCIA
community the same terminology and definitions will be adopted.
Table 1. Types of reserves and definitions.
Terminology Definition
Oers et al. [15] Drielsma et al. [19] A Resource/Reserve Classification for Minerals, USGS [3,20,21].
ultimate reserve crustal content
The quantity of a resource (like a chemical element or compound) that
is ultimately available, estimated by multiplying the average natural
concentration of the resource in the primary extraction media (e.g.,
the earth’s crust) by the mass or volume of these media (e.g., the mass
of the crust assuming a depth of e.g., 10 km) [3].
ultimately extractable
reserve
extractable global
resource
Those reserves that can ultimately be technically extracted may be
termed the “ultimately extractable reserves”. This ultimately
extractable reserve (“extractable global resource”) is situated
somewhere between the ultimate reserve and the reserve base [
20
,
21
].
reserve base mineral resource
Part of an identified resource that meets specified minimum physical
and chemical criteria relating to current mining practice. The reserve
base may encompass those parts of the resources that have a
reasonable potential for becoming economically available within
planning horizons beyond those that assume proven technology and
current economics. The reserve base includes those resources that are
currently economic (reserves) or marginally economic (marginal
reserves), and some of those that are currently subeconomic
(subeconomic resources) (for further definitions see the original
references) [20,21].
economic reserve mineral reserve The part of the natural reserve base which can be economically
extracted at the time of determination [20,21].
The disadvantage of the “reserve base” and “economic reserve” is that estimating the size of
the reserve involves a variety of technical and economic considerations not directly related to the
environmental problem of resource depletion. The estimates, however, are relatively certain, as they
are based on present practice while, on the other hand, they are highly unstable as they continuously
change over time. In contrast, the “ultimately extractable reserve” is more directly related to the
environmental problem of resource depletion, and relatively stable over time. However, it is highly
uncertain how much of the scattered concentrations of elements and compounds will eventually
become available, as technical and economic developments in the far future are unpredictable.
The ultimate reserve and ultimately extractable reserve are expected to differ substantially.
However, data on the ultimately extractable reserve are unavailable and will never be exactly known
because of their dependence on future technological developments. Nevertheless, one might assume
that the “ultimate reserve” is a proxy for the “ultimately extractable reserve”, implicitly assuming that
the ratio between the ultimately extractable reserve and the ultimate reserve is equal for all resource
types. In reality this will not be the case, because the concentration-presence-distribution (see Figure 1)
of different resources will most likely be different [
15
]. Hence, there is insufficient information to
decide which of these reserves gives the best indication of the ultimately extractable reserve. Whilst
Resources 2016,5, 16 5 of 12
we acknowledge some authors propose a mineralogical barrier as described by Skinner [
22
] this has
not been considered in the research.
Guinée and Heijungs [
3
] and Oers et al. [
15
] adopted the “ultimate reserve” as the presumably
best proxy for the “ultimately extractable reserve” in their characterization model for abiotic resource
depletion. They recommended that alternative indicators be used for a sensitivity analysis, like the
“reserve base”, and to a lesser extent the “economic reserve”.
Resources 2016, 5, 16 5 of 12
depletion. They recommended that alternative indicators be used for a sensitivity analysis, like the
“reserve base”, and to a lesser extent the “economic reserve”.
Figure 1. Concentration-presence-distribution of several theoretical resources in the Earth’s crust.
The average Earth crust thickness is assumed to be 17 km. The Earth crust surface is assumed to be
5.14 × 1014 m2. The average earth crust density is 2670 kg·m−3. The ultimate reserve of a resource is the
surface area enclosed by the curve. The size of the other estimates of the reserves is given by the
surface area enclosed by the curve and the given secant with the x-axis [15].
2.1.5. Equations for Characterization Factors
Based on all the choices described above, the characterization model can be described.
The characterization model is a function of natural reserves (stocks/deposits in the environment) of
the abiotic resources combined with their rates of extraction (see Equation (2)). The method has been
made operational for many elements and fossil fuels (actually: the energy content of fossil fuels).
The natural reserves of these resources are based on “ultimate reserves”; that is, on concentrations of
the elements and fossil carbon in the Earth’s crust.
The characterization factor is the abiotic depletion potential (ADP). This factor is derived for
each extraction of elements and fossil fuels and is a relative measure, with the depletion of the element
antimony as a reference (see Equation (2)).
In accordance with the general structure of the LCIA, the impact category indicator result for the
impact category of “abiotic depletion” is calculated by multiplying LCI results, extractions of elements
and fossil fuels (in kg) by the characterization factors (ADPs in kg antimony equivalents/kg extraction,
The choice of the reference substance is arbitrary. Choosing another reference will not change the
relative sizes of the characterization factors. Antimony was chosen as a reference substance because it
is the first element in the alphabet for which a complete set of necessary data (extraction rate and
ultimate reserve) is available, and aggregating the results of these multiplications in one score to obtain
the indicator result (in kg antimony equivalents) (see Equation (1)):
=
×
(1)
with:
=
(2)
where,
ADPi: abiotic depletion potential of resource i (kg antimony equivalents/kg of resource i);
mi: quantity of resource i extracted (kg);
Ri: ultimate reserve of resource i (kg);
DRi: extraction rate of resource i (kg·yr–1) (regeneration is assumed to be zero);
Earth crust mass
(kg)
Concentration
(kg resource/ kg earth crust)
economic reserve
reserve base
ultimately extractable reserve
ultimate reserve
mineral reserve
mineral resource
extractable global resource
crustal content
Figure 1.
Concentration-presence-distribution of several theoretical resources in the Earth’s crust.
The average Earth crust thickness is assumed to be 17 km. The Earth crust surface is assumed to be
5.14 ˆ1014 m2
. The average earth crust density is 2670 kg
¨
m
´3
. The ultimate reserve of a resource is
the surface area enclosed by the curve. The size of the other estimates of the reserves is given by the
surface area enclosed by the curve and the given secant with the x-axis [15].
2.1.5. Equations for Characterization Factors
Based on all the choices described above, the characterization model can be described.
The haracterization model is a function of natural reserves (stocks/deposits in the environment)
of the abiotic resources combined with their rates of extraction (see Equation (2)). The method has
been made operational for many elements and fossil fuels (actually: the energy content of fossil fuels).
The natural reserves of these resources are based on “ultimate reserves”; that is, on concentrations of
the elements and fossil carbon in the Earth’s crust.
The characterization factor is the abiotic depletion potential (ADP). This factor is derived for each
extraction of elements and fossil fuels and is a relative measure, with the depletion of the element
antimony as a reference (see Equation (2)).
In accordance with the general structure of the LCIA, the impact category indicator result for the
impact category of “abiotic depletion” is calculated by multiplying LCI results, extractions of elements
and fossil fuels (in kg) by the characterization factors (ADPs in kg antimony equivalents/kg extraction,
The choice of the reference substance is arbitrary. Choosing another reference will not change the
relative sizes of the characterization factors. Antimony was chosen as a reference substance because
it is the first element in the alphabet for which a complete set of necessary data (extraction rate and
ultimate reserve) is available, and aggregating the results of these multiplications in one score to obtain
the indicator result (in kg antimony equivalents) (see Equation (1)):
abiotic de pletion “ÿ
i
ADPiˆmi(1)
with:
ADPi“DRi{pRiq2
DRr e f {´Rre f ¯2(2)
Resources 2016,5, 16 6 of 12
where,
ADPi: abiotic depletion potential of resource i (kg antimony equivalents/kg of resource i);
mi: quantity of resource i extracted (kg);
Ri: ultimate reserve of resource i (kg);
DRi: extraction rate of resource i (kg¨yr´1) (regeneration is assumed to be zero);
Rre f : ultimate reserve of the reference resource, antimony (kg);
DRre f : extraction rate of the reference resource, Rref (kg¨yr´1).
The first operational set of ADPs was developed by Guinée [
3
]. In 2002, these ADPs were updated
by Oers et al. [
15
]. This update included new extraction rates (DRs) of resources for the base year 1999.
To facilitate sensitivity analysis, alternative ADPs were developed based on different definitions of
reserves, viz. economic reserve, reserve base and ultimate reserve. Oers et al. [
15
] adjusted the ADPs
of fossil fuels and defined a separate impact category for fossil fuels based on the assumption that
fossil fuels are mutually substitutable as energy carriers. However, in the LCA handbook [
16
], fossil
fuels and elements were still considered to be part of one impact category, “abiotic resource depletion”.
The split into two separate impact categories was implemented in 2009.
2.2. Developments after 2002
Since 2002, the ADPs have been reported in a spreadsheet together with characterization factors for
other impact categories. The Centrum voor Milieuwetenschappen Leiden Impact Assessment (CMLIA)
spreadsheet with impact assessment factors as recommended by the Dutch LCA Handbook [
16
] can be
downloaded from the CMLIA website [23].
2.2.1. Update of Impact Categories by CML: Impact Category of “Abiotic Resource Depletion” Split
into Two Separate Impact Categories
In 2009, the impact category of “abiotic resource depletion” was split into two separate
impact categories:
‚“abiotic resource depletion—elements”; and
‚“abiotic resource depletion—fossil fuels”.
The impact category for elements is a heterogeneous group, consisting of elements and compounds
with a variety of functions (all functions being considered of equal importance). The resources in the
impact category of fossil fuels are fuels like oil, natural gas, and coal, which are all energy carriers and
assumed to be mutually substitutable. As a consequence, the stock of the fossil fuels is formed by the
total amount of fossil fuels, expressed in Megajoules (MJ).
Despite the fact that uranium is also an energy carrier, the extraction of uranium is classified under
the impact category of “abiotic resource depletion—elements” and not together with the fossil fuels,
under the impact category of “abiotic resource depletion—energy carriers”. Uranium and fossil fuels
also have other applications besides “energy carrier”. Fossil fuels are considered to be interchangeable
for these other applications, like the production of plastics, while uranium is not. However, one might
argue that the largest application of both fossil fuels and uranium is that of an energy carrier, and for
this reason they should be classified under the same impact category. Future versions of the CMLIA
might be adapted accordingly.
2.2.2. Update of ADP Values by CML
In 2009, the ADPs reported in the Dutch Handbook on LCA [
16
], based on Guinée [
3
], were
updated using the ADPs reported by Oers et al. [
15
] (R and DR data based on 1999 data). Since 2009,
the basic data underlying the ADPs and, thus, the ADPs themselves, have not been updated anymore.
Resources 2016,5, 16 7 of 12
2.2.3. Update of R and DR Values by Others
Some new DR and R data have been reported by Frischknecht et al. [
24
]. However, ADPs derived
from these data have not been implemented in the CMLIA spreadsheet.
Until 2010, the US Geological Survey (USGS) [
20
] reported the extraction rates, (economic) reserve,
and reserve base data for many resources on an annual basis. However, the reserve base data have no
longer been reported since 2010.
3. Discussion and Possible New Approaches in Abiotic Resource Depletion in CMLIA
3.1. Depletion, Scarcity, and Criticality
When trying to assess the “sustainability” of the use of resources by society, different definitions
of the problem of the resource use are used in terms of depletion, scarcity, and criticality.
Depletion of a resource means that its presence on Earth is reduced. This refers to geological
(or natural) stocks. Scarcity of a resource means that the amount available for use is, or will soon be,
insufficient (“demand higher than supply flow”). Criticality of a resource means that it is scarce and, at
the same time, essential for today’s society. In addition to environmental aspects, criticality assessment
often also considers economic, social, and geopolitical issues [9].
Interest in natural resources has recently increased, due to the growing interest in policies
to ensure the security of supply of critical metals like tantalum, indium, neodymium, etc. [
10
,
11
].
It should be noted, however, that this assessment of the criticality of metals is often based on
more criteria than environmental issues alone. The criticality of a metal is based on a mixture of
environmental, geopolitical, social, and economic considerations. Furthermore, the criticality debate
does not necessarily take into account the cradle-to-grave chain perspective; it often only focuses on a
supply of elements at the company or national economy level.
It is our opinion that the environmental impact assessment of LCA should not strive to take into
account all these different aspects of criticality assessment, i.e., environment, economy, and social aspects.
They may be part of a broader life cycle sustainability assessment (LCSA), but even then a general
approach will be difficult, as many of the criticality aspects are highly time- and region-dependent and
even differ for different stakeholders. LCSA is a framework for (cradle-to grave) system analysis which,
besides environmental aspects, may also focus on economic and social issues [
25
,
26
]. However, the impact
categories for “abiotic resource depletion” only deal with the environmental pillar of the sustainability
assessment, and the indicator is based on the problem of depletion only.
3.2. Ultimate Reserve, Reserve Base, and Economic Reserve
The ADPs developed by Guinée [
3
] and Oers et al. [
15
] are based on ultimate reserves as
calculated from the average element concentrations in the Earth’s crust [
27
] assuming a crust mass of
2.31 ˆ1022 kg
(average depth of the earth’s crust (m): 17,000; average density (kg/m
3
): 2670; surface
of the Earth (m2): 5.1 ˆ1014 ) [3].
Drielsma et al. [
19
] stated that “crustal content” (a synonym of ultimate reserve) is a stable,
comprehensive dataset that can be used to derive a physical estimate of resource depletion for abiotic
resources. They base this conclusion on a study by Rudnick and Gao [
28
], which compared several
studies done since the initial study by Clarke and Washington [
29
] on estimates for the total stock of
resources. The study by Rudnick and Gao [
28
] provided updated figures on crustal content that can be
used for updated ADPs based on ultimate reserves.
Given the fact that estimates of economic reserves are far less stable due to technological changes
and economic developments, and that reserve base data are no longer provided by USGS [
20
], the use
of “ultimate reserves” as a basis for the calculation of ADPs seems justified and confirmed.
We, therefore, argued here that the ILCD-recommended characterization model for abiotic
resource depletion [
12
] based on Oers et al. [
15
] should be readjusted to the original reserve definition,
involving ultimate reserve, instead of reserve base.
Resources 2016,5, 16 8 of 12
3.3. Availability in the Broad Sense, ADP Based on Stocks in Environment and Economy
Resources are not only produced from natural resource supplies, but are also recycled from
the growing amount of stocks in the economy and wastes (“urban stocks”). Should these stocks of
materials in the economy, which can potentially be recycled, be included when estimating the total
amount of available resources? In other words, should the reserve be based on the natural reserve only
or also on the reserve in the economy?
As discussed above, resources fulfill specific unique functions in materials and products.
Theoretically, there is no depletion problem as long as the resource after extraction is still available in
the economic stock. As a consequence, resource availability should encompass both natural stocks and
stocks in the economy [15].
Schneider et al. [
30
] made preliminary attempts to derive ADPs based on stocks in both the
economy and the environment, the so-called anthropogenic stock-extended abiotic depletion potential
(AADP). Their first results emphasize the relevance of anthropogenic stocks for the assessment of
abiotic resource depletion. However, a larger set of characterization factors and further research
are needed to verify the applicability of the concept within LCA practice. [
30
]. The AADPs were
updated in 2015, including an approach to the estimation of the ultimately extractable reserves from
the environment. The update resulted in 35 operational AADPs [
31
]. However, gathering information
on anthropogenic stocks still remains a challenge.
The problem could therefore be redefined as follows: abiotic resource depletion is the decrease in
the availability of resources, both in the environment and the economy.
3.4. Emissions from Economic Stocks and Processes as an Indicator of Dilution
If we adopt the broader definition of reserve described above, the de-accumulation parameter
“DR” in the ADP equation becomes meaningless. After all, the extraction (DR) of resources from
environmental sources for use in the economy is just a shift from environmental stock to economic
stock. Thus, when redefining the reserve parameter (R) in the ADP model, the other parameter in the
model, the “extraction rate” (DR) should also be redefined.
As originally suggested by Oers et al [
15
], the emission of resources to the environment, instead of
the extraction rate, might be a promising parameter for use in the characterization model.
Oers et al.
[
15
]
suggested that the loss of resources from economic processes and stocks due to emissions of elements and
compounds to air, water, and soil can be used as a measure of the dilution of resources. The argumentation
is that when environmental and economic stocks are regarded as a total reserve of resources, the problem
of depletion is in fact a dilution problem (see Figure 2). The assumption is that the resources that are
emitted to the environment are too diffuse to be ultimately extractable and, thus, are not part of the
ultimately extractable reserve. Hence, abiotic resource depletion can also be redefined as a problem of
dilution of resources; that is, a reduction of concentrated reserves of resources.
Frischknecht (in: Vadenbo et al. [
7
]) also suggested focusing on the role of borrowing and
dissipative resource use in impact assessment of abiotic resources. Material resources on Earth cannot
be lost (unless converted into energy or lost into space) but might be dispersed. The impact assessment
factors derived by Frischknecht are still based on the original ADP model (using environmental
reserves only), but extraction rates are applied to the dissipative use of resources, which is defined as
the difference between the amounts of resources extracted and recycled, i.e., the aggregated amount
lost during manufacture, use, and end-of-life treatment. This new concept of assessing the dissipative
use of resources using ADP factors is recommended by the new version of the ecoscarcity method [
24
].
Oers et al. [
15
] suggested a different elaboration of the characterization model for the dilution
problem of resources. If depletion of abiotic resources is defined as the dilution of the resources, the
leakage (L) of elements, minerals, and energy (heat) from the economy is suggested as a parameter
for the characterization model, as an alternative to the extraction rate (DR). This leakage of resources
can then be combined with the total reserve of resources in the environment and the economy
(
Rtotal = Renv + Rec
) into a new characterization model for the dilution of abiotic resources. This new
Resources 2016,5, 16 9 of 12
model can still use the original ADP equation, but the parameter extraction rate (DR) is replaced by
Leakage (L), and the reserve parameter (R) refers to reserves in both the economy and the environment.
Making these new characterization factors operational requires an estimation of stocks of elements
in the economy and the total emissions of elements to the environment in the world. A preliminary
approach to estimating these stocks can be based on the method and data described by Schneider [
30
,
31
]
and the United Nations Environment Program (UNEP) (stocks per capita) [
9
]. The total emissions in
the world can be based on the inventories made to derive normalization factors, as in the work by
Wegener Sleeswijk et al. [32].
The result for the impact category of “abiotic resource dilution” can then be calculated by
multiplying the emission, instead of extraction, of elements (in kg) by the characterization factors
(ADPs in kg antimony equivalents/kg emission) and by aggregating the results of these multiplications
in one score to obtain the indicator result (in kg antimony equivalents).
Resources 2016, 5, 16 9 of 12
the world can be based on the inventories made to derive normalization factors, as in the work by
Wegener Sleeswijk et al. [32].
The result for the impact category of “abiotic resource dilution” can then be calculated by multiplying
the emission, instead of extraction, of elements (in kg) by the characterization factors (ADPs in kg
antimony equivalents/kg emission) and by aggregating the results of these multiplications in one score
to obtain the indicator result (in kg antimony equivalents).
Figure 2. Relevant parameters for the abiotic resource depletion model, reserves in economy and
environment and annual extraction or emission of the resource (adapted from [15]).
4. Conclusions
It is impossible to define one correct method for assessing the problem of depletion of abiotic
resources, since the choice of relevant parameters that make up the model will depend on the problem
definition, and the correctness of the parameters cannot be verified empirically.
However, the definition of the problem and the choices made to define the characterization
model will result in different sets of characterizations.
The model of abiotic resource depletion as defined in the ADP [3,15] is a function of the annual
extraction rate and geological reserve of a resource.
In the model as presently defined, the ultimate reserve is considered the best estimate of the
ultimately extractable reserve and also the most stable parameter for the reserve parameter.
However, data for this parameter will by definition never be available. As a proxy, we suggest the
ultimate reserve (crustal content).
annual
extraction
(DR)
reserve (Renv)
(concentrated resource)
Figure 2.
Relevant parameters for the abiotic resource depletion model, reserves in economy and
environment and annual extraction or emission of the resource (adapted from [15]).
4. Conclusions
It is impossible to define one correct method for assessing the problem of depletion of abiotic
resources, since the choice of relevant parameters that make up the model will depend on the problem
definition, and the correctness of the parameters cannot be verified empirically.
Resources 2016,5, 16 10 of 12
However, the definition of the problem and the choices made to define the characterization model
will result in different sets of characterizations.
The model of abiotic resource depletion as defined in the ADP [
3
,
15
] is a function of the annual
extraction rate and geological reserve of a resource.
In the model as presently defined, the ultimate reserve is considered the best estimate of the
ultimately extractable reserve and also the most stable parameter for the reserve parameter. However,
data for this parameter will by definition never be available. As a proxy, we suggest the ultimate
reserve (crustal content).
Extraction rates and reserves are regularly updated by USGS [
19
], partly as a result of changes
in technology and new insights. As a consequence, characterization factors should also be regularly
updated, but this is unfortunately no longer being done.
The impact category of abiotic resource depletion as defined by CML [
3
,
15
] comprises only the
depletion of environmental resources. Criticality of resources is not part of the problem definition.
We recommend not including criticality in environmental LCA, as it deals with more than just
environmental aspects. It could be included in the broader LCSA framework, which also tries to
incorporate economic and social assessment into life cycle thinking.
A possible new development for the characterization model defined by Oers et al. [
15
] is the
redefinition of resource depletion as a dilution problem. This implies the inclusion of reserves in the
economy into the reserve parameter and using leakage from the economy, instead of extraction rate, as
a dilution parameter. However, this idea remains to be worked out, and no operational characterization
factors are as yet available.
Acknowledgments:
Part of the development of the ADP is based on the project “Abiotic resource depletion in
LCA; Improving characterization factors for abiotic resource depletion as recommended in the new Dutch LCA
handbook”. This project was commissioned by the Dutch Ministry of Infrastructure and the Environment.
Conflicts of Interest:
The authors declare no conflict of interest. The sponsors had no role in the design of the
study; in the collection, analysis, or interpretation of data; in the writing of the manuscript; or in the decision to
publish the results.
Abbreviations
The following abbreviations are used in this manuscript:
AADP
anthropogenic stock extended abiotic depletion potential
ADP
abiotic depletion potential
CML
Centrum voor Milieuwetenschappen Universiteit Leiden (Institute of Environmental
Sciences, Leiden University)
CMLIA
Centrum voor Milieuwetenschappen Universiteit Leiden Impact Assessment
CRIRSCO
Committee for Mineral Reserves International Reporting Standards
DR
extraction rate of resource, originally defined as annual de-accumulation, with
de-accumulation defined as the annual extraction (e.g., in kg/yr) minus the annual
regeneration (e.g., in kg/yr) of a resource, the latter of which is assumed to be zero
ILCD
International Reference Life Cycle Data System
LCA
life cycle assessment
LCIA
life cycle impact assessment
LCSA
life cycle sustainability assessment
PEF
product environmental footprint
R reserve of resource
SETAC
Society of Environmental Toxicology and Chemistry
UNEP
United Nations Environment Program
USGS
United States Geological Survey
WIA
Working Group on Life Cycle Impact Assessment
Resources 2016,5, 16 11 of 12
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