America’s top beef buyers have failed to tackle deforestation in South America despite some companies’ pledges to source “deforestation-free” beef, according to a report by an environmental advocacy group.

The growing global appetite for beef is the single biggest driver behind the disappearance of the planet’s rainforests—and that is likely to expand as the world hurtles toward a population of 10 billion in 2050.

Those forest suck up millions of tons of planet-warming carbon dioxide every year and are critical to solving the global climate crisis. Ending deforestation and restoring damaged forests could deliver as much as 40 percent of the emissions cuts needed to meet the goals of the Paris accord, the international climate agreement that will go into force next month.

The study, by the Union of Concerned Scientists, tracked the beef industry’s progress toward “deforestation-free” beef, ranking 13 U.S. companies on the strength of their efforts over the past few years in South America, where deforestation rates have skyrocketed over the last four decades.

While the beef industry and its customers, including major buyers like McDonald’s and Wal-Mart, have made some commitments to sourcing beef raised on land that hasn’t been cleared or burned for pasture, UCS researchers said they’re not going far enough.

Burger King, ConAgra, Kroger and Pizza Hut each earned a score of zero for failing to make any commitment toward buying “deforestation-free” beef. McDonald’s, Wal-Mart and Mars earned the report’s top scores. They have made public commitments to buying from suppliers that have pledged to raise cattle on land that hasn’t been freshly cleared.

Burger King, ConAgra, Kroger, Pizza Hut and McDonald’s did not respond to requests for comment.

“McDonald’s is very progressive and doing much more than other companies, and Wal-Mart also got a good score. Wal-Mart has had a sustainability program for years,” said Chris Wille, a co-founder of the Rainforest Alliance, an environmental advocacy group, which did not participate in the UCS study. “When they or McDonalds make the tiniest move, it rocks the whole industry.”

UCS estimated that 10 percent of global annual carbon dioxide emissions is linked to the slashing of tropical rainforests. And a United Nations analysis in 2015 showed that the amount of deforestation caused by beef production is more than double caused by soy, palm oil and wood products—the next largest drivers of tropical deforestation.

Yet the beef industry has only recently started to limit its impact. Over the past several years, a number of collaborative efforts have outlined pledges to limit deforestation—and accurately guage companies’ attempts—but most of the corporate pledges have been aimed at soy, palm oil and paper. A report by the Supply Change project found that 61 percent of the companies active in palm oil have committed to deforestation efforts, whereas only 15 percent of the industries linked to cattle had done so as of last March.

“The disparity is alarming because it is estimated that cattle production causes 10 times more deforestation than palm,” the report said.

“The cattle sector is the last to really address the need for some kind of guidance and metrics, and the last where new omnibus commitments are being made,” said Wille. “It’s really the laggard industry.”

The UCS report said even the companies getting high marks could be buying indirectly from ranchers that clear land. That’s because cattle are bred, raised and slaughtered by different ranchers in different places, so even when a company buys cattle from a “deforestation-free” supplier, that supplier may have gotten his cattle from elsewhere.

“Despite [our] progress, we recognize that there are still major risks of deforestation within our beef supply,” Wal-Mart acknowledged in a 2016 company report. “Before cattle arrive at our slaughterhouse, it is possible that they might be traded from high-risk ranches to ‘approved’ ranches or slaughterhouses.”

Wal-Mart, widely considered a leader in supply chain sustainability, also said ranchers “can simply re-register their operations under different names to access clean supply chains,” and that a major deforestation pledge by major suppliers only extends to the Brazilian Amazon.

“We are exploring ways to deepen transparency within our beef-monitoring system by extending our reach to other sensitive biomes,” the company said.

Between 1990 and 2005, beef production was responsible for 71 percent of the deforestation in South America. In Brazil, the world’s second-largest beef producer after the U.S., 80 percent of the forests cleared were linked to beef production. Ranchers have cleared millions of acres across the continent for grazing land, but also for soy, most of which is fed to livestock, indirectly compounding the problem.

“We did the damage 150 years or 200 years ago here in the U.S.,” Wille said. “It’s ongoing now in Brazil.”

In 2009, in response to pressure from advocacy groups led by Greenpeace, four of the largest meatpackers operating in Brazil signed a public agreement to stop buying beef from recently deforested land in the Brazilian Amazon. Between 2005 and 2014, deforestation rates in the region dropped 70 percent. This is at least in part because of the industry’s actions, according to the UCS report.

In 2014, criteria for sustainable beef production, with deforestation-free measures, was formally approved by the Global Roundtable for Sustainable Beef, a coalition of retailers, producers, processors and environmental groups. Those steps are now being developed by individual member countries, including Brazil and the U.S.

At a conference held this month by the Global Roundtable, conservationist groups and the industry acknowledged progress, particularly in the Brazilian Amazon. Now their focus is turning to other massive ecosystems, including the Cerrado, a swath of savanna mostly in Brazil, and the Chaco, a woods and grassland ecosystem mostly in Paraguay and Argentina.

“Issues in the Amazon have not been resolved, but there’s progress,” said Sharon Smith, who leads the tropical forest and climate initiative at UCS. “The conversation is expanding to how we begin tackling these other ecosystems.”

Earlier this year, an effort called the Forests and Agriculture Markets Initiative launched with the mission of securing deforestation-free commitments from the beef and soy industries in areas beyond the Brazilian Amazon.

“The ecosystems of the Cerrado and Chaco aren’t dense forests like the Amazon, but they store carbon, provide habitats for wildlife and provide other ecosystem services just like the Amazon,” said Carlos Saviani, vice president for food sustainability at the World Wildlife Fund.

But conservation groups say the industry will continue to threaten South American forests unless global agreements set stronger criteria, with more oversight of exactly where beef is coming from.

“What we need to see is joint collective action, modeled using the lessons from the [2009 Brazilian] Cattle Agreement, that extends to more meatpackers, more ecosystems, like the Cerrado,” Smith explained, “and all suppliers including indirect suppliers.”