Emission factors – what are they and what are they for?


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What are emission factors?

Carbon emission factors, sometimes called carbon conversion factors, are used to calculate the carbon emissions arising from an activity. For the greatest accuracy, the best way to measure carbon emissions would be to monitor them directly as they are emitted, for example by placing a sensor in a power station chimney.  Given the large number of different sources of carbon emission that can be associated with a particular activity, this method would quickly become impractical.  To simplify the calculation, emission factors were developed by measuring the average carbon emissions associated with a particular activity.Given the complexities of measuring carbon emissions directly, developing emission factors is a technical and scientific challenge.  Over the past 15 years, numerous scientific studies have been conducted by teams of climate and energy experts from around the world, predominantly driven by the Intergovernmental Panel on Climate Change (IPCC), but also by national governments and academic researchers.  Countries that have signed up to the United Nations Framework Convention on Climate Change (UNFCCC) are required to create National Inventory Reports detailing all the greenhouse gas emissions arising from their country.  In developing these reports, the countries rely on a specific set of emission factors developed by the IPCC for use by UNFCCC countries.

Who sets them and why aren’t they all the same?

More recently, there has been growing interest in understanding carbon emissions on a smaller scale; businesses, government departments and individuals are increasingly keen to understand what their impact on climate change is, what it arises from, and therefore what they can do to reduce it.  This interest has led to the development of emission factors aimed at enabling a consistent analysis and a variety of work has been published by various academics and consultancies.  The first comprehensive collection of these emission factors, together with guidance on how to apply them, was the Greenhouse Gas (GHG) Protocol, developed by the World Resources Institute and the World Business Council for Sustainable Development. Since its first publication in 2001, the GHG Protocol has become the best known standard for calculating carbon emissions and its associated emission factors are the most widely used international carbon methodology.

As a standard intended to be applicable worldwide, it has some limitations due to the generic nature of the emission factors that it uses and subsequent standards have been developed in an attempt to address these areas.  Noteworthy amongst these efforts are the emission factors developed by the UK Department of Environment Food and Rural Affairs (Defra) as part of their guidelines on how to report corporate carbon emissions in the UK.  This set of factors has been developed to be specific to the UK situation and consequently differs in some respects to those in the GHG Protocol.

Other countries have also developed specific emission factors for use in that particular country.  For example the French environment agency, ADEME, has developed the Bilan Carbone carbon accounting methodology, which includes one of the most comprehensive sets of emission factors available.

As a result of these efforts, a wide range of emission factors exists.  Differences between them can be due to specific conditions in each country, but also due to differing scientific approaches used in developing the factors.  For this reason it is important to state clearly which set of emission factors has been used when calculating a carbon footprint and to select the set that is most appropriate for the intended purpose.

Which emission factors should I use?

The best emission factor methodology to use will depend on why you want to measure your carbon emissions.  If your organisation only operates in one country, then the national set of emission factors is probably the most appropriate.  In the UK, this would be the emission factors developed by Defra.  If your organisation has operations in several countries, then the GHG Protocol would be better as it includes emission factors recognised in all countries worldwide. Using this methodology for all your sites would ensure consistency in the carbon footprint calculation in all countries and enable like for like comparisons.

What do I do if an activity is not covered by the existing methodologies?

The main standards such as Defra and GHG Protocol cover the majority of activities that a business would want to report, but they are not intended to be universal. There are some activities which do result in greenhouse gas emissions for which they do not provide emission factors.  That is not to say that those emission factors do not exist – due to the wide variety of government and academic research efforts, emission factors are available for a great deal of activities beyond what is in the mainstream methodologies.  These factors can be used in conjunction with Defra or GHG, provided that the report is clear on where they have been used. The key issue is to ensure that the source of the emission factors is reputable and therefore ensure the factors will provide an accurate portrayal of the actual greenhouse gas emissions resulting from the activity.

Greenstone’s consultants are experts in the application of emission factors and can provide assistance in understanding the best option to use for your organisation.

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