Last month, at a star-studded photo call in London, Britain’s vegetarians-in-chief, Stella, Mary and Sir Paul McCartney, launched a campaign to persuade non-vegetarians to go veggie for one day a week. In case you missed it, the initiative — Meat Free Mondays (MFM) — is a kind of start-the-week dress-down Friday that promotes greens rather than jeans. The headline message is simple: eating less meat will help save the planet.

Even a few years ago, most of us would have scoffed. Vegetarians were a bunch of hippies with an overdeveloped sense of farmyard sentimentality. Nobody really took them seriously. But attitudes have changed, and the news that meat farming is an important contributor to global warming has boosted their cause. Vegetarianism is looking relevant, even cool. Scan the MFM campaign website and there’s a list of glossy celebrity vegetarians — Brad Pitt, Natalie Portman, Cameron Diaz, Gwyneth Paltrow and Peaches Geldof among others — all ready to stand up and be counted. The animal-rights organisation Peta recently signed Che Guevara’s glamorous granddaughter Lydia as the face of its vegetarian campaign. In an image even the most red-blooded gaucho will find hard to resist, Ms Guevara will appear on posters dressed in little more than a carrot-filled bullet belt and beret. The slogan: “Join the vegetarian revolution.”

From the rape of the oceans to the plight of pigs, we’re agonising over our relationship with the food chain more than ever. But vegetarianism is still a difficult case to argue. Many — me included — have always been infuriated by the emotive “animals are people too” logic of some ethical veggies, and the line, recently trotted out by a Peta official, that “the best way to save animals is not to eat them”. Save them from what? The beasts that end up on our plates are, by and large, born, raised and slaughtered for that purpose. If we didn’t eat them, they wouldn’t exist — either that or we’d be overrun by the jobless progeny of liberated farm animals. Don’t get me wrong: it matters to me how animals are treated. I’m right behind all those Jamie Oliver and Hugh Fearlessly-Eatsitall campaigns for happy porkers and cheerful chickens. But, ultimately, I care because they’re going to end up as my dinner. I think a happy life and happy death make a difference. I’m also inclined to believe that eating cheap, industrial meat doesn’t do much for your chances of a long and healthy retirement. But like the welfare issue, that’s a reason to eat better meat less often, not give it up entirely.

Like many people, though, I’ve found myself moving gradually closer to a state of semi-vegetarianism. We are becoming a nation of flexitarians, pescetarians and weekend carnivores. In the past week, I’ve had fish four times and a hunk of red meat only once. Give or take the bit of bacon I threw into a soup, the rest has been pretty much veggie. Meat Free Mondays don’t seem such a hardship.

And the environmental arguments are compelling. According to the UN, the livestock industry is responsible for 18% of total global greenhouse gas emissions, from feed, fertiliser, transport and waste products. That’s more than from all the world’s planes, trains and cars combined. The animals’ farts, belches and accumulated poop alone contribute 37% of global emissions of methane, a gas 20 times more warming in greenhouse terms than carbon dioxide.

Then there’s the land issue. An estimated 70% of the Amazon rainforest lost each year is turned into pasture for beef cattle. One-third of all arable land in the world is used to grow feed for livestock. It’s hardly an efficient way to feed the planet. You can get 10 times more edible protein from an acre of crops if you don’t turn them into a Sunday roast en route. There are 1 billion starving people in the world — and growing. You do the math.

Of course, you may find all this dull and worthy — a bit vegetarian, in fact. But think of it like this: we’re in a recession, good meat is expensive and vegetables are cheap. It’s not for nothing that people are growing chard in their window boxes, or queuing up for allotments (there’s a 10-year waiting list in some parts of Britain). There are moves in the City of London to set up community vegetable gardens in the building lots left vacant by property developers.

All in all, summer 2009 looks like a good time to dabble with vegetarianism. An abundance of aubergines, tomatoes, courgettes, lettuce, cucumbers, fruit and berries means cutting down on meat isn’t difficult, and a decent veg box can feed a family for the best part of a week. Vegetarians used to be culinary second-class citizens. But, as a nation, we’re exploring the simple joy of veg more than we have for decades. Ideas such as seasonality and provenance are gradually finding their way into the food mainstream. Who knows? Perhaps the day when we can forsake the flesh for one day a week isn’t so far away.

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