The man behind one of the most influential reports on climate change, Lord Stern, has highlighted the impact meat production has on greenhouse gas emissions. Part of it comes through methane made by the animals as they digest food. So which farm animals expel the most methane?
A diet that relies heavily on meat production results in higher emissions than a typical vegetarian diet, says Lord Stern.
The author of the 2006 Stern Review into the cost of climate change attacked the "enormous pressure" meat production puts on the world's resources and said people were becoming increasingly aware about "low carbon consumption".
He told the BBC that cutting greenhouse gas emissions was important across the board, in areas such as electricity, transport and food.
In a 2006 report, the UN Food and Agriculture Organization (FAO) concluded that worldwide livestock farming generates 18% of the planet's greenhouse gas emissions. By comparison, it said, all the world's cars, trains, planes and boats accounted for a combined 13% of greenhouse gas emissions.
The greenhouse gas emissions associated with meat consumption has many components, the largest of which is land use change - the clearing of forests for pasture or for the production of soya for animal feed. Other elements that have an impact on emissions include the rearing and slaughter of livestock, and the transport, refrigeration and cooking of meat.
There is also the nitrous oxide, a greenhouse gas, in the manure of animals reared for meat and the methane, another greenhouse gas, in their flatulence. Molecule for molecule, methane has a much larger warming effect than carbon dioxide.
Western cows emit only 15% of methane expelled by cows
As the diagram above shows, methane emission is dramatically higher in cows (primarily from belching) than other animals. But cutting back on eating meat is not the simple answer, say scientists.
For a start, many of the cows responsible for producing methane are not reared to be eaten, according to Elaine Matthews, a methane expert at Nasa's Goddard Institute for Space Studies.
The breeds favoured in non-western countries are often bred for other uses, such as work, and these non-western cows are far more numerous than the larger dairy varieties reared in North America and Europe.
The larger western cows actually produce more methane per cow than their smaller non-western breeds, but because there are fewer of them, they only account for about 15% of all the methane produced by cows in general.
Meat output 'doubling'
Ms Matthews also says the quantity of methane they produce depends on the quality of food they are given. Cows that eat grain, she says, produce less methane than cows grazing on wild grass.
HAVE YOUR SAY
The simple fact of the matter is we don't need to eat meat, so why do we? Without it we can save releasing dangerous gases and save the cruelty of the meat trade
And methane is not the most important consideration in relation to livestock, says Friends of the Earth - it's the intensity with which they are reared.
According to the environmental pressure group, methane from livestock accounts for about 6% of greenhouse gas emissions, with 6% from CO2 released when forests are cleared for pasture and to produce soy for feeds.
What is clear is that people are eating more meat and dairy products every year.
Global meat production is projected to more than double from 229 million tonnes in 1999/2001 to 465 million tonnes in 2050, while milk output is set to climb from 580 to 1043 million tonnes.
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