Food prices do not reflect all the environmental costs of food production and trade. These costs include such things as the environmental damage due to fertilizer, pesticide and water use, land conversion and food transportation. Some people argue that we should buy locally produced food in order to avoid the environmental costs of transportation.
At Gristmill, Clark Williams-Derry reviews quantitative data on the the relative costs of food production and transportation. In many cases the environmental costs of transportation are very small compared to the costs of food production, and do not justify buying local. In other cases the environmental costs of food transportation are substantial relative to the costs of food production. Williams-Derry believes that in these cases it would be good to buy local. I disagree.
We lower the environmental costs of food production by growing each kind of foodstuff where it is more efficient (on average) to do so. For example, we tend to match plant varieties to climates and soils. If we grew everything everywhere, so that everybody could always buy local, we would need more pesticides, fertilizer, water, energy and land to produce the same amount of food. The rise in the environmental costs of production under a "buying local" regime would more than offset the environmental benefits of reduced transportation.