As crazy as it sounds, cow 'farts' are indeed a source of global warming. When cows fart, they release methane (CH₄) into the atmosphere. In the US, about 20% of the methane emitted comes directly from cows. Although methane isn't the #1 cause of global warming, consider this analogy. When you boil water, the easiest way to burn yourself is the burner, but given enough time and allowing the water to heat up, the water will burn you too. If you turn off the burner, the source of heat will soon be gone, but the water will stay hot and will burn you for a while after the heat source is gone. So, even though methane isn't the #1 cause of global warming, the more methane that is added to the atmosphere, the longer we will feel the effects. For more information, check out this website:

You're right that cows release greenhouse gases, but cow farts are not the number one cause of global warming.

Bacteria in the stomachs of cows and other ruminants (animals with chambered stomachs) produce methane, a strong greenhouse gas, that the animals release mostly by burping but sometimes also by farting. Ruminant livestock (domestic farm animals), including cows and sheep, do release a significant amount of methane. In fact, according to the U.S. Environmental Protection Agency, ruminant livestock are the largest source of methane from "human-related activities" around the world.

However, the methane from ruminant livestock burps and farts is not the number one cause of global warming. There is a far greater amount of carbon dioxide released to the atmosphere from the burning of fossil fuels than the release of methane from livestock and other sources. Carbon dioxide plays a bigger role in global warming than does methane.

The Earths atmosphere is composed of many different types of gases. Some of these, called greenhouse gases are extremely important to the global climate. These gases include carbon dioxide (CO₂) and methane (CH₄) and help regulate the Earths surface temperature by absorbing and re-emitting infrared energy, reducing the rate that heat energy leaks from Earth into the cold expanse of space. If we didnt have these gases in the atmosphere, it would be too cold for life, as we know it, it exists. However, too much of these gases cause the planet to retain more of its radiant heat. The result is a change in climate. Although we hear much about the release of carbon dioxide into the atmosphere (by the burning of fossil fuels etc), your question relates primarily to the release of methane. Methane is very good at retaining heat (20 times better than carbon dioxide), so can be an important factor in the atmosphere. But what does this have to do with cows? Well, cows belong to a group called the ruminants (which also include sheep, goats, camels, llamas and deer) that are distinguished by the way in which they digest their food. In these plant-eating species, their digestive system is composed of several chambers in which their food is broken down into its digestible components. This process relies upon the presence of billions of bacteria in the stomach that feed on the plant matter, thereby releasing the nutritious components that the cow needs to survive. A side effect of this process is that the bacteria release methane as a waste product as they feed, and this methane is burped (not farted) out of the stomach by the cow into the atmosphere. Now a single adult cow can burp out about 280 liters (or 74 gallons) methane each day, and it is estimated that there is over a billion ruminants on Earth. This can quickly add up. It has been estimated that live-stock related methane emission contributes about 20% of all the methane released into the atmosphere each year (methane released during the process of energy extraction from natural gas fields also contributes about the same amount), and thus is a major component of human-related methane emission. Interestingly, in Kangaroos, which also have a multi-chambered stomach, methane is not a problem, as the bacteria in theirstomachs do not produce it. Scientists are currently trying to figure out how to transplant the non-methane producing bacteria found in Kangaroos into the stomachs of cattle as a way of drastically reducing the amount of methane being emitted into the atmosphere.

I have a hard time believing this, frankly, just because cows do not produce that much volume.

Farts release methane - which is a fairly potent greenhouse gas, but it is not one of the two most potent greenhouse gasses: water vapor and carbon dioxide. The Earth is getting warmer, but we aren't even sure that isn't a natural process (e.g. if the Sun is getting brighter, and it may be, then the Earth will warm up). Barring that, humans aren't contributing much to water vapor except indirectly through other greenhouse gasses, but humans certainly are contributing a lot of carbon dioxide into the atmosphere. Most of that, such as I understand, comes from coal-burning power plants, and secondarily to gasoline engines. In the long term, the primary source of carbon dioxide in the atmosphere is volcanoes, but they contribute less as rapidly as we do, I think.Most of the methane I think comes from fermentation outside of ruminants (cows).

So I don't think that cows are the primary cause,although I don't think also that anybody knows what the primary cause is.