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20 November 2009
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Consumer goods in some countries are being increasingly labelled with
indications to show to buyers how much equivalent carbon dioxide is
required for the user to make use of the product. In the UK, for
example, a joint effort by the Carbon Trust, the British Standards
Institute and the Department for Food, Environment and Rural Affairs has
produced a
document that explains the methodology of determining the footprint
of a product from cradle to grave. As an example, they take the
croissant, hypothetically calculating 1.2 kg of carbon dioxide is
emitted for a packet of 12 croissants, using the following scenario:
An estimate is made of each item but, as is mentioned, the
diagram is simplified. Some components are omitted altogether:
Other raw materials include butter, which due to
its high emissions factor represents a higher proportion of the total
footprint than that suggested by its mass (and thus a higher proportion of
the overall product footprint than is suggested by these
results).
This illustrates the difficulty of calculating the carbon
footprint for a product. Is the wheat used grown locally, with low transport
costs, or imported from Argentina? What were the emissions for ploughing,
harrowing, sowing and harvesting? What pesticides and fertilisers were used?
I suggest that the croissant bakery could not do better than make a wild
guess, although the document cites what appears to be an arbitrary figure of
500 kg of equivalent emitted carbon dioxide per tonne of wheat, to which is
added an equally arbitrary 10 kg/tonne of wheat for the 100 km transport
from the farm to the flour mill. And so it goes on, all the way to the
retail outlet, where it is assumed that all the croissants are sold by the
sell-by date and that the consumer did not need to transport them home (what
is the distance from the supermarket to the home and what means of transport
are used?).
These questions are partially addressed in Appendix IV
which suggests that Monte Carlo analyses of probabilities be applied to
each variable to determine the uncertainty factor and then calculate the
overall probability. This is a rather glib answer, given that they did
not do this to their example. It would therefore appear that the
uncertainty factor for the example may really place the packet of 12
croissants on a rather flat Gaussian curve centred at 1.2 kg of carbon
dioxide but with a high probability between at least 600 g and 2.4 kg.
Is this kind of label really informative?
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