The Christians stole the winter solstice from the pagans, and capitalism stole it from the Christians. But one feature of the celebrations has remained unchanged: the consumption of vast quantities of meat. The practice used to make sense. Livestock slaughtered in the autumn, before the grass ran out, would be about to decay, and fat-starved people would have to survive a further three months. Today we face the opposite problem: we spend the next three months trying to work it off.

Our seasonal excesses would be perfectly sustainable, if we weren't doing the same thing every other week of the year. But, because of the rich world's disproportionate purchasing power, many of us can feast every day. And this would also be fine, if we did not live in a finite world.

By comparison to most of the animals we eat, turkeys are relatively efficient converters: they produce about three times as much meat per pound of grain as feedlot cattle. But there are still plenty of reasons to feel uncomfortable about eating them. Most are reared in darkness, so tightly packed that they can scarcely move. Their beaks are removed with a hot knife to prevent them from hurting each other. As Christmas approaches, they become so heavy that their hips buckle. When you see the inside of a turkey broilerhouse, you begin to entertain grave doubts about European civilisation.

This is one of the reasons why many people have returned to eating red meat at Christmas. Beef cattle appear to be happier animals. But the improvement in animal welfare is offset by the loss in human welfare. The world produces enough food for its people and its livestock, though (largely because they are so poor) some 800 million are malnourished. But as the population rises, structural global famine will be avoided only if the rich start to eat less meat. The number of farm animals on earth has risen fivefold since 1950: humans are now outnumbered three to one. Livestock already consume half the world's grain, and their numbers are still growing almost exponentially.

This is why biotechnology - whose promoters claim that it will feed the world - has been deployed to produce not food but feed: it allows farmers to switch from grains which keep people alive to the production of more lucrative crops for livestock. Within as little as 10 years, the world will be faced with a choice: arable farming either continues to feed the world's animals or it continues to feed the world's people. It cannot do both.

The impending crisis will be accelerated by the depletion of both phosphate fertiliser and the water used to grow crops. Every kilogram of beef we consume, according to research by the agronomists David Pimental and Robert Goodland, requires around 100,000 litres of water. Aquifers are beginning the run dry all over the world, largely because of abstraction by farmers.

Many of those who have begun to understand the finity of global grain production have responded by becoming vegetarians. But vegetarians who continue to consume milk and eggs scarcely reduce their impact on the ecosystem. The conversion efficiency of dairy and egg production is generally better than meat rearing, but even if everyone who now eats beef were to eat cheese instead, this would merely delay the global famine. As both dairy cattle and poultry are often fed with fishmeal (which means that no one can claim to eat cheese but not fish), it might, in one respect, even accelerate it. The shift would be accompanied too by a massive deterioration in animal welfare: with the possible exception of intensively reared broilers and pigs, battery chickens and dairy cows are the farm animals which appear to suffer most.

We could eat pheasants, many of which are dumped in landfill after they've been shot, and whose price, at this time of the year, falls to around £2 a bird, but most people would feel uncomfortable about subsidising the bloodlust of brandy-soaked hoorays. Eating pheasants, which are also fed on grain, is sustainable only up to the point at which demand meets supply. We can eat fish, but only if we are prepared to contribute to the collapse of marine ecosystems and - as the European fleet plunders the seas off West Africa - the starvation of some of the hungriest people on earth. It's impossible to avoid the conclusion that the only sustainable and socially just option is for the inhabitants of the rich world to become, like most of the earth's people, broadly vegan, eating meat only on special occasions like Christmas.

As a meat-eater, I've long found it convenient to categorise veganism as a response to animal suffering or a health fad. But, faced with these figures, it now seems plain that it's the only ethical response to what is arguably the world's most urgent social justice issue. We stuff ourselves, and the poor get stuffed.

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