If you buy a packet of Waitrose blueberries from Chile, it’s a crime against humanity. If you nibble mange tout from Africa, you’re practically murdering the planet. And if you eat apples from New Zealand, well, you’re in league with the devil.

Why? Food miles, of course. It’s obvious that if you buy food from thousands of miles away, the transport alone must consume vast amounts of energy, thus fuelling climate change and global meltdown. As any concerned citizen knows, think green, think local.

Or think again. Researchers are finding that food miles are far from the whole story when assessing the environmental impact of what we eat. At a conference last month on the economics of food, Chris Foster of Manchester Business School presented some startling conclusions from a review of the evidence.

The biggest environmental impact of many food products, he said, came from their production. Bulk transport by land or sea was of “low significance”. And he suggested that policy-makers should “critically unpick the ‘local food’ agenda”.

Foster points out that local production and a distribution system involving lots of vans and cars miss the environmental benefits of economies of scale. Just over a ton of goods moved six miles as part of a 22-ton lorry load generates about 14oz of CO2; moved in 50 cars, each carrying 40lb, it generates about 22lb of CO2 To many environmentalists he sounds like a heretic to be burnt at the stake (with local deadwood, naturally, and carbon capture). Food miles have, in their eyes, been a concern ever since a campaigning environmental group reported in the early 1990s that the distance travelled by our food had increased by half, but the quantity had remained roughly the same. That was partly caused by growing imports and was partly a result of supermarkets trucking food to central depots and then sending it back for sale at stores near the place where it was produced.

More recently, as fears of climate change have hotted up, foreign food miles have been an easy concept for consumers to grasp. Local good – long-haul bad.

To get a broader view, researchers now prefer what they call the “life cycle assessment” (LCA) of food products. LCA tries to encapsulate the whole environmental impact of growing, transporting, selling and consuming a product – from farm to fork.

The results are often counter-intuitive. Tomatoes grown in the natural heat of Spain have less “global warming potential” (GWP) than out-of-season British tomatoes grown in heated greenhouses.

Even more surprisingly, researchers in New Zealand claim that antipodean lamb and apples use less energy – even after being transported 12,000 miles – than the same products from Britain. A study by Lincoln University in New Zealand compared the use of fuel, electricity, pesticides, fodder, transportation, storage and other items and calculated that a ton of New Zealand apples generated the equivalent of 407lb of CO2 compared with almost 600lb for UK apples.

The difference was even more marked in lamb. The study claimed that a ton of New Zealand lamb carcass generated more than half a ton of compared with about three tons for British lamb. Much of the hugeCO2 disparity was down to the use of electricity and fertiliser in rearing the British lambs. The shipping of New Zealand lamb to the UK accounted for . only 273lb of CO2 Peter Gordon, a New Zealand chef who runs Providores restaurant in Marylebone, central London, believes that such considerations justify using imported products. “We source lamb from New Zealand as well as Wales. Food miles is a great term, but in reality the big issue is sustainability,” he says. “Consumers will look at a pineapple from Ghana and won’t buy it because it has terrible food miles. But the Ghanaian farmer has a tiny carbon footprint.”

Various bodies, including the Department for Environment, Food and Rural Affairs and the Carbon Trust, are trying to formulate a method for calculating LCAs for consumer labelling. A pilot scheme involving Walkers Crisps, Boots and Innocent drinks started last year and has expanded to include food giants such as Tesco and Coca-Cola.

Tomorrow the trust will announce that seven new companies, including British Sugar and Morphy Richards, are joining the scheme, either to test the measuring of their products’ carbon footprints or adopt the labelling.

Before you give up all hope of shopping ethically, hold fast. There are some simple guidelines. According to Tara Garnett of the Food Climate Research Network, 90% of the greenhouse gas emissions associated with meat occur during its production; meat is a large source of methane and nitrous oxide, gases which have a . In other GWP far higher than CO2 words, it doesn’t matter so much where the meat comes from, but how much you consume. “In a way the message is: eat less meat,” she says.

Air-freighted food is, in almost all cases, bad. It accounts for less than 1% of food transport but 11% of the from all food transport. So if it has arrived by plane, rather than seaCO2 or road, avoid it.

Packaging (or lack of it) is another area where consumers can readily make a difference, along with waste. As much as one-third of food bought in the UK is not eaten, which means that it generates greenhouses gases in its production and its decay. So shop little and often, use your leftovers and compost scraps.

However, for those who really want to make a difference, the LCA is the future. So please note: during the production of this article, no blueberries or other air-freighted products were consumed.

Additional reporting: Roger Waite

The green teen myth

“The environment’s screwed, trust me,” announces my 16-year-old friend when I ask him whether he wants to join the rather humble environmental society of my school. He asks the opinion of another friend who agrees that “yes, it’s f*****”. I continue my efforts to recruit more people but it’s depressing, writes Ottilie Wilford.

Growing up in my household, in which my father insists on driving his 4x4 to the gym every day – a 10-minute journey – I have come to accept that adults are mostly beyond repair. It’s up to our generation to preserve this beautiful planet.

However, the truth of the matter is that only very few of us actually care.

How can it be possible for me to attend a school that holds some of the brightest minds in the country and still feel that I am surrounded by idiots? My one eco-friend agrees that the rest are “in denial, they are lazy and like to live in their comfort zone, they like to pretend everything’s all right when it’s not”.

I’m always telling people to spurn plastic bags, to turn off lights, to reuse paper. But my attempts to make being eco-friendly more accessible to my age group seem futile. Take shopping, for example: for the most part I try to buy clothes secondhand as an alternative to shopping at global chain stores. But my efforts to convert my friends have been met with remarks such as “Why can’t Primark just go vintage?”

I sometimes feel as if I’m talking to people with their iPods on full volume, but I am determined to get their attention. I know that people may find me strange or exasperating, but I don’t care.

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