In still another study, of Navy midshipmen with acne, four weeks of eating at least three bars of chocolate daily had no effect on skin lesions. It is known that methane, which is produced by the bovine digestive tract, is a greenhouse gas, contributing to the greenhouse effect, the trapping of some of the sun's radiant energy so that it cannot be reflected back into space. Methane is produced in the airless environment of the gut when bacteria break down cellulose in the cow's diet. Methane is far more efficient at trapping heat than the same weight of carbon dioxide, and some estimates say that at present rates of increase in its emission, it would contribute as much as 20 to 25 percent of the share of warming caused by human activity. However, experts widely agree that carbon dioxide is the biggest factor in the greenhouse effect because of the huge amount that is produced, accounting for an estimated half the human-caused effect.
More methane comes from rice paddies and forest fires.
Methane is also released by geological processes like volcanic eruptions and may also be locked in frozen tundra. And finally, it is released from coal mines and landfills.