And we already know this is an issue. While there are plenty of problems in modern agriculture systems, and ultimately I think retaining at least some degree of local production is a good thing, the environmental impacts of farming, when honestly evaluated, don’t come out to a simple “local/organic good, conventional/far-away bad” result. Economies of scale and efficient supply-chains can reduce environmental impacts and generate a smaller carbon footprint. Local small scale farmers may be doing their work with, for instance, old tractors that burn more gasoline or diesel fuel and are then transporting them to local farmer’s markets in smaller trucks, again which increases the carbon footprint per poundage of product.
Similarly conventional agriculture has recent developments that allow things like no-till or low-till farming methods, which is better for soil health and prevents run off.
There are things we (and the agricultural system) can learn from each other if we use rational methods and rely less on emotional arguments. While I think there are some things worth promoting that food miles do address, ultimately it does more to elicit an emotional or nostalgic response among a subset of consumers that may not be based on sound science in terms of environmental impacts.