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The vast majority of the methane in the atmosphere comes from bacterial production in permafrost and by the production of oil. This urban legend about cows gets sillier every year. Yes cows do produce methane--so do we. But what they produce is nowhere near the 75% you claim. Furthermore, this biogenic methane is a drop in the bucket to what may be released from the methane ices (or clathrates) buried in the ocean. If global warming continues on its seemingly inevitable path, the release of the enormous amounts of methane from the ocean will be a major ecological disaster the like of which hasn't been seen in 65 million years. Man does not live by bread alone.