By Richard Black
Environment correspondent, BBC News
Livestock's Long Shadow calculated meat-related emissions from field to abattoir
UN specialists are to look again at the contribution of meat production to climate change, after claims that an earlier report exaggerated the link.
A 2006 report concluded meat production was responsible for 18% of greenhouse gas emissions - more than transport.
The report has been cited by people campaigning for a more vegetable-based diet, including Sir Paul McCartney.
But a new analysis, presented at a major US science meeting, says the transport comparison was flawed.
Sir Paul was one of the figures launching a campaign late last year centred on the slogan "Less meat = less heat".
But curbing meat production and consumption would be less beneficial for the climate than has been claimed, said Frank Mitloehner from the University of California at Davis (UCD).
"Smarter animal farming, not less farming, will equal less heat," he told delegates to the American Chemical Society (ACS) meeting in San Francisco.
"Producing less meat and milk will only mean more hunger in poor countries."
Leading figures in the climate change establishment, such as Intergovernmental Panel on Climate Change (IPCC) chairman Rajendra Pachauri and Lord (Nicholas) Stern, have also quoted the 18% figure as a reason why people should consider eating less meat.
Apples and pears
The 2006 report - Livestock's Long Shadow, published by the UN Food and Agriculture Organization (FAO) - reached the figure by totting up all greenhouse-gas emissions associated with meat production from farm to table, including fertiliser production, land clearance, methane emissions from the animals' digestion, and vehicle use on farms.
But Dr Mitloehner pointed out that the authors had not calculated transport emissions in the same way, instead just using the IPCC's figure, which only included fossil fuel burning.
Land in the Amazon Basin is routinely cleared for cattle ranching
"This lopsided 'analysis' is a classical apples-and-oranges analogy that truly confused the issue," he said.
One of the authors of Livestock's Long Shadow, FAO livestock policy officer Pierre Gerber, told BBC News he accepted Dr Mitloehner's criticism.
"I must say honestly that he has a point - we factored in everything for meat emissions, and we didn't do the same thing with transport, we just used the figure from the IPCC," he said.
"But on the rest of the report, I don't think it was really challenged."
FAO is now working on a much more comprehensive analysis of emissions from food production, he said.
It should be complete by the end of the year, and should allow comparisons between diets, including meat and those that are exclusively vegetarian.
Organisations use different methods for apportioning emissions between sectors of the economy.
In an attempt to capture everything associated with meat production, the FAO team included contributions, for example, from transport and deforestation.
By comparison, the IPCC's methodology collects all emissions from deforestation into a separate pool, whether the trees are removed for farming or for some other reason; and does the same thing for transport.
This is one of the reasons why the 18% figure appears remarkably high to some observers.
The majority of the meat-related emissions come from land clearance and from methane emissions associated with the animals' digestion.
Other academics have also argued that meat is a necessary source of protein in some societies with small food resources, and that in the drylands of East Africa or around the Arctic where crop plants cannot survive, a meat-based diet is the only option.
Dr Mitloehner contends that in developed societies such as the US - where transport emissions account for about 26% of the national total, compared with 3% for pig- and cattle-rearing - meat is the wrong target in efforts to reduce carbon emissions.