By Brad Plumer

Dec. 5, 2018

WASHINGTON — If the world hopes to make meaningful progress on climate change, it won’t be enough for cars and factories to get cleaner. Our cows and wheat fields will have to become radically more efficient, too.

That’s the basic conclusion of a sweeping new study issued Wednesday by the World Resources Institute, an environmental group. The report warns that the world’s agricultural system will need drastic changes in the next few decades in order to feed billions more people without triggering a climate catastrophe.

The challenge is daunting: Agriculture already occupies roughly 40 percent of the world’s land and is responsible for about a quarter of humanity’s greenhouse gas emissions. But with the global population expected to grow from 7.2 billion people today to nearly 10 billion by 2050, and with many millions of people eating more meat as incomes rise, that environmental impact is on pace to expand dramatically.

Based on current trends, the authors calculated, the world would need to produce 56 percent more calories in 2050 than it did in 2010. If farmers and ranchers met that demand by clearing away more forests and other ecosystems for cropland and pasture, as they have often done in the past, they would end up transforming an area twice the size of India.

That, in turn, could make it nearly impossible to stay below 2 degrees Celsius of global warming, the agreed-upon international goal, even if the world’s fossil-fuel emissions were rapidly phased down. When forests are converted into farmland, the large stores of carbon locked away in those trees is released into the atmosphere.

“Food is the mother of all sustainability challenges,” said Janet Ranganathan, vice president for science and research at the World Resources Institute. “We can’t get below 2 degrees without major changes to this system.”

Less meat, but also better farming

The new study, the result of six years’ worth of modeling work conducted in partnership with French agricultural researchers, is hardly the first to warn that feeding the world sustainably will be a formidable task. But the authors take a different view of the most plausible solutions.

In the past, researchers who have looked at the food problem have suggested that the key to a sustainable agriculture system is to persuade consumers to eat far less meat and waste far less of the food that’s already grown.

The new report, however, cautions that there may be limits to how much those strategies can achieve on their own. The authors do recommend that the biggest consumers of beef and lamb, such as those in Europe and the United States, could cut back their consumption by about 40 percent by 2050, or down to about 1.5 servings a week on average. Those two types of meat have especially large environmental footprints.

But the authors are not counting on a major worldwide shift to vegetarianism.

“We wanted to avoid relying on magic asterisks,” said Timothy D. Searchinger, a researcher at Princeton University and the World Resources Institute and lead author of the report. “We could imagine a significant shift from beef to chicken, and that by itself goes a long way.” (Poultry production has about one-eighth the climate impact of beef production.)

So, in addition to actions on diet and food waste, the researchers also focused on dozens of broad strategies that could allow farmers and ranchers to grow far more food on existing agricultural lands while cutting emissions, a feat that would require a major shift in farming practices worldwide and rapid advances in technology.

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For example, they note, in parts of Brazil, the best-managed grazing lands can produce four times as much beef per acre as poorly managed lands — in part owing to differences in cattle health and how well the grass is fertilized. Improving productivity across the board could help satisfy rising meat demand while lessening the need to clear broad swaths of rain forest.

The authors also pointed to possible techniques to reduce the climate impact of existing farms. For instance, new chemical compounds could help prevent nitrogen fertilizers from producing nitrous oxide, a potent greenhouse gas. And scientists are exploring feed additives that get cows to burp up less methane, another big contributor to global warming.

The report notes that producing 56 percent more calories without expanding agricultural land could prove even more difficult if, as expected, rising temperatures reduce crop yields. But, Mr. Searchinger said, many of the recommendations in the report, such as breeding new, higher-yielding crop varieties or preventing soil erosion, could also help farmers adapt to climate change.

Conserving the world’s remaining forests

The researchers emphasize that strategies to improve the productivity of existing croplands and pastures will have to be paired with more rigorous conservation policies to protect existing forests in places like Brazil or sub-Saharan Africa. Otherwise, farmers will just find it more profitable to clear more forests for agriculture — with dire climate consequences.

“In the past, we’ve often seen agricultural policies and conservation policies moving in parallel without a lot of interaction,” said Linus Blomqvist, director of conservation at the Breakthrough Institute, who was not involved in the study. “The big challenge is to link the two, so that we get more intensive farming without using more land.”

In another contentious recommendation, the report’s authors call for a limit on the use of bioenergy crops, such as corn grown for ethanol in cars, that compete with food crops for land.

Money is also a hurdle. The report’s authors call for large increases in research funding to look at ideas like fertilizers that can be made without the use of fossil fuels, organic sprays that can reduce waste by preserving fresh food for longer, and genetic editing techniques that might produce higher-yielding crops. They also urge new regulations that would encourage private industry to develop sustainable agricultural technologies.

Over the past three years, 51 countries have spent roughly $570 billion a year to support food production, said Tobias Baedeker, an agricultural economist at the World Bank, which contributed to the new study.

If those subsidies were overhauled so that they helped support more sustainable practices, Mr. Baedeker said, “we could have a real game-changer on our hands.”