Tara Garnett, director of the Food Climate Research Network, University of Surrey, says we could be paying too much attention to food miles and missing the real culprit

The Stern Report has rewritten the economic agenda. Ignoring climate change is not an option. Every section of society must do something about it, and fast.

That includes the food industry. More and more of us now know that food contributes substantially - about 20% - to our climate changing emissions.

The automatic villain is 'food miles' and its demons - Kenyan dwarf beans, Christmas strawberries and cauliflowers being trucked half way round Britain for packing, only to return to a supermarket down the road from where they are grown.

And it's true, food miles and their environmental impact are on the increase. Worse, the volume of food travelling by air, while still small, is growing rapidly. According to Defra, though we only fly in 1% of our food, this tiny quantity accounts for 11% of all CO2 associated with food transport.

The problem is, paying too much attention to food miles can distract attention from the real culprit. Study after study shows that meat and dairy products account for half of all food-related greenhouse-gas emissions - in other words, some 10% of the UK's entire climate- changing emissions. That's nearly as much as emissions from all the cars (13%) on our roads today.

Energy is used to grow and produce animal feed, to transport it, to house livestock, and to chill and distribute the resulting meat and dairy products.

All these stages in the supply chain will produce CO2. However, buying locally-reared beef is not going to solve the problem, since the main source of livestock-related emissions is, well, biological.

Put bluntly, cows and sheep burp and fart a lot and in so doing they produce methane, which has a warming effect 21 times more potent than carbon dioxide. Livestock of all kinds are also responsible for emissions of nitrous oxide, which has a warming potential 310 times greater than CO2.

So can we actually alter basic animal biology to reduce emissions? We can tinker a bit with the feed, with the way we breed, house and manage livestock and with what we do with the manure and urine it produces.

This is all useful, and Defra is funding various projects to look at what we can do.

But at the end of the day, it's all down to numbers. If we are to reduce greenhouse gas emissions, we will need to rear fewer animals and consume less meat and milk.