• Livestock products provide one-third of humanity's protein intake.
• Livestock production — including food, fiber, fertilizers, energy and labor — accounts for 40 percent of agricultural GDP and creates livelihood for 1 billion of the world's poor, while employing 1.3 billion people.
• An estimated 20 percent of the total land for livestock production is being degraded by grazing activities; 70 percent of land degradation occurs in arid regions.
• Nearly 30 percent of Earth's ice-free surface is devoted to livestock production, while only 8 percent is devoted to crops consumed directly by people.
• Livestock production deeply impacts air quality, being responsible for over 60 percent of global anthropogenic ammonia emissions (from fertilizers), which contribute to acid rains and acidification of ecosystems.
• Livestock production is associated with 18 percent of greenhouse gas emissions, including 9 percent of carbon dioxide, 37 percent of methane and 65 percent of nitrous dioxide.
• There is increased risk of diseases transported from livestock to humans.

So, a meat-free world would have more forests, less demand for fresh water, less use of pesticides and antibiotics, more grains for people and less emission of greenhouse gases. Apparently, a no-brainer.

But cutting meat completely would not be as good a solution as it seems.

As people eat less meat, they need to eat more plants, so farming areas need to be converted accordingly. In some areas where the land is not easily arable (such as mountainous regions with dry climate), grazing cattle, sheep and goats is actually converting inedible grass into edible meat and milk. Pigs and chickens could be fed leftover foods; leftover food amounts to an absurd one-third of total food produced in the world. In some poor regions, meat and milk consumption is the only reliable source of protein and fat. Also, in poor regions, many people work with livestock. They could be redirected and trained for farming, but due to limitations with soil quality this may not be an easy shift.

Switching to a vegetarian (no-meat, but egg and dairy are OK) as opposed to a strict vegan diet is also impractical, since, in the case of livestock, cows must calve every year to keep producing milk and 50 percent of their offspring are males. What do we do with them?

It's possible that genetic engineering will take care of this issue, producing only cows tweaked to be milk-producing factories. Chickens could be tampered with as well. But how moral would such technology be?

Circling back to the question in the title, it is clear that less meat is good morally and environmentally, but no meat may not be as good as some may think. Some poor regions in the world need all the meat they can get. The same in areas where there is no arable land and grazing is efficient. Culture also plays an essential role, and meat-eating is firmly ingrained in many parts of the world. Try convincing a Maasai from Kenya that he can't eat meat or drink milk and blood. (Their diet is slowly changing to include more grains.)

For those of us who can afford a nourishing plant-based diet, there is little reason to remain a carnivore. But try telling that to a BBQ chef from South Carolina or São Paulo.

Marcelo Gleiser is a theoretical physicist and writer — and a professor of natural philosophy, physics and astronomy at Dartmouth College. He is the director of the Institute for Cross-Disciplinary Engagement at Dartmouth, co-founder of 13.7 and an active promoter of science to the general public. His latest book is The Simple Beauty of the Unexpected: A Natural Philosopher's Quest for Trout and the Meaning of Everything. You can keep up with Marcelo on Facebook and Twitter: @mgleiser

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