AUSTRALIANS chew through more red meat a head than Americans, and we export more again. So attached are we to red meat and dairy products that the sheep and cattle population of this country outnumbers the human population by five to one, and 56 per cent of Australia's land mass is devoted to grazing.

As grass makes its way through the four-stomach digestive process of these ruminants, it ferments, and the animals burp, fart, urinate and defecate with such gusto that they pump out, on official figures, between 11 per cent and 15 per cent of Australia's total greenhouse gas emissions, about the same as every car, truck and bus in Australia.

To solve this problem, our questioner, vegan Paul Mahony, says there is only one option: a general move towards a plant-based diet.

Nutritionally, we could do it. All the nutrients found in meat can be found in vegetarian alternatives. And while eating lean red meat is an easy way to take in vital protein, vitamins and minerals, Australians eat 45 kilograms of red meat a person every year, much more than necessary.

So is it possible, or desirable, to create a nation of vegans? The British government is trying to reduce meat consumption, mainly for health reasons. But Australians are kings of the barbecue, weaned on the myths of the drover. Should we limit, even eliminate, our red meat consumption to help climate change?

The Meat and Livestock Association says no. ''Why wouldn't anyone living in this great country desire a balanced diet that includes red meat?'' says marketing general manager Glen Feist. ''That anyone could presume to tell someone else what to eat in a country where food is so bountiful and healthy is outrageous.''

But by simply existing, sheep, cattle, goats and buffalo pump out large volumes of methane and nitrous oxide. Methane is produced during digestion - what the scientists call ''enteric fermentation'' - and is 21 times stronger as a greenhouse gas than carbon dioxide. It stays for less time in the atmosphere (about 12 years compared with carbon dioxide, a proportion of which can last thousands of years) but while methane is there, it traps more heat.

Nitrous oxide is produced when animal manure and urine decomposes. It is produced in far lower quantities than methane, but stays in the atmosphere for 114 years and is 310 times better at trapping heat than carbon dioxide.

But the picture becomes even more dire if you include, as Mahony does, the clearing of forests to create land for ruminants. Deforestation unlocks carbon dioxide from trees as they decompose, and releases it into the atmosphere. If you add that into the equation, according to Department of Climate Change figures, almost 20 per cent of Australia's emissions can be attributed to agriculture, most of which is to raise meat.

(The 30 per cent referred to by Mahony comes from a CSIRO report that used information from the 1990s. But in the past two decades deforestation for agriculture has been outlawed, halving emissions. The clearing that takes place in Australia now is, by and large, cutting back the regrowth from land already cleared.)

Throughout the rest of the world, deforestation is a very big problem. An area the size of Greece was cleared of trees each year between 2000 and 2010 - seven soccer fields every minute - with South America and Africa the worst offenders. Much of that land is cleared for grazing. The senior scientist at the World Preservation Foundation, Gerard Wedderburn-Bisshop, says livestock now outnumber wildlife by eight to one, eat six times the amount the dinosaurs did, and five times what humans consume. To deforestation, methane and nitrous oxide, he adds another powerful warming agent, ''black carbon'', the soot released when forests and savannahs are burned to make way for grazing animals. Black carbon, he says, blows onto the ice sheets in Antarctica and causes rapid melting. All these are ''extremely potent, shorter-lived climate forcers'', according to Wedderburn-Bisshop.

But the world, instead, is increasing its herds of sheep and cattle, and the demand for red meat from the developing world is rising.

But people in these countries are not likely to aspire to the same red meat habit as in the Western world because they prefer the white meats, pork and chicken, which produce barely measurable methane emissions.

Even in the West, the red meat habit is waning. In Australia, the consumption of beef peaked in 1977, when we ate 70 kilograms a person each year. Last year, according to the Australian Bureau of Agricultural and Resource Economics, we ate just half that, 34.1 kilograms. Lamb and mutton consumption is at just one-quarter of its peak, down from 43.6 kilograms a person in 1971 to 11.5 kilograms last year. At the same time the consumption of chicken in the West has grown quickly.

Ruminants' environmental footprint goes well beyond their greenhouse gas emissions. They are inefficient converters of food to energy, consuming, on some estimates, up to 54 kilograms of feed to produce just one kilogram of beef. And when they graze in arid zones, cattle degrade the quality of soil and increase the size of deserts, a process called ''desertification''.

''It is pretty obvious that if you want to feed a growing world population, 9.5 billion people by 2050, the most energy-efficient way is to grow crops,'' says Associate Professor Richard Eckard, of the Primary Industry Climate Challenges Centre, at Melbourne University.

''The second-most efficient way is to feed crops to monogastric [single-stomach] livestock, pigs and chickens, and the least efficient is to feed crops to a ruminant, then to feed humans,'' he says.

Bruce Poon, the research manager for a lobby group, Vegetarian Victoria, believes that to achieve our greenhouse gas reduction targets we must not only reduce the number of grazing animals but also regrow forests on a large scale.

More than 90 per cent of the forests cut down in Australia historically were to make way for animal agriculture. As trees regrow, they suck carbon back out of the atmosphere and lock it into the tree in a process called sequestration.

Our politicians recognise the problem, but do not agree with the vegetarian lobby's prescription. (Read their full responses on our website theage.com.au). Quite apart from the economic value of animal agriculture - $18 billion a year, including $15 billion in exports - governments are unpopular enough without invading the plates and palates of their constituents and trying to ban the barbecue.

Climate Change Minister Greg Combet denied that the government was ignoring the ''elephant in the room'', but pointed to the $429 million it was putting towards research to reduce methane and other emissions, and incentives for farmers themselves to reduce their stock emissions with better animal husbandry.

Opposition spokesman Greg Hunt said it was likely that agriculture would ''attract a significant proportion of emissions reduction'' money under the Coalition's direct action policy.

"The answer is incentives for cleaner production, not killing off the national cattle herd,'' he said.

And Greens deputy leader Christine Milne said her party wanted to see agriculture included in carbon accounting once farm emissions could be accurately counted.

Poon believes agriculture should be included in the federal government's carbon trading scheme (currently excluded) and farmers charged for all the environmental damage done by grazing animals..

''If you had all that: pollution, water, carbon taxes, no one would eat meat because it would be too frigging expensive,'' he said.

The dividend would be a much easier road to a carbon-free economy. Mahony cites a study by the Dutch Environmental Assessment Agency, which showed a transfer to a completely vegan diet would reduce climate change mitigation costs by about 80 per cent.

Poon acknowledges that his policy prescription to reduce meat consumption is ''extremely threatening, even for people who want to believe it politically, because it sounds like - to the best politician in the world - that I'm trying to tell people what to eat. It won't happen.''

But Mahony draws an analogy with an emergency of another century.

''Look back to World War II, when there was food rationing in Britain because there was a tangible threat. They were in desperate straits. We could do that again.

''The problem is that people don't equate climate change to anywhere near that kind of threat, where I say it is. And if we did have to change our diet, that's a small price to pay.''

The Climate Agenda was a partnership between The Sunday Age and website Oursay.org, which let readers vote for the 10 questions they wanted the paper to answer. For more information go to theage.com.au/national/climate-agenda/