Of the many terms attached to our burgers and steaks, "sustainable" and "grass-fed" often sit next to each other. But a new study finds that raising livestock on grassy pastures is far from sustainable and doesn't have the climate benefits proponents have claimed.

"Can we eat our way out of the climate problem by eating more grass-fed beef?" Tara Garnett, of the Food Climate Research Network at Oxford University in the UK, and her colleagues asked. The answer, they found, is no.

Eating grass-fed beef doesn't get climate-conscious carnivores off the hook.

Cattle and other ruminants have long been considered a major source of greenhouse gases, largely because of the methane they emit through belching and the carbon dioxide released when forests are cleared for grazing or growing feed.

But in recent years, researchers have found that cattle raised on pastures, munching on grasses and treading the ground underneath them, have the potential to sequester carbon in the soil. Some have made the argument that certain kinds of managed grazing practices not only provide the key to feeding calorie-dense beef and dairy products to a growing global population—predicted to hit 9.8 billion by 2050—but are critical to controlling greenhouse gas emissions because of their ability to store carbon in the soil and potentially offset their emissions

Not everyone is convinced.

"This is an area of contestation," Garnett said. "We wanted to look at the evidence."

The Argument for Grass-Fed Beef

With the world's population climbing and as more countries, notably China, develop the wealth and appetite for animal products, food and agricultural researchers have puzzled over how to feed everyone.

Some proponents of grass-fed beef have argued that managed grazing on pasture provides not just more protein for a meat-hungry planet, but an essential climate benefit.

Here's how that argument goes:

Plants grow, they take carbon out of the atmosphere, then they die. Their roots and aboveground biomass contain carbon. If that carbon is left undisturbed, then the carbon stays in the ground in a stable form. If animals nibble away at plants, that stimulates growth, causing plants to put down deep roots that contain carbon. At the same time, animals eat the plants and excrete manure, which contains carbon and nitrogen—a process that returns carbon and nitrogen to the soil and fosters more plant growth, sequestering more carbon.

So, the argument goes, it's better to eat grass-fed, carbon-sequestering ruminants than "monogastric" creatures like pigs and chickens whose diets will require more grain and more carbon-storing forests cleared to grow those grains as demand for meat grows.

Problems with the Grass-Fed Beef Equation

"But," Garnett said, "there are a lot of buts."

Conditions—weather, rainfall, soil consistency, soil nutrients, plant species, stocking rates (the number of cattle per acre)—all need to be just right. They seldom are, the study says, which means that research emphasizing the possible climate benefits of pasture-fed ruminants has likely been overstated.

Garnett and her colleagues found that the carbon-sequestering potential of pasture-raised ruminants is quite limited.

Ruminants, they point out in a report published Monday, contribute 80 percent of total livestock emissions. But, they find, even under "very generous assumptions," grazing management could only offset up to 60 percent of average annual emission from the grass-fed sector, and only up to 1.6 percent of total human-caused greenhouse gas emissions.

To meet the growing demand for protein from grass-fed animals, the report said, "we would have to massively expand grazing land into forest and intensify existing grassland through the use of nutrient inputs, which among other things, would cause devastating CO2 releases and increases in methane and nitrous oxide emissions," both potent climate-warming gases.

The report finds that if projections for animal product consumption stay unchanged, livestock would take up one-third of the total emissions budget under the Paris climate agreement's 2-degree warming limit.

"Increasing grass-fed ruminant numbers is, therefore, a self-defeating climate strategy," the report concluded. The report, called Grazed and Confused, didn't determine whether grass-fed beef is better or worse than feed-lot beef; instead it looked at the ability of grass-fed production systems to sequester carbon and what that would mean for the future.

No Silver Bullet for Beef-Based Diets

The report's authors say the planet's consumers need to cut their intake of all animal products, no matter how they were produced.

"We can't escape the fact that if we're going to have a hope in hell of cutting our climate emissions, then we need to stop our consumption of animal products," Garnett said. "The high consumers of meat and dairy need to be cutting back their consumption. And that holds, whatever the animal type and whatever the system in which it's been produced."

Jonathan Kaplan, the food and agriculture program director at the Natural Resources Defense Council (NRDC), which has looked at the issue extensively but was not involved in the Grazed and Confused report, said there's research demonstrating that pasture-raised livestock sequester carbon, "but the case for arguing that it's a silver bullet to take care of beef's impact is not there."

NRDC has concluded that, pound-for-pound, beef releases about 34 times more greenhouse gases than legumes, including lentils and black beans. The group also recently found that a decline in U.S. beef consumption contributed to a 9 percent decline in greenhouse gas emissions.

"Our take is that grass-fed is better than conventional," Kaplan added, noting other benefits unrelated to greenhouse gas emissions, including better animal welfare and less water pollution. "But plants are better than either."