Carbon Audit

Carbon Audit Overview

Baseline greenhouse gas (GHG) audits have been produced for each island, which consider the main emission sources and sinks for each island. The inventories consider the key sectors of Energy; Transport; Waste; Agriculture; and Land Use, Land Use Change and Forestry (LULUCF). The audits follow the Global Protocol for Community-Scale Greenhouse Gas Inventories and  IPCC Guidelines for National Greenhouse Gas Inventories. 

Sectors considered by the CNI GHG audits

In addition to the sectors included in the GHG Protocol and IPCC Guidelines, the Scottish Blue Carbon Forum is working to include ‘blue carbon’ – or carbon deriving from marine habitats – in the UK Greenhouse Gas Inventory. A ‘Habitat Suitability Study’ is in development within the CNI project framework to estimate blue carbon potential for the islands. 

It is important that the goal of the GHG audit is well defined before compilation. Accuracy is important; however, the objective is not to exhaustively gather the most accurate data for its own sake. Rather, the CNI GHG audits provide a tool for community engagement to inform action towards decarbonisation. The audits play a significant role in GHG management. Fundamentally, the baseline emissions estimates for each island allow the local communities to target areas where interventions may be made to reduce emissions. By updating the inventory after a given period, the community can monitor their progress towards emissions reduction goals. 

Three main gases are considered: Carbon Dioxide (CO2), Methane (CH4), and Nitrous Oxide (N2O), which derive from both natural and anthropogenic sources. Fluorinated gases (F-gases) may also be considered, which consist mostly of Chlorofluorocarbons (CFCs). F-gases derive only from man-made sources and although present in smaller quantities than the three major GHGs, are potent contributors to global heating. 

Each gas can be described by their Global Warming Potential (GWP), a weighted measure which approximates its contribution to global heating considering various factors over given timescales, relative to the reference gas, CO2. Due to the complexity comparing the impacts of GHGs, it is standard practice to express emissions in terms of CO2 equivalent (CO2e), which multiplies the emissions of non-CO2 gases by their GWP. GWPs are periodically revised to reflect emerging scientific practice. Recently published figures are available from the IPCC Sixth Assessment Report (AR6) and BEIS Technical Reports of GHGs. 

Activity Data

To estimate emissions from a given activity, various approaches may be used depending on the sector. Each sector considered includes various sub-sectors, under which emitting activities are categorised. These may include domestic or non-domestic fuel usage, modes of transportation such as road-based or waterborne transport, land-use habitats such as woodlands or peatlands, or types of livestock. It is important to identify which sub-sectors are occurring or applicable to the GHG inventory. 

At a high level, emissions are calculated by multiplying activity data by an appropriate emission factor, explained below:


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GHG Emissions

Measured in metric tonnes of Carbon Dioxide equivalent (tCO2e)


Activity Data

e.g. quantity of fuel consumed, habitat extent and condition. Taken from local, regional and national data.


Emission Factor

GHG emissions per unit of activity (tCO2e/unit). Provided by BEIS, IPCC, amongst other sources.


Activity data describe the quantity or scale of the human activity that causes an emission source or sink. Examples of activity data include vehicle mileage, the quantity of fuel consumed, or the extent or condition of a habitat. Generally, activity data sources are taken from a wide range of national and regional datasets depending on the activity. Only emissions and changes to carbon stocks which derive from human activity are considered. 

Activity data can be taken from a wide range of sources, though certain types of data are preferred and deliver the most accurate emissions estimates. Direct consumption data, such as the quantity of fuel consumed, are preferred over more modelled data that require more assumptions, such as estimating emissions from waterborne travel using passenger data. Therefore, where possible activity data should be obtained from meter readings, organisational records, and sales from suppliers. Generally, basing emissions estimates on financial spending data will lead to the least accurate estimates. 

In many cases, however, such data may not be available at an island level, so aggregated figures at the national or local authority level must be used. In these scenarios, the data for wider geographic boundaries should be scaled by an appropriate factor. For example, factors can include scaling by population, the number of meters, land area, or the number of registered vehicles. 

Emission Factors

Emission factors for each activity are typically expressed as the quantity of a given GHG emitted per unit of the activity. Generally, emissions factors are taken from national and international guidelines, such as BEIS Conversion Factors for GHG Reporting or the IPCC Emission Factor Database. The use of specific emission factors is encouraged, particularly geographically specific factors for community-scale inventories such as those undertaken for the CNI project. In most cases, emission factors use modelled data. Measured data specific to the activity is preferred; however, due to practical constraints, verified data for each of the wide range of emitting activities considered cannot be achieved. It is important that these sources are verified or supported by peer-reviewed consensus in the scientific community. 

Using BEIS Conversion Factors from 2023, one metric tonne of CO2 is roughly equivalent to: 

Examples of Consumption activities that emit the equivalent of 1 t CO2e.

Emission Scopes

For community-scale GHG inventories, emissions are considered under three scopes. 

Scope 1 includes activities occurring inside the community’s boundary – in the case of the CNI project, this covers emission sources and sinks which occur on the island. 

Scope 2 captures emissions which are imported into the community, though occur outside its geographic boundaries. In this case, Scope 2 emissions come from grid-supplied electricity from the UK National Grid. 

Scope 3 emissions cover those entirely outside the community’s geographic boundary but exist due to consumption within the community. These include emissions from transportation and processing of fuel, and electrical losses from usage in the transmission and distribution system. Scope 3 emissions will also be reported as Scope 1 or 2 within their respective regional boundaries, introducing a risk of double counting. However, these remain important to report to allow effective GHG management.

Diagram illustrating emissions scopes for island communities

Data Quality

The GHG Protocol recommends three tiers of data quality for activity data and emission factors, summarised in the table below. 

Data Tier 

Activity Data 

Emission Factors 

Tier 1 

Highly modelled or uncertain activity data 

Default, or international emission factors 

Tier 2 

Modelled activity data using robust assumptions 

More specific, nationally derived emission factors

Tier 3 

Detailed activity data

Highly specific, or regional emission factors

Driving Principles

The GHG Protocol is built around the following core principles, which aim to ensure the robustness of GHG Inventories and allow wide applicability. 

Core principles of the GHG Protocol

In alignment with these principles, and to ensure effective benchmarking, the audits are intended to be island-led and replicable by local communities. By using local expertise, the data used is both robust and specific to the islands. Where possible, existing audits and supporting data have been used, and data has been provided by island residents or verified by local experts.  

Next Steps

Having completed the initial GHG audits, the emissions inventories are now undergoing a process of verification and refinement to allow accurate benchmarking. In addition to this process, the CNI project is working to build local capacity and skills to take on the work to refine and revise the GHG inventories going forwards and develop effective GHG management plans. 

Energy and Transport Carbon Audits