How will 9 billion or 10 billion people eat without destroying the environment?

The human population has reached 7.6 billion and could number 9 billion or 10 billion by midcentury. All those people will need to eat. A sobering report published Wednesday in the journal Nature argues that a sustainable food system that doesn’t ravage the environment is going to require dramatic reforms - including a radical change in dietary habits.

To be specific: Cheeseburgers are out, fruits and veggies are in.

The 23 authors of the report - hailing from Europe, the United States, Australia and Lebanon - reviewed the many moving parts of the global food system and how they interact with the environment. The authors concluded that current methods of producing, distributing and consuming food are not environmentally sustainable, and that damage to the planet could make it less hospitable for human existence.

A core message from the researchers is that efforts to keep climate change at an acceptable level will not be successful without a huge reduction in meat consumption.

"Feeding humanity is possible. It's just a question of whether we can do it in an environmentally responsible way," said Johan Rockström, an earth scientist at the Potsdam Institute for Climate Impact Research in Germany and a coauthor of the study.

The report comes on the heels of a warning from the U.N. Intergovernmental Panel on Climate Change that global leaders need to take unprecedented action in the next decade to keep the planet’s average temperature from rising more than 1.5 degrees Celsius above preindustrial levels.

Global warming has typically been linked to the burning of fossil fuels, but food production is a huge and underappreciated factor, and the new report seeks to place food in the center of the conversation about how humanity can create a sustainable future.

"Everybody knows that energy has something to do with climate - we need to transform our energy system. There's very few people who realize that it's just as, and maybe more, important to transform our food system," said Katherine Richardson, director of the Sustainable Science Center at the University of Copenhagen in Denmark. Richardson, who was not part of the team producing the new study, added, "The food system is broken and needs to be fixed if we have any hope of feeding 9 to 10 billion."

Already, half the planet's ice-free land surface is devoted to livestock or the growing of feed for those animals, Richardson said. That's an area equal to North and South America combined, she said. Rain forests are steadily being cleared for cropland. And the demand for food is increasing faster than the population: Rising income in China and many other formerly impoverished countries brings with it a higher demand for meat and other forms of animal protein. Some 70 percent of the world's fresh water is already used in agriculture, and the demand for that water will intensify.

The Nature report, titled "Options for keeping the food system within environmental limits," contends that, without targeted changes, pressures on various environmental systems will increase 50 to 90 percent by 2050 compared with 2010. There's no simple solution, the authors write; rather, "a synergistic combination of measures" will be needed to limit the environmental damage.

One obvious measure is a change in diets. Researchers say meat production - which includes growing food specifically to feed to livestock - is an environmentally inefficient way to generate calories for human consumption. Moreover, ruminants such as cows are prodigious producers of methane as they digest food, and methane is a potent greenhouse gas. The report says greenhouse-gas emissions from the global food system could be reduced significantly if people curb red-meat consumption and follow a diet built around fruits, vegetables, nuts and legumes.

To limit greenhouse-gas emissions, "We won't get very far if we don't seriously think about dietary changes to a more plant-based diet," said Marco Springmann, lead author of the report and a senior researcher at the Oxford Martin Program on the Future of Food.

He said that what is good for the planet is good for the eater. For most people consuming a typical Western diet, eating less meat will generally mean better health.

Two representatives of the National Cattlemen's Beef Association, asked to respond to the Nature report, said the U.S. beef industry is focused on improving the efficiency of beef production. The United States had 128 million head of cattle (including dairy cows) in 1976 and 94 million cattle as of this past January, yet it produces just as much beef today as it did in the 1970s, in part because of breeding efforts that boosted the growth rate of the livestock, said Sara Place, the Beef Association's senior director for research on sustainable beef production.

Ashley McDonald, senior director of sustainability for the association, said, "We're trying as an industry to take a proactive stance and really make a commitment to continuous improvement."

The report notes that the current food system is incredibly wasteful, with about one-third of the food produced eventually being discarded. Most of that food waste comes from spoilage. Halving the amount of wasted food would put a dent in the overall environmental problem, they said, and reducing waste by 75 percent is theoretically possible.

The report is agnostic on whether the world should adopt genetically modified organisms (GMOs) in the food supply. The report also does not take a position on population growth. Although birth rates have declined dramatically in many countries - to levels far below the replacement rate - the global population continues to rise. A 2015 U.N. report estimated that the population would reach 9.7 billion by 2050.

Decades ago, the prospect of so many human beings crowding the planet inspired predictions of widespread famine. The "green revolution" in agriculture changed the equations. Still, the food is not evenly distributed. About 3 billion people are malnourished today and 1 billion of them suffer from food scarcity, according to Rockström.

At the core of this research is the argument that Earth has several limits, the "planetary boundaries," that cannot be exceeded without potentially dire consequences. These boundaries - which involve factors such as climate change, loss of biodiversity, deforestation, atmospheric aerosols (smog), stratosphere-ozone depletion and the supply of fresh water - define the "safe operating space" for humanity. Proponents of the hypothesis say that human civilization has thrived in the geological epoch known as the Holocene, covering a period of roughly 11,700 years since the end of the last ice age, but that damage to the environment could put humanity into an existential crisis.

"You can imagine a scenario in which contemporary society starts to unravel" because of degradation in the environment, said Will Steffen, an emeritus professor of Earth-system science at the Australian National University and a proponent of the planetary-boundaries hypothesis. "So it's a long fuse, big bang."

He noted a movement in Australia to promote the consumption of kangaroo meat, since kangaroos are not ruminants and don't have the same ecological footprint.

“It’s a gamier taste, but it’s also a much leaner meat. It takes more talent to cook it to make it easy to chew and digest,” he said, before quickly adding, “I don’t like the thought of the poor little guys getting shot.”