Agriculture is responsible for an estimated 14 percent of the world's greenhouse gases. A significant portion of these emissions come from methane, which, in terms of its contribution to global warming, is 23 times more powerful than carbon dioxide. The U.S. Food and Agriculture Organization says that agricultural methane output could increase by 60 percent by 2030 [Source: Times Online]. The world's 1.5 billion cows and billions of other grazing animals emit dozens of polluting gases, including lots of methane. Two-thirds of all ammonia comes from cows.
Cows emit a massive amount of methane through belching, with a lesser amount through flatulence. Statistics vary regarding how much methane the average dairy cow expels. Some experts say 100 liters to 200 liters a day (or about 26 gallons to about 53 gallons), while others say it's up to 500 liters (about 132 gallons) a day. In any case, that's a lot of methane, an amount comparable to the pollution produced by a car in a day.
Not nearly as much, a high meat diet produces flatulence with 10% methane. Cows produce nearly 1000x more methane than humans per day.
Have some math:
Density of CO2 = 1.98kg/m²
Mass of CO2 (equivalence) produced per person per year = 1.247kg
7 billion people
Global "output" is equivalent to 8.727 million tons of CO2
Total CO2 released by humans in 2010 = 31 billion tons
The equivalence in other greenhouse gases = 13 billion tons
Total CO2 equivalence emitted by humans = 44 billion tons
Human flatulence = about one five-thousandth of our greenhouse gas emissions
Human greenhouse gases have caused 0.8°C of warming
Flatulence will account for about 0.000159°C of this