Here's the science if you're really interested. (From PETA website)
Feeding massive amounts of grain and water to farmed animals and then killing them and processing, transporting, and storing their flesh is extremely energy-intensive. And forests—which absorb greenhouse gases—are cut down in order to supply pastureland and grow crops for farmed animals. Finally, the animals themselves and all the manure that they produce release even more greenhouse gases into our atmosphere.
Greenhouse-Gas Emissions
Carbon dioxide, methane, and nitrous oxide are all powerful greenhouse gases, and together, they cause the vast majority of climate change.
Carbon Dioxide
Burning fossil fuels (such as oil and gasoline) releases carbon dioxide. Since it takes, on average, about 11 times as much fossil fuel to produce a calorie of animal protein as it does to produce a calorie of grain protein, considerably more carbon dioxide is released. Researchers acknowledge that “it is more ‘climate efficient’ to produce protein from vegetable sources than from animal sources.”
Chatham House, an international affairs think tank, has called for a carbon tax on meat to help combat climate change. Of course, eating vegan foods rather than animal-based ones is the best way to reduce your carbon footprint. A University of Chicago study even showed that you can reduce your carbon footprint more effectively by going vegan than by switching from a conventional car to a hybrid.
Methane
The billions of animals who are crammed onto U.S. factory farms each year produce enormous amounts of methane. Ruminants—such as cows, sheep, and goats—produce the gas while they digest their food, and it’s also emitted from the acres of cesspools filled with the feces that pigs, cows, and other animals on these farms excrete. The U.S. Environmental Protection Agency has shown that animal agriculture is globally the single largest source of methane emissions and that, pound for pound, methane is more than 25 times as effective as carbon dioxide at trapping heat in our atmosphere.
According to Vasile Stanescu, a scholar at Mercer University, animals raised by “organic” methods emit even more methane than animals on factory farms do. He believes that so-called “free-range” or “pasture-raised” animals are “significantly worse” in terms of greenhouse-gas emissions.
Nitrous Oxide
Nitrous oxide is about 300 times more potent as a greenhouse gas than carbon dioxide. According to the U.N., the meat, egg, and dairy industries account for an astonishing 65 percent of worldwide nitrous-oxide emissions. (Use the N-Calculator to calculate your nitrogen footprint and to see how you can lower your nitrogen usage.)
What Other Experts Say
The U.N. believes that a global shift toward plant-based food is vital if we are to combat the worst effects of climate change. Globally, animal agriculture is responsible for more greenhouse gases than all the world’s transportation systems combined.