A growing, more affluent population competing for ever scarcer resources could make for an "unrecognisable" world by 2050, researchers say.
The United Nations has predicted the global population will reach 7 billion this year, and climb to 9 billion by 2050, "with almost all of the growth occurring in poor countries, particularly Africa and South Asia", John Bongaarts of the non-profit Population Council said.
To feed all those mouths, "we will need to produce as much food in the next 40 years as we have in the past 8000", said Jason Clay of the World Wildlife Fund at the annual meeting of the American Association for the Advancement of Science.
"By 2050 we will not have a planet left that is recognisable if current trends continue," Mr Clay said.
The swelling population will exacerbate problems, such as resource depletion, said John Casterline, director of the Initiative in Population Research at Ohio State University.
But incomes are also expected to rise over the next 40 years - tripling globally and quintupling in developing nations - and add more strain to global food supplies.
People tend to move up the food chain as their incomes rise, consuming more meat than they might have when they made less money, the experts said.
It takes about 3.4 kilograms of grain to produce half a kilogram of meat, and about 1.36 to 1.81 kilograms of grain to produce half a kilogram of cheese or eggs, experts said.
"More people, more money, more consumption, but the same planet," Mr Clay said, urging scientists and governments to start making changes now to how food is produced.
Population experts, meanwhile, called for more funding for family planning programs to help control the growth in the number of humans, especially in developing nations.
"For 20 years, there's been very little investment in family planning, but there's a return of interest now, partly because of the environmental factors like global warming and food prices," Mr Bongaarts said.
"We want to minimise population growth, and the only viable way to do that is through more effective family planning," Mr Casterline said.
Vegetarianism is looking better and better these days, I used to feel good because I didn't eat that much meat (probably only two meals per week), but now, morally speaking I can't justify that anymore.