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The term 'carbon footprint' has become tremendously popular over the last few years and is now in widespread use across the media – at least in the United Kingdom. With climate change high up on the political and corporate agenda, carbon footprint calculations are in demand. Numerous approaches have been proposed to provide estimates, ranging from basic online calculators to sophisticated life-cycle-analysis or input-output based methods and tools. Despite its ubiquitous use however, there is an apparent lack of academic definitions of what exactly a 'carbon footprint' is meant to be. The scientific literature is surprisingly void of clarifications, despite the fact that countless studies in energy and ecological economics that could have claimed to measure a 'carbon footprint' have been published over decades. This commentary explores the apparent discrepancy between public and academic use of the term 'carbon footprint' and suggests a scientific definition based on commonly accepted accounting principles and modelling approaches. It addresses methodological questions such as system boundaries, completeness, comprehensiveness, units, and robustness of the indicator.
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ISA
UK
Research Report 07-01
0
A Definition of ‘Carbon Footprint’
ISA
UK
Research Report
07-01
Thomas Wiedmann and Jan Minx
ISA
UK
Research Report 07-01
This report has now been published as a book chapter (suggested citation):
Wiedmann, T. and Minx, J. (2008). A Definition of 'Carbon Footprint'. In: C. C. Pertsova, Ecological
Economics Research Trends: Chapter 1, pp. 1-11, Nova Science Publishers, Hauppauge NY, USA.
https://www.novapublishers.com/catalog/product_info.php?products_id=5999.
© June 2007
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UK
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ISA
UK
Research Report 07-01
1
Research Report
07-01
available at www.censa.org.uk
A Definition of 'Carbon Footprint'
Thomas Wiedmann
1)*)
and Jan Minx
2)
1) ISA
UK
Research & Consulting, Durham, DH7 7FB, UK (www.isa-research.co.uk)
2) Stockholm Environment Institute, University of York, Heslington, York, YO10 5DD, UK (www.sei.se)
*) Corresponding author. Email: tommy@isa-research.co.uk
Abstract
The term ‘carbon footprint’ has become tremendously popular over the last few years and is now in
widespread use across the media at least in the United Kingdom. With climate change high up on the
political and corporate agenda, carbon footprint calculations are in strong demand. Numerous approaches
have been proposed to provide estimates, ranging from basic online calculators to sophisticated life-cycle-
analysis or input-output-based methods and tools. Despite its ubiquitous use however, there is an apparent
lack of academic definitions of what exactly a ‘carbon footprint’ is meant to be. The scientific literature is
surprisingly void of clarifications, despite the fact that countless studies in energy and ecological economics
that could have claimed to measure a ‘carbon footprint’ have been published over decades.
This report explores the apparent discrepancy between public and academic use of the term ‘carbon
footprint’ and suggests a scientific definition based on commonly accepted accounting principles and
modelling approaches. It addresses methodological question such as system boundaries, completeness,
comprehensiveness, units and robustness of the indicator.
Keywords
carbon footprint, ecological footprint, indirect carbon emissions, indicators, environmental accounting,
input-output analysis, life-cycle analysis, hybrid analysis
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Introduction
‘Carbon footprint’ has become a widely used term
and concept in the public debate on responsibility
and abatement action against the threat of global
climate change. It had a tremendous increase in
public appearance over the last few months and
years and is now a buzzword widely used across
the media, the government and in the business
world.
But what exactly is a ‘carbon footprint’? Despite its
ubiquitous appearance there seems to be no clear
definition of this term and there is still some
confusion what it actually means and measures
and what unit is to be used. While the term itself is
rooted in the language of Ecological Footprinting
(Wackernagel 1996), the common baseline is that
the carbon footprint stands for a certain amount of
gaseous emissions that are relevant to climate
change and associated with human production or
consumption activities. But this is almost where
the commonality ends. There is no consensus on
how to measure or quantify a carbon footprint. The
spectrum of definitions ranges from direct CO2
emissions to full life-cycle greenhouse gas
emissions and not even the units of measurement
are clear.
Questions that need to be asked are: Should the
carbon footprint include just carbon dioxide (CO2)
emissions or other greenhouse gas emissions as
well, e.g. methane? Should it be restricted to
carbon-based gases or can it include substances
that don’t have carbon in their molecule, e.g. N2O,
another powerful greenhouse gas? One could even
go as far as asking whether the carbon footprint
should be restricted to substances with a
greenhouse warming potential at all. After all,
there are gaseous emissions such as carbon
monoxide (CO) that are based on carbon and
relevant to the environment and health. What's
more, CO can be converted into CO2 through
chemical processes in the atmosphere. Also,
should the measure include all sources of
emissions, including those that do not stem from
fossil fuels, e.g. CO2 emissions from soils?
A very central question is whether the carbon
footprint needs to include indirect emissions
embodied in upstream production processes or
whether it is sufficient to look at just the direct, on-
site emissions of the product, process or person
under consideration. In other words, should the
carbon footprint reflect all life-cycle impacts of
goods and services used? If yes, where should the
boundary be drawn and how can these impacts be
quantified?
Finally, the term ‘footprint’ seems to suggest a
measurement (expression) in area-based units.
After all, a linguistically close relative, the
‘Ecological Footprint’ is expressed (measured) in
hectares or 'global hectares'. This question,
however, has even more far-reaching implications
as it goes down to the very decision whether the
carbon footprint should be a mere ‘pressure’
indicator expressing (just) the amount of carbon
emissions (measured e.g. in tonnes) or whether it
should indicate a (mid-point) impact, quantified in
tonnes of CO2 equivalents (t CO2-eq.) if the impact
is global warming potential, or in an area-based
unit if the impact is ‘land appropriation’.
Many of these questions have been discussed in
the disciplines of ecological economics and life-
cycle assessment for many years and therefore
some answers are at hand. So far, however, they
have not been applied to the term carbon footprint
and thus a clear definition is currently missing.
This report addresses the questions above and
attempts a clarification. We provide a literature
overview, propose a working definition of the
term 'carbon footprint' and discuss methodological
implications.
A brief literature review
A literature search in June 2007 for the term
"carbon footprint" (i.e. where these two words
stand next to each other in this order) in all
scientific journals and all search fields covered by
Scopus
1
and ScienceDirect
2
for the years 1960 to
1
Scopus (www.scopus.com) is currently the largest
abstract and citation database of peer-reviewed
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2007 yielded 42 hits; 3 from the year 2005, 8 from
2006 and 31 from 2007. Most articles deal with the
question of how much carbon dioxide emissions
can be attributed to a certain product, company or
organisation, although none of them provides an
research literature. Scopus is updated daily and
covers 30 million abstracts of 15,000 peer-reviewed
journals from more than 4,000 publishers ensuring a
broad interdisciplinary coverage.
2
ScienceDirect (www.sciencedirect.com) contains over
25% of the world's science, technology and medicine
full text and bibliographic information, including a
journal collection of over 2,000 titles as well as online
reference works, handbooks and book series.
unambiguous definition of the term carbon
footprint.
In most cases 'carbon footprint' is used as a generic
synonym for emissions of carbon dioxide or
greenhouse gases expressed in CO2 equivalents.
Some articles, however, discuss the implications of
precise wording. Geoffrey Hammond writes
(Hammond 2007): "…The property that is often
referred to as a carbon footprint is actually a
'carbon weight' of kilograms or tonnes per person
or activity." Hammond argues "…that those who
favour precision in such matters should perhaps
campaign for it to be called 'carbon weight', or
some similar term."
Table 1: Definitions of 'carbon footprint' from the grey literature
Source Definition
BP (2007)
"The carbon footprint is the amount of carbon dioxide emitted due to your daily
activities – from washing a load of laundry to driving a carload of kids to school."
British Sky
Broadcasting
(Sky) (Patel 2006)
The carbon footprint was calculated by "measuring the CO2 equivalent emissions from
its premises, company-owned vehicles, business travel and waste to landfill." (Patel
2006)
Carbon Trust
(2007)
"… a methodology to estimate the total emission of greenhouse gases (GHG) in carbon
equivalents from a product across its life cycle from the production of raw material
used in its manufacture, to disposal of the finished product (excluding in-use
emissions).
"… a technique for identifying and measuring the individual greenhouse gas emissions
from each activity within a supply chain process step and the framework for
attributing these to each output product (we [The Carbon Trust] will refer to this as the
product’s ‘carbon footprint’)." (CarbonTrust 2007, p.4)
Energetics (2007)
"… the full extent of direct and indirect CO2 emissions caused by your business
activities."
ETAP (2007)
"…the ‘Carbon Footprint’ is a measure of the impact human activities have on the
environment in terms of the amount of greenhouse gases produced, measured in
tonnes of carbon dioxide."
Global Footprint
Network (2007)
"The demand on biocapacity required to sequester (through photosynthesis) the
carbon dioxide (CO2) emissions from fossil fuel combustion." (GFN 2007; see also text)
Grub & Ellis
(2007)
"A carbon footprint is a measure of the amount of carbon dioxide emitted through the
combustion of fossil fuels. In the case of a business organization, it is the amount of
CO2 emitted either directly or indirectly as a result of its everyday operations. It also
might reflect the fossil energy represented in a product or commodity reaching
market."
Paliamentary
Office of Science
and Technology
(POST 2006)
"A ‘carbon footprint’ is the total amount of CO2 and other greenhouse gases, emitted
over the full life cycle of a process or product. It is expressed as grams of CO2
equivalent per kilowatt hour of generation (gCO2eq/kWh), which accounts for the
different global warming effects of other greenhouse gases."
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Haven (2007) mentions the carbon footprint
analysis of an office chair as a "life-cycle
assessment which took into account materials,
manufacture, transport, use and disposal at every
stage of development."
3
This hints at a more
comprehensive approach, rarely described in other
articles. However, there is no definition or
methodological description. Eckel (2007) points out
that the "Assessment of a business' carbon
footprint is not just calculating energy
consumption but also with increasing every scrap
of data from every aspect of the business
practices." Again, no clear scope of analysis is
provided.
While academia has largely neglected the
definition issue, consultancies, businesses, NGOs
and government have moved forward themselves
and provided their own definitions. In the grey
literature is a plethora of descriptions, some of
which are presented in Table 1.
In the UK, the Carbon Trust
4
has aimed at
developing a more common understanding what a
carbon footprint of a product is and circulated a
draft methodology for consultation (Carbon Trust
2007, see definition in Table 1). It is emphasised
that only input, output and unit processes which
are directly associated with the product should be
included, whilst some of the indirect emissions
e.g. from workers commuting to the factory are
not factored in.
Life-cycle thinking can be found in many other
documents and seem to have developed into one
characteristic of carbon footprint estimates. A
standardisation process has been initiated by the
Carbon Trust and Defra aimed at developing a
Publicly Available Specification (PAS) for LCA
methodology used by the Carbon Trust to measure
3
Note that a carbon footprint of a product derived in
such a way cannot just be added to the carbon
footprint of an office using this chair as this would
lead to double counting. Furthermore, double (or
multiple) counting would occur if companies
involved in the life cycle chain of the chair
(manufacturing, transport, disposal) reported their
full emissions (see e.g. Hammerschlag and Barbour
2003, Lenzen 2007 and Lenzen et al. 2007).
4
The Carbon Trust is a private company set up by the
UK government "to accelerate the transition to a low
carbon economy."
the embodied greenhouse gases in products
(DEFRA 2007). Below, we discuss the pro's and
con's of various methodologies.
The Global Footprint Network, an organisation
that compiles 'National Footprint Accounts' on an
annual basis (Wackernagel et al. 2005) sees the
carbon footprint as a part of the Ecological
Footprint. Carbon footprint is interpreted as a
synonym for the 'fossil fuel footprint' or the
demand on 'CO2 area' or 'CO2 land'. The latter one
is defined as "The demand on biocapacity required
to sequester (through photosynthesis) the carbon
dioxide (CO2) emissions from fossil fuel
combustion. [It] includes the biocapacity,
typically that of unharvested forests, needed to
absorb that fraction of fossil CO2 that is not
absorbed by the ocean." However, while
individual documents have used such a land-
based definition, for example the Scottish Climate
Change Strategy (see Scottish Executive 2006), it
has not changed the common understanding of the
carbon footprint as a measure of carbon dioxide
emissions or carbon dioxide equivalents in the
literature.
A definition of 'carbon footprint'
We propose the following definition of the term
'carbon footprint':
"The carbon footprint is a measure of the
exclusive total amount of carbon dioxide
emissions that is directly and indirectly caused
by an activity or is accumulated over the life
stages of a product."
This includes activities of individuals, populations,
governments, companies, organisations, processes,
industry sectors etc. Products include goods and
services. In any case, all direct (on-site, internal)
and indirect emissions (off-site, external,
embodied, upstream, downstream) need to be
taken into account.
The definition provides some answers to the
questions posed at the beginning. We include only
CO2 in the analysis, being well aware that there are
other substances with greenhouse warming
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potential. However, many of those are either not
based on carbon or are more difficult to quantify
because of data availability. Methane could easily
be included, but what information is gained from a
partially aggregated indicator, that includes just
two of a number of relevant greenhouse gases? A
comprehensive greenhouse gas indicator should
include all these gases and could for example be
termed 'climate footprint'. In the case of 'carbon
footprint' we opt for the most practical and clear
solution and include only CO2.
The definition also refrains from expressing the
carbon footprint as an area-based indicator. The
'total amount' of CO2 is physically measured in
mass units (kg, t, etc) and thus no conversion to an
area unit (ha, m
2
, km
2
, etc) takes place. The
conversion into a land area would have to be
based on a variety of different assumptions and
increases the uncertainties and errors associated
with a particular footprint estimate (see e.g.
Lenzen 2006). For this reason accountants usually
try to avoid unnecessary conversions and attempt
to express any phenomenon in the most
appropriate measurement unit (e.g. Keuning 1994;
Stahmer 2000). Following this rationale a land-
based measure does not seem appropriate and we
prefer the more accurate representation in tonnes
of carbon dioxide.
Whilst it is important for the concept of 'carbon
footprint' to be all-encompassing and to include all
possible causes that give rise to carbon emissions,
it is equally important to make clear what this
includes. The correct measurement of carbon
footprints gains a particular importance and
precariousness when it comes to carbon offsetting.
It is obvious that a clear definition of scope and
boundaries is essential when projects to reduce or
sequester CO2 emissions are sponsored. When
accounting for indirect emissions, methodologies
need to be applied that avoid under-counting as
well as double-counting of emissions, therefore the
word 'exclusive' in the definition.
5
Furthermore, a
full life-cycle assessment of products means that all
the stages of this life cycle need to be evaluated
correctly (with “full meaning untruncated”). In
the following section we discuss the
methodological implications of these requirements.
5
Compare with the discussion of 'shared reponsibility'
as outlined by Lenzen et al. (2007).
Methodological issues
The task of calculating carbon footprints can be
approached methodologically from two different
directions: bottom-up, based on Process Analysis
(PA) or top-down, based on Environmental Input-
Output (EIO) analysis. Both methodologies need to
deal with the challenges outlined above and strive
to capture the full life cycle impacts, i.e. inform a
full Life Cycle Analysis/Assessment (LCA). Here,
only a brief impression of some of their main
merits and drawbacks can be provided.
Process analysis (PA) is a bottom-up method,
which has been developed to understand the
environmental impacts of individual products
from cradle to grave. The bottom-up nature of PA-
LCAs (process-based LCAs) means that they suffer
from a system boundary problem - only on-site,
most first-order, and some second-order impacts
are considered (Lenzen 2001). If PA-LCAs are used
for deriving carbon footprint estimates, a strong
emphasis therefore needs to be given to the
identification of appropriate system boundaries,
which minimise this truncation error. PA-based
LCAs run into further difficulties once carbon
footprints for larger entities such as government,
households or particular industrial sectors have to
be established. Even though estimates can be
derived by extrapolating information contained in
life-cycle databases, results will get increasingly
patchy as these procedures usually require the
assumption that a subset of individual products
are representative for a larger product grouping
and the use of information from different
databases, which are usually not consistent (see
e.g. Tukker and Jansen 2006).
Environmental input-output (EIO) analysis
provides an alternative top-down approach to
carbon footprinting (see e.g. Wiedmann et al.
2006). Input-output tables are economic accounts
providing a picture of all economic activities at the
meso (sector) level. In combination with consistent
environmental account data they can be used to
establish carbon footprint estimates in a
comprehensive and robust way taking into
account all higher order impacts and setting the
whole economic system as boundary. However,
this completeness comes at the expense of detail.
The suitability of environmental input-output
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analysis to assess micro systems such as products
or processes is limited, as it assumes homogeneity
of prices, outputs and their carbon emissions at the
sector level. Although sectors can be disaggregated
for further analysis, bringing it closer to a micro
system, this possibility is limited, at least on a
larger scale. A big advantage of input-output
based approaches, however, is a much smaller
requirement of time and manpower once the
model is in place.
The best option for a detailed, yet comprehensive
and robust analysis is to combine the strength of
both methods by using a hybrid approach (Bullard
et al. 1978, Suh et al. 2004, Heijungs and Suh 2006),
where the PA and input-output methodologies are
integrated. Such an approach allows to preserve
the detail and accuracy of bottom-up approaches
in lower order stages, while higher-order
requirements are covered by the input-output part
of the model. Such a Hybrid-EIO-LCA method,
embedding process systems inside input-output
tables, is the current state-of-the art in ecological
economic modelling (Heijungs and Suh 2002,
Heijungs et al. 2006, Heijungs and Suh 2006). The
literature is just emerging and few practitioners so
far have acquired the skills to carry out such a
hybrid assessment. However, rapid progress and
much improved models can be expected over the
next few years.
The method of choice will often depend on the
purpose of the enquiry and the availability of data
and resources. It can be said that environmental
input-output analysis is superior for the
establishment of carbon footprints in macro and
meso systems. In this context a carbon footprint of
industrial sectors, individual businesses, larger
product groups, households, government, the
average citizen or an average member of a
particular socio-economic group can easily be
performed by input-output analysis (e.g. Foran et
al. 2005, SEI et al. 2006, Wiedmann et al. 2007).
Process analysis has clear advantages for looking
at micro systems: a particular process, an
individual product or a relatively small group of
individual products.
Practical examples
To date carbon footprints have been established for
countries and sub-national regions (SEI and WWF
2007), institutions such as schools (GAP et al.
2006), products (Carbon Trust 2006), businesses
and investment funds (Trucost 2006).
In this section we present two practical examples
of a carbon footprint analysis that adhere to the
definition suggested above. Both analyses were
undertaken by researchers of the Stockholm
Environment Institute at the University of York,
employing an input-output based approach.
The 'UK Schools Carbon Footprint Scoping Study'
(GAP et al. 2006) estimates that all schools in the
United Kingdom had a carbon footprint of 9.2
million tonnes of carbon dioxide in 2001, equating
to 1.3% of total UK emissions. Only around 26% of
this total carbon footprint can be attributed to on-
site emissions from the heating of premises,
whereas the other three quarters are from indirect
emission sources, such as electricity (22%), school
transport (14%), other transport (6%), chemicals
(5%), furniture (5%), paper (4%), other
manufactured products (14%), mining and
quarrying (2%) and other products and services
(3%).
The second example is a calculation of the carbon
footprint of UK households, taking into account
direct and indirect emissions occurring on UK
territory due to consumption activities of UK
residents as well as the (indirect) emissions that are
embodied in imports to the UK. The results,
presented in the 'Counting Consumption' report
(SEI et al. 2006), suggest that the carbon footprint
of an average UK household was 20.7 tonnes of
CO2 in 2001. A breakdown of this total is presented
in Figure 1.
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-
5
10
15
20
Private cars
Direct fuel use in the home
Electricity in the home
Aviation and public transport
Recreation, leisure and tourism
Food, drink and catering
Household appliances
Clothing and footwear
Health, hygiene and education
Other goods
Other services
Figure 1: Carbon dioxide emissions associated with UK household consumption in 2001
(tonnes of CO2 per household) (SEI et al. 2006)
Direct emissions occur through heating and car
use. Indirect emissions are the emissions that occur
during the generation of electricity and the
production of goods and services (whether they
are produced in the UK or in other countries).
They make up 70 per cent of the almost 21 tonnes
of CO2 per household. Transport (private cars,
aviation and public transport) accounts for 28% of
total emissions. Electricity use in the home and use
of fuels for space and water heating in the home
account for almost one third of the emissions.
These findings have also been published by the UK
Department for the Environment, Food and Rural
Affairs (DEFRA) in the 'The Environment in your
Pocket' publication (DEFRA 2006).
Conclusions
A review of scientific literature, publications and
statements from the public and private sector as
well as general media suggests that the term
'carbon footprint' has become widely established in
the public domain albeit without being clearly
defined in the scientific community. In this report
we suggest a definition of the term 'carbon
footprint' and hope to stimulate an academic
debate about the concept and process of carbon
footprint assessments.
We argue that it is important for a 'carbon
footprint' to include all direct as well as indirect
CO2 emissions, that a mass unit of measurement
should be used, and that other greenhouse gases
should not be included (or otherwise the indicator
should be termed 'climate footprint'). We discuss
the appropriateness of two major methodologies,
process analysis and input-output analysis, finding
that the latter one is able to provide
comprehensive and robust carbon footprint
assessments of production and consumption
activities at the meso level. As an appropriate
solution for the assessment of micro-systems such
as individual products or services we suggest a
Hybrid-EIO-LCA approach, where life-cycle
assessments are combined with input-output
analysis. In this approach, on-site, first- and
second-order process data on environmental
impacts is collected for the product or service
system under study, while higher-order re-
quirements are covered by input-output analysis.
6
Whatever method is used to calculate carbon
footprints it is important to avoid double-counting
along supply chains or life cycles. This is because
there are significant implications on the practices
of carbon trading and carbon offsetting
(Hammerschlag and Barbour 2003, Lenzen 2007,
Lenzen et al. 2007).
6
For example by using the Bottomline
3
tool
(see www.bottomline3.co.uk).
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... In these years, industrial development lagged behind and the developing countries of the Arab world, East Asia, Pacific, Middle East, and North Africa remained below the world's carbon emissions. As the years progressed, developed countries made efforts to reduce carbon emissions, such as the Kyoto Protocol, and while complying with these rules, they greatly reduced carbon emissions, while developing countries completed their industrial development, greatly (2007) The carbon footprint measures the biocapacity demand from burning fossil fuels in terms of the amount of forest area required to capture these CO 2 emissions Wiedmann and Minx, (2008) A carbon footprint is a measure of the specific total amount of carbon dioxide emissions that are directly or indirectly caused by an activity or accumulated throughout a product's life stages MCI (2008) The carbon footprint is the total amount of CO 2 and other greenhouse gases emitted over the entire life cycle of a product or service Tatar (2012) The Carbon Footprint is a measure of the damage caused by human activities to the environment in terms of the amount of greenhouse gases produced, measured in units of carbon dioxide. In other words, it is a measure of one's personal share in global warming Kaypak (2013) The carbon footprint is the measure of human damage to nature in terms of the amount of greenhouse gases, measured in units of carbon dioxide (CO 2 ). ...
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Preface. 1. Introduction. 2. The basic model for inventory analysis. 3. The refined model for inventory analysis. 4. Advanced topics in inventory analysis*. 5. Relation with input-output analysis*. 6. Perturbation theory. 7. Structural theory. 8. Beyond the inventory analysis. 9. Further extensions*. 10. Issues of implementation*. A. Matrix algebra. B. Main terms and symbols. C. Matlab code for most important algorithms. References. Index.