By Richard Black
Environment correspondent, BBC News website
Livestock production has a bigger climate impact than transport, the UN believes
People should consider eating less meat as a way of combating global warming, says the UN's top climate scientist.
Rajendra Pachauri, who chairs the Intergovernmental Panel on Climate Change (IPCC), will make the call at a speech in London on Monday evening.
UN figures suggest that meat production puts more greenhouse gases into the atmosphere than transport.
But a spokeswoman for the UK's National Farmers' Union (NFU) said methane emissions from farms were declining.
Dr Pachauri has just been re-appointed for a second six-year term as chairman of the Nobel Prize-winning IPCC, the body that collates and evaluates climate data for the world's governments.
"The UN Food and Agriculture Organization (FAO) has estimated that direct emissions from meat production account for about 18% of the world's total greenhouse gas emissions," he told BBC News.
"So I want to highlight the fact that among options for mitigating climate change, changing diets is something one should consider."
Climate of persuasion
The FAO figure of 18% includes greenhouse gases released in every part of the meat production cycle - clearing forested land, making and transporting fertiliser, burning fossil fuels in farm vehicles, and the front and rear end emissions of cattle and sheep.
Dr Pachauri has chaired the Nobel Prize-winning body since 2002
The contributions of the three main greenhouse gases - carbon dioxide, methane and nitrous oxide - are roughly equivalent, the FAO calculates.
Transport, by contrast, accounts for just 13% of humankind's greenhouse gas footprint, according to the IPCC.
Dr Pachauri will be speaking at a meeting organised by Compassion in World Farming (CIWF), whose main reason for suggesting people lower their consumption of meat is to reduce the number of animals in factory farms.
CIWF's ambassador Joyce D'Silva said that thinking about climate change could spur people to change their habits.
"The climate change angle could be quite persuasive," she said.
"Surveys show people are anxious about their personal carbon footprints and cutting back on car journeys and so on; but they may not realise that changing what's on their plate could have an even bigger effect."
There are various possibilities for reducing the greenhouse gas emissions associated with farming animals.
They range from scientific approaches, such as genetically engineering strains of cattle that produce less methane flatus, to reducing the amount of transport involved through eating locally reared animals.
"The NFU is committed to ensuring farming is part of the solution to climate change, rather than being part of the problem," an NFU spokeswoman told BBC News.
"We strongly support research aimed at reducing methane emissions from livestock farming by, for example, changing diets and using anaerobic digestion."
Methane emissions from UK farms have fallen by 13% since 1990.
But the biggest source globally of carbon dioxide from meat production is land clearance, particularly of tropical forest, which is set to continue as long as demand for meat rises.
Ms D'Silva believes that governments negotiating a successor to the Kyoto Protocol ought to take these factors into account.
"I would like governments to set targets for reduction in meat production and consumption," she said.
"That's something that should probably happen at a global level as part of a negotiated climate change treaty, and it would be done fairly, so that people with little meat at the moment such as in sub-Saharan Africa would be able to eat more, and we in the west would eat less."
Dr Pachauri, however, sees it more as an issue of personal choice.
"I'm not in favour of mandating things like this, but if there were a (global) price on carbon perhaps the price of meat would go up and people would eat less," he said.
"But if we're honest, less meat is also good for the health, and would also at the same time reduce emissions of greenhouse gases."