So you’re trying to be an ethical shopper? Your weekly shop now includes organic fruit and vegetables, free-range chicken, Fairtrade wine and coffee, environmentally friendly washing powder - and no ready meals.

But a recent edition of Costing the Earth, the BBC Radio 4 programme, revealed that being an ethical shopper is not only costly but increasingly complex. A microwaveable chicken ready meal, for example, has a lower carbon footprint than cooking your own raw chicken because the roasting process produces more carbon dioxide.

The Department for Environment, Farming and Rural Affairs says that 56 per cent of the carbon emissions associated with food come from its production - including the diesel the farmer uses in his tractor and the carbon emitted by rotting plants. And then there are the worst offenders of all - cows, one of the biggest emitters of greenhouse gases. All that methane they produce is even more harmful to the environment than carbon. On that score, the only way to make your shopping basket greener is to reduce the amount of milk and beef you buy.

These are just some of the dilemmas the Carbon Trust is wrestling with as it tries to come up with a universal carbon labelling system for groceries within the next couple of years. But where does this leave the consumer? Grocery costs are rising and families are pound;1,300 a year worse off, according to a report for the Centre for Policy Studies. Food miles and ethical issues may be the last things on your mind.

One way to count costs - and compare the price of organic products - is to use www.mysupermarket.co.uk, a shopping site that compares prices and in-store deals at rival supermarkets.

You would think that Fairtrade products, which allow the world’s poorest to enjoy some of the benefits of a globalised market, would be immune from this debate but, alas, no. It would be more environmentally sound, for instance, for Chilean Fairtrade wine to be shipped to Britain for bottling - but then that would put lots of people out of jobs in Chile.

On the other hand, produce grown under the Kenyan sun by a farmer who does not even own a car is arguably more carbon-efficient, even with the food miles thrown in, than the same produce being grown in vast energy-hungry greenhouses in Europe.

So what’s a shopper to do? If keeping costs down is your priority, make tracks to Asda, so says mysupermarket.co.uk. If ethical issues are uppermost, seek out your nearest Co-op outlet. The company has tackled its contribution to climate change on every level, winning the British Renewable Energy Pioneer award for 2007 and the Queen’s Award for Enterprise in the sustainable development category.

So, ready meal or free-range chicken tonight? The decision might not be that easy, but it’s certainly food for thought.

You could always ...

- Rent an allotment for between pound;10 and pound;35 a year, depending on where you live. Contact your local council.

- Grow tomatoes, rocket and herbs in your back garden.

- Set up a chicken run. For pound;395 you could have two chickens, a fox-proof run and all the equipment (including eggboxes and a tutorial) to produce eggs in the back garden. See www.omlet.co.uk for advice on keeping chickens.

Find out more

Listen to the Radio 4 programme at www.bbc.co.ukradio4sciencecosting theearth.shtml

Calculate your carbon footprint at www.carbontrust.co.uk

Other links:

www.fairtrade.org.uk

www.soilassociation.co.uk

www.cooponline.coop.