One way to cut back on food miles would be to grow pineapples on Ben Nevis. Technically, it would be possible to do so, if you produced enough heat and covered the growing area with enough glass or plastic. Customers in the local supermarket, were you to undertake such a foolish enterprise, would then have the chance to buy your locally grown pineapples (at an enormous price) with next-to-nil food miles. And some of them might equate "locally grown" with "environmentally friendly."

Food miles are the number of miles a product travels from farm gate to plate, with some of those miles being far more environmentally unfriendly than others. For instance, deciding to walk to the supermarket instead of driving there could well have a bigger impact on the environment than deciding against a particular bag of Kenyan beans because it was grown far away.

Then there is the question of how to carry your food home. The leader of Oxford City Council, John Goddard, this week launched a campaign to make Oxford the UK's first plastic bag-free city. His announcement of the initiative coincided with an event at the Town Hall called Climate Change Oxford: What You Can Do Now, aimed at encouraging practical action to tackle the issue.

Mr Goddard said: "We need to encourage shoppers to say 'I don't want a plastic bag' and encourage supermarkets to make the shift. What it needs is willing co-operation and shopper pressure."

He added, virtuously enough: "I refuse plastic bags if I am offered one and I want as many shoppers as possible to follow suit."

But what are the environmental consequences of food miles and plastic bags? Oxfordshire company AEA Energy and Environment, part of AEA Technology, has made detailed studies of both for the Government. Its conclusions might help those who stare at supermarket shelves to decide which products are the most socially and environmentally sensible.

The studies are just two of many undertaken by AEA Energy and Environment, which employs 400 people, most of them at its Harwell headquarters. The food mile report, commissioned by the Department of the Environment, Food and Rural Affairs in 2005, is suddenly topical as this week the UK's largest certifier of organic food, the Soil Association, waded into the debate by announcing that it might withdraw its approval from food air-freighted to the UK (that bag of Kenyan beans).

AEA found that air freight of food is rapidly growing and has a higher environmental impact than any other transport mode. It grew by 140 per cent between 1992-2002. However, only one food mile in 1,000 is travelled by air. The greatest contribution to food miles, and one the consumer cannot gauge when in the supermaket, is when food is carted to and from central depots in lorries. In other words, if our Scottish consumer decided to buy those pineapples because they had been produced locally, it might turn out that they had been to a distribution centre in Strathclyde before reaching his supermarket.

Lorry miles have increased by 36 per cent since 1991 and doubled since 1974. In 2002 food transport accounted for 25 per cent of all UK lorry miles - although increased efficiency meant that the amount of carbon dioxide emitted per item of food did not increase.

So where does all this leave the consumer? Buy from as close as possible if whatever you are buying grows well here. For instance, the report points out that except in high summer it could be better from a climate change point of view to buy tomatoes grown in Spain than in Britain, which needs energy and polytunnels to produce them.

And so to the vexed question of plastic bags. AEA's report into the subject was commissioned by the Scottish Exectuve after a member of the Scottish Parliament had tabled a Members Bill, inspired by the so-called PlasTax in the Republic of Ireland, where customers pay for bags in supermarkets.

The report kicks off: "Advocates of a levy on plastic bags cite the main benefits as being reduced littering (including marine litter), reduced use of resources and energy, lower pollutant emissions and increased public awareness of environmental issues.

"Opponents argue that lightweight plastic carrier bags are hygienic, convenient and durable, that they are often reused for other purposes, that they form only a small part of the litter stream and that they have a lower overall environmental impact than paper bags."

The report, produced in 2005, found that provided all bags (not just plastic ones) are taxed, a levy would indeed cut back on consumption of non-renewable energy, atmospheric acidification, the formation of ground level ozone, and the risk of litter. However, it added, surprisingly, that the environmental advantages would be modest: a decrease in total household waste of about 0.23 per cent.

AEA estimated the levy would lead to between 300-700 job losses in Scotland, most of them in small and medium-size enterprises. Main beneficiaries would be the supermarkets, who would not have to buy the bags that many of them now supply free. Instead they would be presented with an opportunity to sell more so-called 'bags for life'.

All in all, Councillor Goddard's common-sense approach makes sense: take your own bag to the shops. Environmentalists Friends of the Earth say that eight billion plastic bags are handed out in UK each year, or 134 to each person.

They add that small gestures like refusing plastic bags will help us to "think green" in bigger issues, such as walking, instead of driving, to the supermarket. And don't buy pineapples grown in Scotland.