Bettina Gartner in the Christian Science Monitor:
As cows digest their food (up to 150 pounds of grass, hay, and silage per day, along with 20 pounds of concentrated feed), myriad microorganisms – bacteria, protozoa, fungi, and archaea – busily break down the fibers and other nutrients in their rumens. In the process, hydrogen and carbon dioxide are released. The archaea (a kind of bacteria) transform the two gases into methane (CH4), up to 100 gallons of it per cow per day, and the cows get rid of it mainly by burping.
How could a burp matter? But it does.
Odorless, colorless methane – the primary of natural gas – is a powerful greenhouse agent. According to the US Environmental Protection Agency, pound for pound methane is about 21 times more effective at warming Earth’s atmosphere than carbon dioxide is. Globally, ruminant livestock – including cattle, goats, and buffaloes – produce about 80 million metric tons of methane a year, accounting for about 28 percent of man-made methane emissions annually.
Recently, researchers from the Japanese National Institute of Livestock and Grassland Science in Tsukuba calculated the environmental impact of a serving of beef and published the result in The New Scientist. According to them, the production of one kilogram of beef (2.2 pounds) results in the emission of greenhouse gases with a warming potential equivalent to 80 pounds of carbon dioxide. In other words: Serving steak to your family is the greenhouse-gas equivalent of driving 155 miles.