Calculating Food Miles
Looking at existing sources and studies, we found that a standard formula had been developed to calculate "weighted average source distances". Translated into regular English, this basically means "the average distance that food travels from where it's produced to where it is consumed".
The standard formula for the WASD is:
WASD = S (m(k) x d(k))
------
S m(k)
where:
k = different location points of the production
m = weight (amount) from each point of production, and
d = distance from each point of production to each point of use (or sale).
This looks more complicated than it is. The basic idea is that you find out, for a particular commodity, the various places that it is imported from (say, for tomatoes, Mexico, California, and Texas) and the amount that comes from each place. Taken together with the distances from those places, this can produce the average distance an imported tomato has come.
This formula, however, didn't cover all of our needs. Particularly, it calculates distance--not greenhouse gas emissions. Also, it doesn't distinguish between different modes of transport (e.g., marine, truck, rail, air) even though these modes have vastly different fuel requirements and hence GHG emission rates. (For a given mass of freight, trucking is relatively energy intensive as compared to, for instance, marine transport.)
So, in addition to calculating a weighted average source distance, we came up with another measure, which we've dubbed a "weighted average emission ratio" (WAER). The formula is as follows:
WAER = S (v(k) x S (d(k)(m) x e(m))
------
S v(k)
where:
k = different location points of the production,
v = value of imports from each point of production
m = mode of transport (air, truck, marine, or rail)
e = GHG emission level, and
d = distance from each point of production to each point of use (or sale).
Basically, we've taken the WASD and added in consideration of what transport mode is used and a way to turn distance into GHG emissoin levels. We've also substituted value for mass in the imported foodstuffs, just because these data are easier to come by. What comes out of this formula is the mass of GHGs produced for a given mass of imported product--a simple ratio that can be easily understood. For example, imported grapes might have an WAER of 1.2, meaning that 1.2 kilograms of greenhouse gases are emitted in transport for every kilogram of grapes imported.
So to calculate food miles, we needed three distinct datasets:
- Imports of locally-produced foods broken down by source country and amount.
- Distances from import sources (countries, provinces, states) broken down by transport mode
- Emission levels for each mode of transport (i.e., the amount of greenhouse gases emitted per kilometre travelled per kilogram transported)