Q: Do we really have to stop eating meat?

A: You may have heard that cow flatulence is contributing to climate change. Here's the good news: Cow farts aren't that bad for the environment.

The bad news: Their burps are a real problem.

Cows are ruminants, meaning that microbes in their multichambered stomachs help them digest by fermenting their food. This process produces the powerful greenhouse gas methane, which gets released into the atmosphere when they burp.

But bovine belching is just the beginning of the long list of problems that eating meat poses for the planet.

The agriculture sector is one of the world's biggest sources of climate-altering gases, the vast majority of which come from meat and dairy production. If cows were their own country, they would be the third-biggest greenhouse gas emitter in the world.

Farming also requires huge amounts of land. Forests, wetlands and savannas are carbon sinks: They absorb and store carbon. But when these ecosystems are chopped down to make way for farms, that land turns into a carbon source.

A 2018 study found that about 12.4 million acres of forest - the equivalent of more than five Yellowstone National Parks - are cut down each year to clear room for industrial agriculture. A whopping 30 per cent of Earth's ice-free land mass is used as pasture for livestock.

Here, again, cows are a major culprit. Because ruminants have slower growth and reproduction rates than other animals, they require more resources to produce something a person can eat. Beef requires about twice as much land per gram of protein as chicken and pork, and 20 times as much land as the equivalent amount of protein from beans.

Because of population increases and changes in diets as the global middle class grows, agriculture's emissions have been rising steadily. By 2050, when the world's population is projected to hit 10 billion, scientists anticipate it will use up most of humanity's annual "carbon budget" - the amount we can emit without exceeding warming targets - just to feed everyone.

That is, unless we change the way we eat.

Individual dietary choices, when combined with more sustainable agriculture policy, can have a profound effect on the global climate outlook, scientists say. This is especially true in rich countries like the United States, which has emitted more greenhouse gases since the Industrial Revolution than any other country in the world.

Americans generate the equivalent of more than 16 metric tonnes of carbon dioxide per person per year, more than three times the global average. (Scientists say planetary per-capita emissions must be cut to about 2 tonnes of carbon dioxide equivalent by 2050 if the worst effects of warming are to be avoided.)

Since cows are the most carbon-intensive part of the food industry - cattle are responsible for 62 per cent of agricultural emissions - eating fewer of them is one of the most powerful steps an individual can take toward protecting the planet.

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A 2017 study found that if every American swapped out all the beef in their diet for beans, it would get the United States halfway to meeting the 2020 greenhouse gas emission targets laid out in the Paris climate accord. (US President Donald Trump has since pulled the United States out of the agreement, but the math still stands.)

Reducing beef consumption has advantages far beyond its effect on the climate, scientists say. Turning ranch land into forest will create new habitats and mitigate the effects of agriculture on water pollution. It will also make many people healthier; many Americans, especially teen boys and men, consume far more protein from meat than nutrition experts recommend.

This doesn't mean we need to give up beef entirely. From pot roast to bulgogi to carne asada, beef dishes are a vital part of people's cultural heritage. Beef is also an important source of protein for millions of people in places where protein is scarce. Advocates for reduced meat consumption acknowledge that food ethics are complicated and often personal.

What we really need, said Janet Ranganathan, vice president for science and research at the World Resources Institute, are "more subtle shifts". A shift away from carbon-intensive beef toward poultry, fish and plant-based foods.

A shift in agriculture practices and food preparation to make the meat we do consume more sustainable. A shift in government policies about farming and food that will allow these changes to happen in an equitable and just way.

"The important thing is," Ranganathan said, "this is all doable".

She's done the math. If Earth's biggest beef eaters limited their consumption to the equivalent of 1.5 hamburgers per week (about half of what the average person in the United States currently eats), the planet could support a population of 10 billion people without having to turn any more forests into farmland.

This shift would avoid about 5.5 billion tons of greenhouse gas emissions each year - the equivalent of emissions from two Indias.

On top of that, if people adopted more sustainable ranching practices and turned another 1.6 million square kilometres of land into forest, they could offset all emissions from the food that is grown. This would make the agriculture industry carbon neutral, according to Richard Waite, a colleague of Ranganathan's at WRI.

There are myriad ways to achieve this.

Industrial farms can make cows more climate-friendly by changing their diets; research in California suggests that adding a small amount of seaweed to a cow's feed can reduce its methane production by half.

Food companies can start to offer products like "blended burgers", which mix other ingredients with beef to make the food more sustainable. Governments could eliminate subsidies for meat and dairy producers and support farmers transitioning to more sustainable crops.

Your hamburger alone is not the cause of climate change. And swapping it for a bean patty won't solve the problem.

"Everyone has to be a part of this."

But Waite described climate change as an "all of the above" issue; it requires that humanity take every available action, at all levels of society.

It's true that governments and big corporations must limit fossil fuel production, curb industrial emissions and invest in green energy alternatives, he said.

"But if we don't reduce emissions from agriculture, we're not going to be able to hit climate targets even if we get it exactly right on fossil fuels.

The world must do "everything at once at a really high level of ambition", Ranganathan added.

"Everyone has to be a part of this."