[...] Think globally, act locally?

Buying direct means producers get a fair price, with no middlemen adding big margins along the distribution chain. Nor has local food been shipped in from the other side of the country or the other side of the world, so the smaller number of "food miles" makes local food greener, too. Local food thus appeals in different ways to environmentalists, national farm lobbies and anti-corporate activists, as well as consumers who want to know more about where their food comes from.

Obviously it makes sense to choose a product that has been grown locally over an identical product shipped in from afar. But such direct comparisons are rare. And it turns out that the apparently straightforward approach of minimising the "food miles" associated with your weekly groceries does not, in fact, always result in the smallest possible environmental impact.

The term "food mile" is itself misleading, as a report published by DEFRA, Britain's environment and farming ministry, pointed out last year. A mile travelled by a large truck full of groceries is not the same as a mile travelled by a sport-utility vehicle carrying a bag of salad. Instead, says Paul Watkiss, one of the authors of the DEFRA report, it is more helpful to think about food-vehicle miles (ie, the number of miles travelled by vehicles carrying food) and food-tonne miles (which take the tonnage being carried into account).

The DEFRA report, which analysed the supply of food in Britain, contained several counterintuitive findings. It turns out to be better for the environment to truck in tomatoes from Spain during the winter, for example, than to grow them in heated greenhouses in Britain. And it transpires that half the food-vehicle miles associated with British food are travelled by cars driving to and from the shops. Each trip is short, but there are millions of them every day. Another surprising finding was that a shift towards a local food system, and away from a supermarket-based food system, with its central distribution depots, lean supply chains and big, full trucks, might actually increase the number of food-vehicle miles being travelled locally, because things would move around in a larger number of smaller, less efficiently packed vehicles.

Research carried out at Lincoln University in New Zealand found that producing dairy products, lamb, apples and onions in that country and shipping them to Britain used less energy overall than producing them in Britain. (Farming and processing in New Zealand is much less energy intensive.) And even if flying food in from the developing world produces more emissions, that needs to be weighed against the boost to trade and development. [...]