With Earth's human population at 7 billion, racing toward more than 9 billion by 2050, environmentalists of a vegetarian bent tell us we should give up eating meat, if we haven't already.
Bill McKibben, in his recent book "Eaarth: Making a Life on a Tough New Planet," chides Americans for eating so much grain-fed beef.
Raising grain is fossil-fuel-intensive -- at least the way we do it these days. And raising beef -- again, the way we do it these days -- is grain-intensive.
Americans eat 200 pounds of grain directly each year and 1,800 pounds "that's been run through an animal first," McKibben says.
He has a point. Half the U.S. corn crop becomes animal feed; most of the rest is brewed into ethanol. Hardly any feeds people directly.
Whether you like marbled steaks or not, the grain-to-animal conversion rate is woefully inefficient -- about 35 pounds of grain to produce a single pound of boneless beef.
Meanwhile, intensive farming across the Midwest destroys wildlife habitat, contaminates groundwater, pollutes streams and lakes, and leaves a dead zone the size of New Jersey in the Gulf of Mexico.
But something has to give: Demand for groceries is soaring -- not just because of the growing world population, but because of growing prosperity.
A burgeoning middle class wants to eat like us -- adding more Big Macs and KFC to their diets, in addition to fresh fruits and vegetables. Food experts estimate that demand for food will double by 2050.
The meat has to go, right?
Let's hope not.
For one thing: The pitch for a vegetarian lifestyle to feed the world is at odds with the belief of many of us, including apparently billions in the developing world, that meat, dairy and eggs are superior and healthy additions to the human diet.
Second, the argument against meat overlooks the fact that animals are useful parts of a system of mixed farming (admittedly a bit of an anachronism in the modern landscape of industrial farming). Chickens and pigs dispose of waste produce and grain.
Cows pasture on land unsuited to crops. Manure provides valuable fertilizer. Animals such as bison graze semi-arid prairie, performing conservation duty while providing an income to Great Plains ranchers.
Fixating on meat as the malady is a good example of the tail wagging the dog. The root problem isn't that we cover our continent with grain to eat more red meat than we need.
The problem is that we grow so much grain we hardly know what to do with it. So we feed it to cattle, put it in soft drinks, and even force ourselves to burn it in our cars.
And why do we produce such an abundance of grain? Because we rig the market to make it so.
The real environmental problem is not that we eat meat; it's that we insist on subsidizing inefficient and destructive grain production.
That's what will have to change in the years ahead if Earth is to remain a pleasant place to live. Fortunately, some agricultural big thinkers think this is possible.
Jason Clay, senior vice president of market transformation for the World Wildlife Fund, says that to preserve a planet that we will recognize in half a century, we must "freeze the footprint of agriculture."
No more destruction of prairies. No more clearing forests. No more draining wetlands. But it can be done, says Clay.
Better crop genetics, from traditional plant breeding to genetically modified organisms, are on track to boost production 50 percent by 2050. Better management practices -- those used by the best farmers in the most productive countries -- could add another 50 percent.
More efficient use of water, fertilizer, pesticides and energy can add 40 percent while curbing some unsustainable practices. Rehabbing degraded land will contribute an additional 25 percent.
Reforming property rights to protect the interests of farmers (primarily in the Third World, where these rights are tenuous) and inventors of ag products (primarily in the First World, where hybrids and GMOs are developed) can increase production 20 percent.
Trimming waste -- crops that rot because they can't get to market, food that spoils in the family refrigerator -- could add another 10 percent.
These reforms, says Clay, have the potential to triple food production on the same amount of land we farm today.
But, warns Clay, "the business-as-usual scenario will not get us where we need to go."
Jonathan Foley, director of the Institute on the Environment at the University of the Minnesota, has been studying with his colleagues a similar scenario -- how to double production, cut greenhouse gases by 80 percent, and stop loss of biodiversity, pollution, and permanent drawdowns of aquifers.
"Those are all unsustainable and we shouldn't keep doing them," says Foley. "Is there any way on earth to imagine doing that simultaneously?"
Then he answers his own questions: "Yes, it might be possible. Here's how you might be able to do it."
First step: Halt the expansion of agriculture to new land. Deforestation releases huge amounts of the greenhouse gases that most scientists believe drive global warming.
Most newly cleared land isn't nearly as productive as long-established farmland, anyway. In the Amazon basin, land is cleared primarily to lay claim to it, the result of a broken land-tenure system. "Brazil is now reforming that dramatically," says Foley.
Moreover, Cargill is working with independent auditors to certify that its soybeans do not come from land that has recently been deforested. Says Foley, "We have to tip our hats to the Cargill guys."
Step two: Focus on underperforming land in Africa, large regions of Latin America, and Eastern Europe.
"We can invest in eking out another 5 percent in Iowa, or we could quadruple the yields in Belarus and Ukraine," Foley says. "And that's without any GMOs, new breeds or anything. If you got Iowa farmers in Ukraine, they could quadruple the production if they had access to markets and farmer co-ops, and if they could farm like they do here on the same kind of climate and soils. It's about markets and institutions."
The third step: the Goldilocks plan.
"How can we get more food per drop of water? How can we get more food per ton of fertilizer?" Foley asks. "Here in Minnesota and Iowa we're using too much fertilizer, and we have the dead zone in the Gulf of Mexico as a result. But in a lot of Africa, we're using way too little. Can we find the Goldilocks fertilizer rates that are just right, where farmers are getting the most food and the least environmental pollution? Right now in Minnesota we're getting the most food, and in Africa they're getting the least pollution, but nobody is getting both."
Step four: Allocate resources to produce the diet we actually eat, rather than grain to feed animals and fuel cars. "Dedicating basically the entire Midwest to growing animal feed and ethanol -- ultimately in a world of 9 billion people we're going to have to ask is that something we can sustain. I don't think that it is.
"A lot of this may come down to the subsidies," says Foley. "Now we're subsidizing big grain and beef production and ethanol. Subsidies are the only thing making that viable right now. If I were a farmer I'd be doing exactly the same thing," he says.
"What we really want is healthy food that people can afford that doesn't hurt the environment. Show me a subsidy right now that rewards that. Our economy needs to change a little bit so farmers can make a decent living doing the right things, which is feeding the world and being stewards of the environment. If farmers can't make a living doing both of those things, then there's something broken with the economy. It's not that the markets are screwed up; it's that the rules we've given the markets are wrong."
The debate over agricultural subsidies is one we'll have again soon. In 2012, the nation's farm bill comes up for renewal. Because it determines the level and nature of crop subsidies over millions of acres, the farm bill is one of the most important pieces of environmental legislation in the country.
So rather than talk about quitting meat, let's have a conversation about something that will really make a difference -- the kinds of incentives that will give us an agriculture for the year 2050.
If we get this right, we stand to have food for 9 billion and still have a healthy planet. In the process, meat will probably cost more and we'll eat a bit less of it. But the meat we do eat will be produced far more efficiently.
Greg Breining writes about travel, science and nature for the Star Tribune, the New York Times, Audubon and other publications. His latest book is "Paddle North: Canoeing the Boundary Waters-Quetico Wilderness."