Guide to PAS 2050 How to assess the carbon ... - Aggie Horticulture

Guide to PAS 2050 55
Appendix IV
Uncertainty
analysis
With zero uncertainty, there is no variation in the
carbon footprint assessments (illustrated below, left). In
this ideal scenario, the two product footprints can be
compared, and users of the footprint information can
be confident their decisions are based on accurate
data.
However, uncertainty creates challenges for comparisons
and decision making as illustrated below, right.
Uncertainty in carbon footprinting comes from two
sources: technical uncertainty and natural variability.
Technical uncertainty is created by limited data quality,
ineffective sampling, wrong assumptions, incomplete
modelling and other flaws in the footprint calculation
itself. These factors are analysed in the uncertainty
calculation described overleaf. Natural variability is
Probability of footprint value
1
Zero uncertainty
400
Product A
Footprint (g CO 2 e)
600
Product B
accounted for in the definition of a product carbon
footprint as an average, or representative figure, so it
does not need to be quantified.
Because the nature of a footprint calculation involves
estimates and judgement, every model input has some
degree of uncertainty associated with it. Each input
has a probability distribution around the mean value,
or the number used in the model. The distribution
curves can take any shape, e.g. normal (as in the
example below).
Probability of footprint value
200 400 500 600 700
Footprint (g CO 2 e)
Uncertainty in this example is the value along the
x-axis greater or less than the products’ footprint
estimates of 400 and 600.
Product A
Product B
Product A has greater uncertainty than Product B.
Higher uncertainty in footprint result = lower
confidence in comparisons