Warning: your festive meal could be more damaging than a long-haul flight

We miscalculate environmental risk: eating certain meats is about the worst thing you can do to the planet

The figures were so astounding that I refused to believe them. I found them buried in a footnote, and assumed at first that they must have been a misprint. So i checked the source, wrote to the person who first published them, and followed the citations. To my amazement, they appear to stand up.

A kilogramme of beef protein reared on a British hill farm can generate the equivalent of 643 kg of carbon dioxide. A kilogramme of lamb protein produced in the same place can generate 749 kg. One kilo of protein from either source, in other words, causes more greenhouse gas emissions than a passenger flying from London to New York.

This is the worst case, and the figure comes from a farm whose soils have a high carbon content. But the numbers uncovered by a wider study are hardly reassuring: you could exchange your flight to New York for an average of 3 kg of lamb protein from hill farms in England and Wales. You'd have to eat 300 kg of soy protein to create the same impact.

In choosing our Christmas dinner - or making any other choice - we appear to take informed and rational decisions. But what looks and feels right is sometimes anything but. In this case, the very features we have been led to see as virtuous (animals wandering freely across the mountains tended by horny-handed shepherds, no concrete and steel monstrosities or any of the other ugliness of modern intensive farming) generate astonishing impacts.

The figures are so high because this form of husbandry is so unproductive. To produce one lamb you need to keep a large area of land bare and fertilized. The animal must roam the hills to find its food, burning more fat and producing more methane than a stalled beast would.

Yes, there are payoffs here. What is good for farmed animals is often bad for the natural world. The cruelties of intensive indoor production are matched by the wreckage of extensive outdoor production. Free-range pig and chicken farming, practised on the current scale, can be environmentally disastrous. Nitrates and phosphates sometimes pour from their paddocks and into the rivers. Unless they are kept at low densities or on well-drained fields, pigs tend to mash the soil: a friend describes some of the farming he's seen as opencast pig mining.

You can raise production - which means fewer greenhouse gases per kilo of meat - by dosing your animals with hormones and antibiotics. But this too has a cost. It's now almost too late, the director of Antibiotic Research UK warned this week, to prevent a global superbug crisis. This is partly because unscrupulous farmers have been chucking shedloads of the antibiotic colistin - the last great hope of killing resistant bacteria - at their animals, as it raises their weight.

But of all forms of production, the most attractive is one of the worst. Hill farming not only makes a wildly disproportionate contribution to climate change; it also trashes our watersheds, increasing the chances of dangerous floods, and destroys what would otherwise be our wildlife refuges: the great empty uplands, in which economic activity is sustained only through lavish farm subsides. It is hard to think of any human activity with a higher ratio of destruction to economic product.

My friends in the industry accuse me of being anti-farmer. It's true that I emphasize the dark side, largely because so few other journalists seem prepared to cover these issues. But I have no visceral dislike of farming - quite the opposite. While visiting a farm on Exmoor last week, I was reminded of all that is beautiful about keeping sheep.

The Arcadian idyll, a conception of the shepherd's life (in both Old Testament theology and Greek pastoral poetry) as the seat of innocence and purity, a refuge from the corruption of the city, resonates with us still. But in the midst of a multifaceted crisis -  the catastrophic loss of wildlife, devastating but avoidable floods, climate breakdown - entertaining this fantasy looks to me like a great and costly indulgence.

As for eating local food, well, in some cases it makes sense. It helps to engender a sense of place and belonging, which should not be lightly dismissed. When we buy seasonal fruit and vegetables from local farmers, it works environmentally as well.

But we've tended to overemphasize food miles and underemphasize other impacts. on average, transport accounts for just 11% of the greenhouse gas emissions caused by the food industry. Pulses shipped from the other side of the world can cause far lower impacts than meat produced here.

A paper published a few days ago suggests that switching from meat to green vegetables would be environmentally damaging. Per calorie, growing lettuces produces more greenhouse gases than rearing pork. But all this establishes is that lettuces are low in calories. You would need to eat 15 kg of lettuce in order to meet your daily energy requirement, which might be reasonable if you were a 200 kg rabbit. As another study remarks, "20 servings of vegetables have less greenhouse gas emissions than one serving of beef".

As the world's people adopt the western diet, a paper in the Climatic Change journal estimates, the methane and nitrous oxide produced by farming could rise to the equivalent of 13bn tonnes of carbon dioxide a year by 2070. This is more than all human activities combined can safely produce without exceeding 2C of global warming. Climate breakdown looks inevitable - unless we all change our diets.

This, above all, means swapping most of the animal protein we eat for vegetable protein. It's not painful, unless we make it so. Many British people used to eat dhal every day. They called it pease pudding, pease pottage or pea soup. As in South Asia, its ingredients varied from place to place and season to season. It's just one component of a diet that offers plenty of variety - without trashing the great variety of life.

I'm not suggesting that you eat no meat or other animal products. I am suggesting that we should all eat far less. Save the splurge for Christmas. And even then, choose carefully.