Why free range meat costs the earth

Feelgood food just got tricky. It was easy when 'good' meant anything which could have stepped off a John Constable canvas: free-range chicken, foraging pigs and grazing cattle. But then climate change came along.

Now it transpires that a meaty diet adds to greenhouse gases. The UN has calculated that livestock warms the planet more than transport.

First a few farmyard facts. Cows and sheep are ruminants, which means their digestion produces much methane, a gas with 20 times the global warming power per puff of carbon dioxide. It comes out in breath, burps and farts. Their manure is also heavy with nitrates which pollute both water and air. Pigs produce less gas but plenty of manure. Chickens eat and waste little.

There is also a vast difference in the efficiency with which these animals turn vegetable fodder into meat protein. Cows and sheep need eight kilos of grain for one kilo of meat, pigs about four kilos, while the most efficient poultry units need a mere 1.6 kg of feed for a kilo of chicken. Clearly, the less land you need to support each animal, the more you have left for anything else, like climate-friendly forests.

Fearful that the 'anti-carbon tyrants' will attempt to wipe their business from the planet, the meat industry has been looking for low greenhouse gas solutions. The problem is that many of them are found indoors.

Housed animals give humans control. The diet can be precisely manipulated to maximise growth and minimise polluting gases. Animals don't waste food energy on running about and keeping warm. Their manure can be collected and burned as a fuel, avoiding contamination of rivers.

A combination of precision husbandry and species advantage puts the humble chicken, raised in an indoor shed, right at the top of the climate chart.
Tom Heap presents 'Costing the Earth', BBC Radio 4, May 8 at 9pm ·