What is the impact of a person becoming a vegetarian on global warming?

Asked by Virginia Troyer, ’85, Santa Cruz, Calif.

At this point you may be thinking: "All right, I could eat a little less meat, but will it really make such a big difference?" I hope to add some meat to that claim here.

Livestock, does it really matter in the scheme of all greenhouse gas emissions?

In short, yes! Depending how the figure is calculated, livestock account for anywhere between 18 and 51 percent of global greenhouse gas (GHG) emissions measured in CO2 equivalent. Even the conservative estimate of 18 percent is a higher share than all transport—cars, trucks, planes, airplanes and mopeds—put together. This number is reported in CO2 equivalent because many of the gases released by agriculture, such as methane, have 23 times the global warming potential (GWP) of CO2. Nitrous oxide, of which livestock is responsible for 65 percent of anthropogenic output, has 296 times the GWP of CO2. Raising farm animals is a huge part of our climate change problem, and cutting back on animal products is one of the biggest, most immediate things we all can do to help.

Changes in land use due to livestock are also a significant contributor to our global carbon footprint. We impact more land with livestock grazing than with any other use, including crops, roads and cities. According to a report by the Food and Agriculture Organization, livestock are the single largest anthropogenic user of land. The total area occupied by grazing is equivalent to 26 percent of the ice-free terrestrial surface of the planet—a full quarter of our livable Earth! Beyond the fact that much of this land was once forest before being converted to pasture, over 20 percent of the pastureland in the world are degraded to some extent by overgrazing, compaction and erosion.  All of these processes release carbon to the atmosphere, and reduce the potential for restoring this carbon in the future.

Why does meat produce more emissions than a vegetarian diet?

The simplest way to think about why meat produces more emissions is in terms of the efficiency of converting grain to edible meat. Or even more, plant protein to animal protein.

Milk Chicken Pork Beef
 lbs. grain 1.0 2.5 4.0 8.0
edible weight
95% 55% 55% 40%
lbs. feed /
lb. edible weight
1.1 4.5 7.3 20.0
- 20% 10% 4%

Let me shed some light on this nifty table. The top row is the amount of grain required to produce a pound of raw animal (or milk). Percent edible weight, well, that is self-explanatory. The third row shows how much grain is required to produce one pound of edible meat. The fourth row is the efficiency with which plant protein is converted to animal protein.

What does it all mean? Well, for one thing, this explains why the U.S. livestock population consumes more than seven times as much grain as is consumed directly by the entire American population (Pimentel 2003). All the fertilizer, tractor fumes, transportation, ground tilling, you name it, that go into producing a pound of grain are literally multiplied by 4.5 to produce a pound of chicken breasts, or by 20 to produce a pound of ground beef.

The greenhouse steak

I know I keep picking on beef, but there's another huge problem with our favorite bovines. When you look at the full lifecycle of cattle, the emissions from producing the feed are only a small portion of the total greenhouse gas output. The matter was raised in a 2006 study published in Nature. Cattle emit a lot more gas than chickens or pigs do, and all those cow farts really add up. According to one of the studies' authors: "Beef is the single food with the greatest impact on the environment. This is true from all perspectives."

Water use and biodiversity

Even though climate change is the hot topic du jour, it not the only thing we should be thinking about. The world is moving towards increasing problems with freshwater scarcity, and we are in an era of unprecedented threats to biodiversity. I won't dwell on these topics, which are each worthy of their own column, but here are a few things to chew on:

Agriculture irrigation accounts for 85 percent of the fresh water use in the United States.

Producing one pound of animal protein requires 100 times more water than producing one pound of grain protein.

According to the Food and Agriculture Association (FAO), "livestock may well be the leading player in the reduction of biodiversity, since it is the major driver of deforestation, as well as one of the leading drivers of land degradation, pollution, climate change, overfishing, sedimentation of coastal areas and facilitation of invasions by alien species."

Going vegetarian is not enough to save the world

It may seem obvious, but eating tofu while still driving a Hummer is not going to help us solve any of our problems. While reducing our meat consumption would be a good idea for our planet and our bodies, I also want to offer a bit of perspective.

For one thing, those of us who happen to live in North America are not the only ones who want to eat more meat. Global meat production is expected to more than double between 2000 to 2050, due in large part to rising demand in China and other developing nations.

Another thing to keep in mind is that eating a typical lacto-ovo vegetarian diet (vegetarian with dairy and eggs), is not the be-all, end-all of fossil fuel reduction. Cheese and yogurt especially require significant amounts of energy, while chicken is not all that bad in terms of GHG emissions

Where that food come from matters a great deal, as well. Eating a grass-fed cow from down the road or a chicken raised in your backyard is significantly different from eating a McDonald's burger that originated in Brazil.

For all the complexities, the bottom line is clear: There are all sorts of things that we can and should think about when it comes to changing our behavior to reduce fossil fuel emissions. And our plates are great place to start.

McNally's Meatless Meal Suggestions:

Making the meatless leap can be daunting for most of us who've grown up with meat as the main star of every meal. I think you will find though, that it's not as hard as you think to cook up some delicious meals. You don't even need to eat tofu. Here are some of my favorite recipes, links and cookbooks to get you going:

"The Best Vegetarian Chili in the World": High in protein and delicious.

Angel Hair Pasta with Pesto: Simple, fast and outrageously tasty. If you are feeling adventurous you can make your own pesto.

Peanut sauce stir-fry: Using peanut butter in sauces adds flavor, creaminess and protein to your meal, and is also quick and easy.

Best cookbooks:

The Enchanted Broccoli Forest by Mollie Katzen (Yes—the Stanford dorm was named after this book!)

Moosewood Restaurant Cooks at Home

For more information, you might want to check out:

Eshel, G. and P.A. Martin. (2006) Diet, Energy and Global Warming. Earth Interactions, 10: 1-17.

Bittman, Mark. (2008) "Rethinking the meat-guzzler." New York Times.

Pimentel, D. and M. Pimentel (2003). Sustainability of meat-based and plant-based diets in the environment. Am J Clin Nutr, 78: 660S-3S

Food and Agriculture Organization. "Livestock's Long Shadow"

Fiala, Nathan. "How Meat Contributes to Global Warming." Scientific American.

Jess McNally plans to receive her bachelor's and master's degrees in earth systems in 2010.