It's a common enough trope: x causes problems so we've got to do something about x. Fair enough you might say, but it does rather depend upon how much of a problem x actually is and how many other problems (of what magnitude) there will be with the solutions. One example might be the vexing issue of food miles. To hear some talk the very idea of eating food from a field you couldn't walk to is what is killing Gaia: more rationally, yes, of course the transport of food creates emissions and yes, there's some reason to think that this might cause problems.
However, we're also aware (Adam Smith pointed out in 1776 that you can grow grapes and make wine in Scotland but why bother when Bourdeaux will happily do it better and more cheaply for you?) that different methods and modes of food production have different emissions patterns as well. So what we really might want to know is what is the relative damage done by the transport and or the growing?
We find that although food is transported long distances in general (1640 km delivery and 6760 km life-cycle supply chain on average) the GHG emissions associated with food are dominated by the production phase, contributing 83% of the average U.S. household’s 8.1 t CO2e/yr footprint for food consumption. Transportation as a whole represents only 11% of life-cycle GHG emissions, and final delivery from producer to retail contributes only 4%.
Not a lot is the answer. In fact, they're a trivial contributor. If a method of production has lower emissions (as, for example, we've found out that Spanish tomatoes as against UK hothouse ones, or New Zealand lamb as against UK grown), even if only slightly lower, then consumption of those transported and even imported foods lowers total emissions.
Good, so that's the end of food miles as a measurement of the sanctity or not of consumption habits. There's really only one thing left for the locovores to insist upon, which is that we should eat only those items which are grown locally and in season: no hothouses, no artificial conditions and no imports. I can't see that catching on to be honest (does anyone know of anywhere in the UK that you can raise tofus? And what do they graze upon anyway?) but if it does I'm going to write a recipe book.
"101 Exciting Things to do with Turnips", because for months of the year on our damp and chilly islands, that's all there is to eat that's local and not hothoused.