Perhaps I should explain. There's a roadside fruit stall that sets up once a week on the route we cycle to work. Japanese fruit is really good, and we like to pick up a selection of whatever is in season. The stall mostly sells fairly local produce which seems to cover a wide range of items, but grapefruit is strangely absent - they are all imported from Florida, which seems odd for a country with such a strong tradition of citrus growing (eg satsuma). There also don't seem to be any local suppliers of that staple food of cyclists, the banana.
This NewScientist article from a few months back describes some interesting research on the hidden environmental costs of food production in the UK. It demonstrates that the total air and ship miles of imported food is actually an extremely small part of the overall problem, with lorry transport in the UK doing the lion's share of the damage. The real surprise to me, though, was the fact that the average person's shopping trips, 221 per year of 6.4km length (and almost all by car) causes about half as much harm as the lorries do. On reflection though, a 40-tonne lorry has a much larger payload as a proportion of vehicle weight than a 1-tonne family car with a bag of shopping, and the car trip will also take place in congested urban environments, so the per-tomato environmental cost of each mile in the car will be much higher than for the lorry.
Back to the melons. The sheer effort of transporting them a few miles also made me think about the extent to which agriculture relies on oil as a raw input. A bit of surfing finds energy input:output ratio estimates of about 10:1 overall from some alarmist "peak oil" type sites (ie 10 times as much fossil fuel energy is used in production as is created in food calories), and more industry-friendly estimates for the most efficient foods (bread) give a more optimistic picture, such as this estimate of an energy ratio of 1:5 in favour of food output. Probably the truth is somewhere between the two. In any case, even the most optimistic estimates underline to what extent our society has been based on ever-increasing increasing supplies of cheap energy.