How Much Meat Should We Eat? | Science - Smithsonian Magazine

Research indicates that those of us in the West should eat less meat to live more sustainably. Igor Golovniov / SOPA Images / LightRocket via Getty Images

As governments drag their feet in responding to climate change, many concerned people are looking for actions they can take as individuals — and eating less meat is an obvious place to start. Livestock today account for about 14.5 percent of global greenhouse gas emissions, more than all the world’s cars and trucks combined.

Those numbers are daunting already, but the situation could grow worse: Our appetite for meat is increasing. The United Nations forecasts that the world will be eating 14 percent more of it by 2030, especially as middle-income countries get wealthier. That means more demand for pasture and feed crops, more deforestation and more climate problems. For people alarmed about climate change, giving up meat altogether can seem like the only option.

But is it? A growing body of research suggests that the world could, in fact, raise a modest amount of beef, pork, chicken and other meat, so that anyone who wants could eat a modest portion of meat a few times a week — and do so sustainably. Indeed, it turns out that a world with some animal agriculture in it likely would have a smaller environmental footprint than an entirely vegan world. The catch is that hitting the environmental sweet spot would require big changes in the way we raise livestock — and, for most of us in the wealthy West, a diet with considerably less meat than we eat today.

“The future that sounds sustainable to me is one where we have livestock, but it’s a very different scale,” says Nicole Tichenor Blackstone, a food systems sustainability researcher at Tufts University in Boston. “I think the livestock industry’s going to have to look different.”

Feeding animals to feed ourselves

One big reason for meat’s outsized environmental impact is that it’s more efficient for people to eat plants directly than to feed them to livestock. Chickens need almost 2 pounds of feed to produce each pound of weight gain, pigs need 3 to 5 pounds, and cattle need 6 to 10 — and a lot of that weight gain is bones, skin and guts, not meat. As a result, about 40 percent of the world’s arable land is now used to grow animal feed, with all the attendant environmental costs related to factors such as deforestation, water use, fertilizer runoff, pesticides and fossil fuel use.

But it’s not inevitable that livestock compete with people for crops. Ruminants — that is, grazing animals with multiple stomachs, like cattle, sheep and goats — can digest the cellulose in grass, straw and other fibrous plant material that humans can’t eat, converting it into animal protein that we can. And two-thirds of the world’s agricultural lands are grazing lands, many of which are too steep, arid or marginal to be suitable for crops. “That land cannot be used for any other food-growing purpose other than the use of ruminant livestock,” says Frank Mitloehner, an animal scientist at the University of California, Davis.

Of course, those grazing lands could revert to natural forest or grassland vegetation, taking up atmospheric carbon in the process. This carbon-capturing regrowth could be a major contributor to global climate-mitigation strategies aimed at net-zero greenhouse gas emissions, researchers say. But that’s not necessarily incompatible with moderate levels of grazing. For example, some research suggests that replacing croplands with well-managed grazing lands in the southeastern US captures far more carbon from the atmosphere.

Livestock can also use crop wastes such as the bran and germ left over when wheat is milled to white flour, or the soy meal left over after pressing the beans for oil. That’s a big reason why 20 percent of the US dairy herd is in California’s Central Valley, where cows feed partly on wastes from fruits, nuts and other specialty crops, Mitloehner says. Even pigs and chickens, which can’t digest cellulose, could be fed on other wastes such as fallen fruit, discarded food scraps and insects, which most people wouldn’t eat.

The upshot is that a world entirely without meat would require about one-third more cropland — and therefore, more energy-intensive fertilizer, pesticides and tractor fuel — to feed everyone, says Hannah van Zanten, a sustainable food systems researcher at Wageningen University in the Netherlands. But only if we’re talking about meat raised the right way, in the right amounts.

Livestock also bring other benefits. Meat provides balanced protein and other nutrients such as iron and vitamin B12 that are more difficult to get from a vegan diet, especially for poorer people who can’t always afford a variety of fresh vegetables and other nutritious foods, says Matin Qaim, an agricultural economist at the University of Bonn, Germany, who coauthored a look at the sustainability of meat consumption in the 2022 Annual Review of Resource Economics. Livestock, he notes, are the main source of wealth for many otherwise poor people in traditional pastoral cultures. And on small, mixed farms, animals that graze widely and then deposit their manure in the farmyard can help to concentrate nutrients for use as fertilizer in the family’s garden.

Moreover, many of the world’s natural grasslands have evolved in the presence of grazers, which play a key role in ecosystem function. Where those native grazers no longer dominate — think of the vanished bison from the American prairies, for example — domestic livestock can fill the same role. “Grasslands are disturbance-dependent,” says Sasha Gennet, who heads the sustainable grazing lands program for the Nature Conservancy. “Most of these systems evolved and adapted with grazing animals and fire. They can benefit from good livestock management practices. If you’re doing it right, and you’re doing it in the right places, you can have good outcomes for conservation.”

For all these reasons, some experts say, the world is better off with some meat and dairy than it would be with none at all — though clearly, a sustainable livestock system would have to be much different, and smaller, than the one we have today. But suppose we did it right? How much meat could the world eat sustainably? The answer, most studies suggest, may be enough to give meat-eaters some hope.

Looking at the whole plate

Interdisciplinary researcher Vaclav Smil of the University of Manitoba got the ball rolling in 2013 with a back-of-the-envelope calculation published in his book, Should We Eat Meat? Let’s assume, he reasoned, that we stop clearing forest for new pastureland, let 25 percent of existing pastures revert to forest or other natural vegetation and feed livestock as much as possible on forage, crop residues and other leftovers. After making those concessions to sustainability, Smil’s best guesstimate was that this “rational” meat production could yield about two-thirds as much meat as the world was producing at the time. Subsequent studies suggest that the real number might be a bit lower, but still enough to promise a significant place for meat on the world’s plate, even as the population continues to grow.

If so, there are several surprising implications. For one thing, the total amount of meat or dairy that could be produced in this way depends strongly on what else is on people’s plates, says van Zanten. If people eat a healthy, whole-grain diet, for example, they leave fewer milling residues than they would on a diet heavy in refined grains — so a world full of healthy eaters can support fewer livestock on its leftovers. And little choices matter a lot: If people get most of their cooking oil from canola, for example, they leave less nutritious meal for feed after pressing out the oil than if they get their oil from soy.

A second surprise is the nature of the meat itself. Sustainability experts typically encourage people to eat less beef and more pork and chicken, because the latter are more efficient at converting feed into animal protein. But in the “livestock on leftovers” scenario, the amount of pork and chicken that can be raised is limited by the availability of milling residues, food scraps and other food wastes. In contrast, cattle can graze on pasture, which shifts the livestock balance back somewhat toward beef, mutton and dairy products.

Much would have to change to make such a world possible, van Zanten notes. To maximize the flow of food wastes to pigs and chickens, for example, cities would need systems for collecting household wastes, sterilizing them and processing them for feed. Some Asian countries are well ahead on this already. “They have this whole infrastructure ready,” van Zanten says. “In Europe, we don’t.” And much of our current animal agriculture, based on grain-fed livestock in feedlots, would have to be abandoned, causing significant economic disruption.

Moreover, people in wealthy countries would have to get used to eating less meat than they currently do. If no human-edible crops were fed to livestock, van Zanten and her colleagues calculated, the world could only produce enough meat and dairy for everyone to eat around 20 grams of animal protein per day, enough for a three-ounce piece of meat or cheese (about the size of a deck of cards) each day. By comparison, the average North American now chows down on about 70 grams of animal protein a day — well above their protein requirement — and the average European on 51.

That’s a hefty reduction in meat — but it would bring significant environmental benefits. Because livestock would no longer eat feed crops, the world would need about a quarter less cropland than it uses today. That surplus cropland could be allowed to regrow into forest or other natural habitat, benefitting both biodiversity and carbon balance.

A “circular food economy” provides a way to sustainably include meat in the world’s diet. In this scenario, livestock eat no crops edible to people. Instead, they graze on grasslands and eat crop residues and food waste that people cannot or will not eat. Such a system would allow everyone in the world to eat a small serving of meat or other animal products a few times each week, researchers have calculated. Source: Adapted from H.H.E. Van Zanten et al. / Global Change Biology 2018 / Knowable Magazine

There’s another dimension to meat’s sustainability, though. The gut microbes that let grazing animals digest grasses and other human-inedible forage release methane in the process — and methane is a potent greenhouse gas. Indeed, methane from ruminants accounts for about 40 percent of all livestock-related greenhouse gas emissions. Animal scientists are working on ways to reduce the amount of methane produced by grazers (see box). At present, however, it remains a serious problem.

Paradoxically, raising cattle on grass — better for other dimensions of sustainability — makes this problem worse, because grass-fed cattle grow more slowly. Grass-fed Brazilian cattle, for example, take three to four years to reach slaughter weight, compared to 18 months for US cattle finished on grain in feedlots. And that’s not all: Because the grain-fed animals eat less roughage, their microbes also produce less methane each day. As a result, grass-fed beef — often viewed as the greener option — actually emits more methane, says Jason Clay, senior vice president of markets for the World Wildlife Fund-US.

Even so, raising livestock on leftovers and marginal grazing lands not suitable for crops eliminates the need to grow feed crops, with all their associated emissions, and there will be fewer livestock overall. As a result, greenhouse gas emissions may end up lower than today. For Europe, for example, van Zanten and her colleagues compared expected emissions from livestock raised on leftovers and marginal lands against those from animals fed a conventional grain-based diet. Livestock on leftovers would produce up to 31 percent less greenhouse gas emissions than the conventional approach, they calculated.

Most animal products generate more emissions of greenhouse gases than plant foods do. Grazers such as cattle, sheep or goats are the biggest emitters even after discounting the methane they produce. Pigs and, especially, chickens generate much smaller amounts of greenhouse gases for a given weight of meat. Adapted from M.C. Parlasca and M. Qaim / AR Resource Economics 2022 / Knowable Magazine

Some sustainability experts also argue that as long as grazing herds aren’t increasing, methane may be less of a worry than previously thought. Molecule for molecule, methane contributes about 80 times more warming than carbon dioxide does in the short term. However, CO₂ persists in the atmosphere for centuries, so newly emitted CO₂ always makes the climate crisis worse by adding to the stock of CO₂ in the atmosphere. In contrast, methane lasts only a decade or so in the atmosphere. If livestock levels remain constant over the span of decades, then the rate at which old methane washes out of the atmosphere will be about equal to the rate at which new methane is emitted, so there would be no additional burden on climate, says Qaim.

But with climate experts warning that the world may be fast approaching a climate tipping point, some experts say there’s good reason to reduce meat consumption well below what’s sustainable. Completely eliminating livestock, for example, would allow some of the land now devoted to feed crops and pastures to revert to native vegetation. Over 25 to 30 years of regrowth, this would tie up enough atmospheric CO₂ to completely offset a decade’s worth of global fossil fuel emissions, Matthew Hayek, an environmental scientist at New York University, and his colleagues reported in 2020. Add to that the rapid reduction in methane no longer emitted by livestock, and the gains become even more attractive.

“We need to be moving in the opposite direction than we are now,” says Hayek. “The things that are going to do that are aggressive, experimental, bold policies — not ones that try to marginally reduce meat consumption by 20 or even 50 percent.”

Editor’s note: This article was amended on August 19, 2022, to correct which scientist made the point that if livestock levels remained constant over decades, there would be no additional methane burden on climate. The correct attribution is Matin Qaim.

Knowable Magazine is an independent journalistic endeavor from Annual Reviews.

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