For decades, environmental arguments against eating meat have been largely the preserve of vegetarian websites and magazines. Just two years ago it seemed inconceivable that significant numbers of western Europeans would be ready to down their steak knives and graze on vegetation for the sake of the planet. The rapidity with which this situation has changed is astonishing.

The breakthrough came in 2006 when the UN Food and Agriculture Organisation (FAO) published a study, Livestock's Long Shadow, showing that the livestock industry is responsible for a staggering 18% of all anthropogenic greenhouse gas emissions. This is only the beginning of the story. In 2008, Brazil announced that in the 12 months to July it had lost 12,000 sq km (3m acres) of the Amazon rainforest, mainly to cattle ranchers and soy producers supplying European markets with animal feed. There is water scarcity in large parts of the world, yet livestock-rearing can use up to 200 times more water a kilogram (2.2lbs) of meat produced than is used in growing wheat. Given the volatile global food prices, it seems foolhardy to divert 1.2bn tonnes of fodder – including cereals – to fuel global meat consumption, which has increased by more than two and half times since 1970.

Vegetarians have been around for a very long time – Pythagoreans forbade eating animals more than 2,500 years ago – but even as the environmental evidence mounted, they didn't appear to be winning the argument. Today in Britain just 2% of the population is vegetarian.

Thankfully, a more pragmatic alternative to total abstinence now seems to be emerging. In September 2008, Dr Rajendra Pachauri, chair of the UN Intergovernmental Panel on Climate Change (IPCC), a vegetarian himself, called on people to take personal responsibility for the impacts of their consumption.

"Give up meat for one day [a week] initially, and decrease it from there," he said. "In terms of immediacy of action and the feasibility of bringing about reductions in a short period of time, it clearly is the most attractive opportunity." This week the Belgian city of Ghent met his demands by declaring Thursday a meat-free day. Restaurants, canteens and schools will now opt to make vegetarianism the default for one day a week, and promote meat-free meals on other days as well.

This is not the first institutional backing for such a move. In Britain, the NHS now aims to reduce its impact on the environment partly by "increasing the use of sustainably sourced fish and reducing our reliance on eggs, meat and dairy". Last year, Camden council in London announced that it would be issuing a report calling for schools, care homes and canteens on council premises to cut meat from menus and encourage staff to become vegetarian. (In the end the initiative was shot down by Conservative councillors who insisted that people should not be deprived of choice.) While in Germany the federal environment agency in January called on Germans to follow a more Mediterranean diet by reserving meat only for special occasions.

These initiatives may sound novel, but in fact they reinstate what was for centuries an obligatory practice across Europe. The fasting laws of the Catholic church stipulated that on Fridays, fast days, and Lent, no one could eat meat or wine; on some days, dairy products and fish were also banned. Even after the Reformation Elizabeth I upheld the Lenten fast, insisting that while there was no religious basis for fasting, there were sound utilitarian motives: to ­protect the country's livestock from over-exploitation and to promote the fishing industry (which had the ancillary benefit of increasing the number of ships available for the navy).

Towards the end of the 18th century, two consecutive bad harvests in Europe created shortages. There was a huge public clamour for the wealthy to cut down on their meat consumption in order to leave more grain for the poor. The idea that meat was a cruel profligacy became current, and led Percy Bysshe Shelley to declare that the carnivorous rich literally monopolised land and food by taking more of it than they needed. "The use of animal flesh," he said, "directly militates with this equality of the rights of man."

In the wake of last year's food crisis and with mounting concern over global warming, we appear to have reached a similar crisis moment.

The vegetarian argument is complicated, however, by the fact that in terms of environmental impact, no two pieces of meat are the same. A hunk of beef raised on Scottish moorland has a very different ecological footprint from one created in an intensive feedlot using concentrated cereal feed, and a wild venison or rabbit casserole is arguably greener than a vegetable curry. Likewise, countries have very different animal husbandry methods. For example, in the US, for each calorie of meat or dairy food produced, farm animals consume on average more than 5 calories of feed. In India the rate is a less than 1.5 calories. In Kenya, where there isn't the luxury of feeding grains to animals, livestock yield more calories than they consume because they are fattened on grass and agricultural by-products inedible to humans.

In a paper published last month in the American Journal of Clinical Nutrition, food ecologist Annika Carlsson-Kanyama showed that kilo for kilo, beef and pork could produce 30 times more CO² emissions than other protein rich foods such as beans. On the other hand, the paper also indicated that poultry and eggs had much lower ­emissions than cheese, which was among the highest polluters. So do meat-free days, and arguments for vegetarianism in general, take adequate consideration of these subtleties, or should we all be chucking out the cheese and going vegan?

"A vegetarian day is a simple message that people can understand," says Carlsson-Kanyama, "though probably what we ultimately need to do is eat less animal products overall."

Alex Evans, fellow at the Centre on International Cooperation at New York University, points out that more and more people – including Sir Nicholas Stern, the author of a 2006 review on the economics of global warming – accept that the only equitable way of achieving an international agreement on climate change is for rich and poor nations to converge on an equal per capita "fair share" of carbon emissions. "The same ought to apply to food," Evans says, "but currently there is no agreed method for calculating what is my 'fair share' of the world's food supply – in particular how much meat."

Based on the global food production figures published by the FAO, I did a few preliminary calculations. Global average consumption of meat and dairy products including milk was 152kg a person in 2003. Average EU and US consumption, by contrast, was over 400kg, while Uganda's was 45kg. In order to reach the equitable fair share of global production, rich western countries would have to cut their consumption by 2.7 times – and this doesn't include the fact that the butter will have to be spread even more thinly if the global population really does increase by another 2.3 billion by 2050.

However, still further reductions would be necessary because global meat production is already at unsustainable levels. The IPCC among other bodies, has called for an 80% reduction of greenhouse gas emissions by 2050. Since high levels of meat and dairy ­consumption are luxuries, it seems reasonable to expect livestock production to take its share of the hit. For rich ­western countries this would mean decreasing meat and dairy consumption to significantly less than one tenth of current levels, the sooner the better.

It is all very well for 2% of the population to live in a monastic state of meatlessness while everyone else gorges their way towards environmental meltdown or the nearest heart clinic. Vegetarianism is good for the willing ­minority, but not much use as a campaign tool. Beginning as Ghent has done, with one meat-free day a week, is a historically-proven idea palatably re-fashioned for the age of eco-consciousness. It also appears to be gaining popular approval, even if McDonald's need not fear for the survival of its Big Mac, yet.

• Tristram Stuart's Waste: Uncovering the Global Food Scandal, will be published by Penguin in July. His history of vegetarianism, The Bloodless Revolution, was published by HarperCollins in 2006

• This artciel was amended on Sunday 18 May 2009. We originally put Tristram Hunt on the byline for the above article. The writer was in fact Tristram Stuart. This has been corrected.