The carbon footprint of a household is the quantity of carbon emissions produced by a given family unit, directly and indirectly. Direct emissions include emissions from energy used directly in homes (for space heating, lighting, and so on) and for personal transportation (such as personal vehicle use and personal aviation). Indirect (or embedded) emissions are carbon emissions produced upstream in the production and distribution of the goods and services purchased by households.This article addresses concepts of calculating a carbon footprint, and specialises the treatment to households in the United Kingdom (UK).
A carbon footprint can be measured in a variety of different units, sometimes leading to ambiguities. Some carbon footprints estimate carbon dioxide emissions from energy use only, whereas others include other greenhouse gases (GHGs), generally as a basket of six GHGs (carbon dioxide, methane, nitrous oxide, hydro-fluorocarbons, perfluorocarbons and sulphur hexafluoride). In this case the carbon footprint is measured in terms of carbon dioxide equivalent (CO2e or CO2equiv). A further complication arises as traditionally some carbon footprints are measured in terms of carbon (C), whereas others are measured in terms of CO2.
An ecological footprint is a similar concept to a carbon footprint, but in this case the measurement is in terms of the area of land and water attributable to a household’s activities.
Household or per capita?
Carbon footprints are often estimated on a personal (per capita) basis instead of a household basis. The household basis is generally considered preferable because many emissions arise at a household level, such as those from energy used for space heating. Household estimates may be simply divided by the number of people in the household to give per capita emissions. However this approach is questionable, particularly in the case of infants and children. Should each child take full responsibility for an equal share of the emissions from petrol used to drive to the supermarket for a weekly food shop? Or should the emissions be allocated according to the weight of food consumed by each member of the household? Such questions are the reason why, in general, it is more meaningful to estimate footprints on a household basis.
Methodology for household carbon footprinting
When estimating the average carbon footprint of a UK household there are two elements to consider, as mentioned above: direct emissions and indirect (embedded) emissions. Direct emissions are recorded in, for example, the UK Environmental Accounts, and so estimation of these emissions for the purposes of carbon footprinting is relatively straightforward. Estimation of indirect emissions is more challenging and is the subject of the remainder of this section.
In order to estimate indirect emissions it is necessary to assess all the supply-chain emissions involved in the production and distribution of all goods and services purchased by a typical household. To illustrate what we mean by supply-chain emissions, consider the purchase of a cotton T-Shirt by a UK resident. The supply-chain stages involved include: growing a crop of cotton (perhaps in, say, India); fertilizer might be applied using a tractor; the cotton is harvested and woven into cloth, which is then fabricated into the T-shirt in a factory; the T-shirt is then shipped to the UK. In this simple example the supply-chain emissions include:
- Emissions from growing the cotton
- Emissions from the fuel used in the tractor
- Emissions that arise in the production of fertilizer
- Emissions from heating and lighting the factory, and powering the machinery
- Emissions from the fuel used transporting the T-shirt to the UK
This list is by no means exhaustive: we might perhaps include, for example, the emissions that arise in making the steel from which the tractor is manufactured.
This example demonstrates that the task of estimating indirect emissions is highly challenging and requires considerable amounts of data for a single item. When this is to be done for the entire range of goods and services purchased by an average UK household, as required for carbon footprinting, the data demands are even higher.
There are two basic methods for estimating indirect emissions:
Life Cycle Analysis (LCA):
This is a bottom-up method which looks at the major processes in the production and distribution of an item or service. Using the example above, LCA systematically estimates the emissions that arise in each specific stage of the supply-chain of the T-shirt.
Environmental Input-Output (EIO) Analysis:
This is an economy-wide approach that is used to map resource flows and emissions through an economy. It is a top-down approach based on national input-output tables. In these tables industry is divided into sectors (such as Agriculture, Forestry, and Fishing). The tables show the sales and purchases between industry sectors, and purchases by final consumers (which include households and government). This economic information is combined with carbon emissions data for each industry sector. Imports are accounted for by using international datasets.
The preferred approach for carbon footprinting is Environmental Input-Output Analysis, as Life Cycle Analysis has two major drawbacks for the purposes of carbon footprinting. First, it suffers from boundary cut-off errors. These can be illustrated by referring again to the T-shirt example: should the emissions caused in manufacturing the steel to make the tractor be included, or may they be considered negligible and therefore outside the system boundary? Decisions such as this concerning where the boundary is drawn are critical for the accuracy of LCA studies. The second drawback is that LCA is commonly applied to single products or services, whereas a carbon footprint must, by definition, include entire consumption patterns.
Both these limitations are overcome by Environmental Input-Output, but, needless to say, EIO also has drawbacks. These include the assumption that each industry sector is assumed to be homogenous, and also EIO does not take account of economies of scale. A further drawback is that production of input-output tables is highly time-intensive and costly, and they are therefore produced relatively infrequently. Consequently carbon footprinting studies are often based on relatively old data.
For a more detailed description of EIO methodology the reader is referred to the book by Miller and Blair listed at the end of this article. For detailed descriptions of the methodology used in producing the findings presented below the reader is referred to the papers by Druckman and Jackson, also listed below.
So for what activities are UK households generating the most carbon? Is most carbon released to the atmosphere for subsistence, such as warming dwellings and feeding families? Or is, for example, more used in pursuit of leisure activities? The average carbon footprint of a UK household is estimated to be 26 tonnes CO2e in 2004. Of this, emissions from direct energy use accounted for 35% of the total (gas 13%; electricity 10%; other household fuels 2%; personal transport fuels 10%). Emissions embedded in products and services account for the remaining 65%.
In answer to the questions posed above, Figure 1 shows the proportions of the average UK household carbon footprint (in terms of GHGs) allocated to high level functional uses. The rationale for the high level functional use categories chosen is to reflect the range of material, social and psychological needs that are associated with modern lifestyles. Some of these are basic functional needs for material subsistence, protection and health. Others are associated more with social needs such as communication and education. Others cover a range of social and psychological motivations for leisure, relaxation, and interacting with friends and family. In carrying out allocation to these categories the carbon emissions from burning gas, for example, are first attributed to the various functions for which the gas is used, such as water heating, space heating and cooking. The emissions are then allocated to the appropriate high level functional use categories such as ‘Food and Catering’, ‘Clothing and Footwear’, ‘Health and Hygiene’, and ‘Space Heating’.
When we re-allocate carbon emissions to high level functional uses, as shown in Figure 1, the highest quantity of GHG emissions are attributable to Recreation and Leisure, and this category accounts for a quarter of total emissions. The next highest category is Food and Catering (22%), and Space Heating is responsible for 13%.This shows that while around a quarter of the carbon footprint of a typical UK household is devoted to recreation and leisure, considerable amounts of carbon are locked up in activities that are more concerned with subsistence: the need to be warm, fed, and clean.
The importance of including a range of GHGs rather than simply focusing on carbon dioxide is demonstrated by considering the carbon emissions attributed to food and catering, as most non-carbon dioxide greenhouse gases GHGs (in particular methane and nitrous oxide) are emitted in the production of food. If the pie-chart were to be drawn for carbon dioxide emissions alone, the percentage attributed to Food and Catering would be around just 15% of the total, compared to the 22% shown for GHGs above.
A word of warning: consumption versus production accounting
In a world where statistics get thrown around it is worth alerting the reader to a particular pitfall where misunderstandings can arise. When the “per capita” emissions of a country are discussed it is important to verify whether they have been assessed using the consumption perspective, as used in the carbon footprint explained above, or the production perspective. In the production perspective all emissions that arise within the UK are included, regardless of the destination of the final goods and services in the production of which they arise. Thus the production perspective includes emissions embedded in exports but excludes those in imports, and the difference between production and consumption emissions are the emissions associated with UK international trade. Per capita UK carbon emissions estimated on a production basis is not the same as the UK per capita carbon footprint calculated using the consumption perspective: the carbon footprint is higher because the UK has exported many of its carbon intensive industries overseas. The reverse is true for countries such as China, where emissions arising in the production of carbon intensive goods for export are higher than the carbon emissions embedded in their imports.
Druckman, A. and T. Jackson (2009a). Mapping our carbon responsibilities: more key results from the Surrey Environmental Lifestyle MApping (SELMA) framework. RESOLVE Working Paper 02-09, University of Surrey, Guildford, UK. Available from http://www.surrey.ac.uk/resolve/Docs/WorkingPapers/RESOLVE_WP_02-09.pdf.
Druckman, A. and T. Jackson (2009b). "The carbon footprint of UK households 1990-2004: a socio-economically disaggregated, quasi-multiregional input-output model." Ecological Economics 68 (7): 2066–2077.
Miller, R. E. and P. D. Blair (2009). Input-output Analysis: Foundations and Extensions. Cambridge, UK. 2nd revised edition, Cambridge University Press
^ To convert from carbon to carbon dioxide it is necessary to multiply by 44/12 (the ratio of the atomic weights of two oxygen atoms (2 x 16) plus one carbon atom (12), to one carbon atom).
^ Druckman and Jackson (2009a)
^ In fact electricity is not strictly a direct energy, as energy is generated at power stations while electricity is itself just an energy carrier. It is, however, included in this category as this is how it is commonly perceived by consumers, and it is subject to household decisions concerning its use and savings in a similar manner to gas.
^ See Druckman and Jackson (2009b).
^ The production perspective is used in accounting for the purposes of the Kyoto treaty.