By Tom Heap
Costing the Earth, Radio 4
Cows consume 8kg of grain for 1kg of meat
Feel-good 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. No one noticed at first, still concentrating their fire on the obvious targets like 4x4s, long flights and coal power stations; but our meaty diet is laden with greenhouse gases, and trying to reduce them throws up some unpalatable choices.
It has prompted the Vegetarian Society to take out adverts in the paper declaring that our carnivorous tastes are a "silent but deadly" assault on our climate.
First a few farmyard facts. Cows and sheep are ruminants which means their digestion produces much methane, a gas with about 20 times the global warming power per puff than 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 they turn vegetable fodder into meat protein; and the less land you need to feed each animal, the more you have left for anything else - like climate-friendly forests.
Cows and sheep need 8kg of grain for every 1kg of meat they produce, pigs about 4kg. The most efficient poultry units need a mere 1.6kg of feed to produce 1kg of chicken.
The UN's food and agriculture organisation has added all this up and decreed that livestock warms the planet more than transport.
So in fear that the "anti-carbon tyrant" might wipe their business from the planet, the meat industry has been looking for low greenhouse gas (GHG) solutions, and the problem is that many of them are found indoors.
Housing animals gives humans control. The diet can be precisely manipulated to maximise growth and minimise polluting gases.
Animals do not waste food energy on running about and keeping warm. Their manure can be collected and burned as a fuel, avoiding damaging evaporation and seepage into rivers.
In the future, it is hoped that sealed barns would have exhaust vents where the harmful gases could be captured before they entered the atmosphere.
This combination of precision husbandry and species advantage is what puts commercial indoor poultry sheds at the top of the climate chart.
Peter Bradnock of the British Poultry Council says: "Organic poultry meat has about 45% more global warming potential than indoor-reared poultry meat.
"If you're rearing outside, then the bird is using a little more of its feed to keep itself warm, or simply to keep itself cool in hot climates."
There is a further hiccup with the vegetarian option: most of those who avoid meat source their protein from dairy foods.
And dairy animals pump out gases and gobble up supplementary feed just like the rest.
If you are avoiding meat for climate reasons, you should be shunning dairy too.
But as abandoning it is unlikely, are there any reduced GHG options? Yes; but once again, they are not the rustic visions you see in the adverts.
Jon Moorby of the Institute of Grassland and Environmental Research at the University of Aberystwyth believes that intensive indoor dairy farms are more climate friendly than their outdoor brethren.
"In general, intensive dairy farms are actually quite good for the environment, because it allows us to control what we do with what comes out of the cow much better than in a more extensive system," he says.
"With the animals being inside all the time, it allows us to control the manure and slurries from them much better than we can when they're outside."
But that goes contrary to a basic belief that animals should be allowed to range outdoors in as natural an environment as possible.
"Your Holstein dairy cow is almost like your Formula One car of the bovine world, and you wouldn't keep your Formula One car outside under a bit of plastic, would you?" counters Jon Moorby.
"So you've got to nurture them. Being inside, especially when it's bad weather, is the best thing for them."
Find that all too distasteful? Then try this: upland-grass-fed sheep and cattle graze on land where little else will grow and their feeding habits actually encourage wildlife.
So they may be a bit gassy but at least they are not demanding that distant lands be ploughed, and they are nibbling a home for birds, butterflies and beetles.
Vegetarians argue that the simple answer is to avoid meat; but altering the diet of billions is a tall order and would rock the lives of millions of farmers.
But in negotiating the place of animals in a low carbon world, we should remember that, unlike cars, they are life forms and deserve respect.
Costing The Earth will be broadcast at 2100 BST on Thursday, 8 May, on BBC Radio 4 and repeated at 1500 BST on Friday, 9 May, at 1500 BST. You can also listen online for 7 days after that at Radio 4's Listen again page.