"Food miles" are a concept that is often tossed around in environmentally concerned circles these days. You may have even seen celebrity chefs discussing the concept of food miles during their cooking shows, or seen advertisements that make you aware of the food miles issue.
Basically, the "food miles" argument states that food grown locally is more environmentally friendly. The reason usually given for this is that imported food (e.g. New Zealand lamb, apples and onions imported to the UK) requires a lot of fossil fuels (and produces a lot of carbon emissions) to get it to the market. Locally grown food, however, requires less fossil fuel and therefore puts out less carbon/greenhouse emissions and is better on a global scale.
However, the food miles argument has a few flaws in the reasoning. One popular image that has buzzed around the media regarding the "New Zealand apples to the UK" issue is the idea that the produce is carried by air. Air transport is a very high producer of greenhouse gases (although the aviation industry is working on improving this) and the concept of produce being literally flown from one side of the world to the other is repellent to many environmentally aware people. However, regarding the examples of New Zealand lamb, apples and onions going to the UK, these items are not transported by air at all. Instead, they are freighted by ship, which uses less fossil fuel (and hence has less emissions) per kg of produce. And, as a few people have pointed out, the amount of fossil fuel used to import produce pales beside the amount of fossil fuel consumed taking the produce from the point of entry into the country to the depot, to the supermarket and then to the consumer's home!
Secondly, the food miles/carbon emissions argument only considers one aspect of the lifecycle of produce - the process of getting the produce from A to B. One study (which, it should be admitted, was undertaken by an agricultural university within New Zealand) questioned the food miles argument from this perspective, taking the whole life cycle into account as it compared locally grown lamb, apples and onions (plus a few others) in the UK with the same items imported from New Zealand. This study showed that the methods used for producing these food items in New Zealand had much lower carbon emissions than the UK methods, so much so that they cancelled out the emissions produced by shipping the produce. For example, sheep in the UK are often zero-grazed in barns over winter, requiring extra feed to be bought (and brought) in from off the farm. In New Zealand, by contrast, sheep are kept outdoors year round, with supplementary feed usually being produced on the farm and requiring minimal transport to bring the food to the animals.
However, it's only fair to mention here that I'm a New Zealander myself and I've spent a few summers working in the horticultural sector!
The real strength of the food miles argument does not lie in the greenhouse gases/fossil fuels argument but in the aspect of health and quality of produce. Locally produced food is always fresher, which means that it usually has more vitamins and antioxidants in a useable form. This means that they're better for you. Besides, fruit and vegetables that have been grown locally are probably allowed to ripen naturally for longer, rather than being picked under-ripe to reduce spoilage in transit. They may also not undergo certain treatments to increase their shelf life.
What's more, locally grown vegetables usually vary according to season. What's in season and grown locally is usually cheaper. This is a way to save money and to add some variety into your diet. While you will not be able to buy locally grown tomatoes in the middle of winter, you will find other things that are tasty and healthy which are in season and locally grown (e.g. pumpkins, swedes or beetroot). You can also take advantages of in-season specials to preserve produce for later use (freezing is easy).
According to traditional Chinese medicine, in season foods have the right "stuff" in them (I forget the correct term used by TCM) to enhance your health at that time of the year. Winter vegetables will help you deal with winter ailments, according to this theory, and summer vegetables are best for summer health. Whether you follow the tenets of TCM or not, it is certainly true that the root crops available locally in winter are ideal for warming stews and roasting, while summer vegetables are better for cool salads.
What's more, buying locally is also good for your local community. You are supporting farmers in your area and boosting your local economy. Farmers' markets are the best place for this, as you get to meet the person involved with growing the food you buy and can build a relationship with them. This is great for fostering community spirit. What's more, farmers' markets use less packaging (less waste) than supermarket goods and often give a fairer price to the grower, even though you may not notice much difference in price.