Skäl 1: Global uppvärmning

Overheating the planet

We humans eat about 230m tonnes of animals a year, twice as much as we did 30 years ago. We mostly breed four species – chickens, cows, sheep and pigs – all of which need vast amounts of food and water, emit methane and other greenhouse gases and produce mountains of physical waste.

But how much stress does our meat-eating put on ecological systems? The answer is a lot but the figures are imprecise and disputed. In 2006, the UN calculated that the combined climate change emissions of animals bred for their meat were about 18% of the global total – more than cars, planes and all other forms of transport put together.

The authors of the report, called Livestock's Long Shadow, did not just count the methane from the belching, farting cattle, but the gases released from the manures that they produce, the oil burned taking their carcasses to markets often thousands of miles away, the electricity needed to keep the meat cool, the gas used to cook it, the energy needed to plough and harvest the fields that grow the crops that the animals eat, even pumping the water that the cattle need.

The figure was revised upward in 2009 by two World Bank scientists to more than 51%, but attempts to fully account for meat-eating are condemned as simplistic. Should the studies have been based on giant US factory farms, or on more sustainable breeding in Europe? Should you include all the knock-on emissions from clearing forests? What about the fertiliser used to grow the crops to feed to the animals, or the emissions from the steel needed to build the boats that transport the cattle; or the "default" emissions – the greenhouse gases that would be released by substitute activities to grow food if we were to give up meat? And is it fair to count animals used for multiple purposes, as they mostly are in developing countries, from providing draught power to shoe leather or transport, and which only become meat once they reach the end of their economic lives?

It's an accounting nightmare but depending on how it's done, livestock's contribution to climate change can be calculated as low as 5-10% of global emissions or as high as 50%. Last year, a Food Climate Research Network report concluded that UK meat and dairy consumption was responsible for 8% of the country's total greenhouse gas emissions. But however it's counted, livestock farming ranks as one of the three greatest sources of climate changing emissions and one of the largest contributors to environmental degradation.