The results of your GHG inventory are not completely accurate. Quantifying these uncertainties isn't easy, as not all emission factor databases provide an uncertainty and there is a lack of consistency between the databases that do provide one.
Uncertainty Sources
There are several sources of uncertainty.
Inherent uncertainty linked to the computation of an emission factor
Lack of granularity of existing emission factors
Lack of consistency between similar emission factors provided by different sources
Incorrect selection of an emission factor
Granularity and precision of activity data (primary and secondary data)
Methodological choices
What is currently included in the uncertainties that are displayed?
Currently, only the uncertainty linked to the emission factor is included in the computation of uncertainties.
Activity data uncertainties (for instance: uncertainty on the total distance traveled by the vehicle fleet) will be included soon.
How are emission factors uncertainties computed?
As not all emission factor databases provide an uncertainty and there is a lack of consistency between the databases that do provide one, Greenly has implemented an internal process to link all emission factors to an uncertainty.
There are 5 levels of uncertainty: 5%, 15%, 30%, 50% and 80%. The level of uncertainty is selected depending on the type of emission factor and on an emission factor confidence score.
Confidence score (between 0 and 1)
Greenly Monetary Emission Factors
These ratios are computed by dividing emission factors provided by renowned databases (e.g. Ecoinvent, IEA) by the average price of the product, service or activity.
The confidence score is then selected depending on the bias of primary data that is used to compute the ratio and the variance of the category (e.g. a very specific sector with highly homogeneous products with have a low variance).
Input-output Monetary Emission Factors
These ratios are computed using input-output tables for a given sector and country.
The confidence score is then selected depending on the variance of the sector and on the relevance of the sector for the Greenly category.
Company Specific Monetary Emission Factors
These ratios are computed by dividing the GHG inventory of a company by its revenue. Sectoral averages can also be computed.
The confidence score is then selected depending on the reliability of the revenue and on the reliability, transparency and completeness of the GHG inventory.
Activity Emissions Factors
These ratios are provided by external sources.
The confidence score is selected depending on the reliability of the source. For example, a renowned and trustworthy database will have the highest score and a non peer-reviewed article a low score.
Uncertainty
Once the confidence score is computed for an emission factor, the uncertainty level is selected depending on the type of emission factor and the confidence score.
💡 Examples
An activity emission factor with a confidence score of 1 will have an uncertainty of 5%
A Greenly monetary emission factor with a confidence score of 0.1 will have an uncertainty of 80%
How are aggregated uncertainties computed?
The uncertainty linked to the total GHG emission amount, or the amount by Scope or regulatory category, is computed using a propagation of uncertainty formula.