HOUSTON – Three years ago, I stood atop the Franklin Mountains at dusk, gazing over El Paso, Texas and gritty Ciudad Juárez, its third-world neighbor south of the border. I had just taken a gondola ride up the mountain, but as the lights in the houses of some 2.5 million people flickered on, I started to feel uneasy.

There I was: Comfortable, warm and happily digesting a hamburger, when right across the Rio Grande people lived in desperate conditions with rampant crime. Something about this juxtaposition of indulgence and poverty made me edgy.

Already, our planet’s 6.8 billion people include 1 billion hungry and 1.6 billion overweight, and scientists’ best predictions have the population rising to 9 billion by 2050 before leveling off. How will we feed so many people without utterly ravaging the Earth?

Here’s the dilemma: As people improve their lot, first they start consuming more food, primarily grains and tubers, and then diets shift to energy-rich vegetable oils, sugars, and meat. Raising these foods on large scales – particularly meat – requires more land, water and energy, and it creates more pollution than grain crops or veggies alone.

“We are in essence eating the world’s tropical rainforests and savannas,” University of Minnesota ecology professor David Tilman told me. But it doesn’t have to be this way. “There is no reason for even one more acre of rainforest to be cut. If we farmed them properly, the lands that have already been cleared could fully meet global food demand for at least the next 50 years,” he said.

Tilman and colleagues modeled how our diet will affect the world by 2050, warning that agriculturally-driven environmental change will rival that from a warming climate. If trends continue, people will be exposed to more pesticides, and we will run out of fresh water for irrigation. Increased fertilizer use will salinize soils and raise the number of aquatic low-oxygen “dead zones.” The loss of natural ecosystems to agriculture will exceed the land area of the United States, leading to biodiversity loss and species extinctions. They conclude that food demand could be lowered “if the trend toward diets rich in meat were reversed.”