We all have read about those wacky atmospheric scientists who wanted to know how much methane in the atmosphere came from ruminants, that is, cattle, goats, sheep, buffalo and the like. It made wonderful derisive news for politicians, comedians, pundits, late-night talk-show hosts, editors and other evil-doers, bent on tickling the fancy of an all-too-easy-to-fool public. "Governments spend millions to study cow farts," read the headlines.

At times, I thought I was the only person on the planet interested in the outcome. In case you've forgotten the results (!), the best guess as to methane's role in global warming is 15 - 18% of the effect.

Methane is about 21 times as potent a greenhouse gas as is carbon dioxide. Ruminant livestock produce 80 million tons of methane annually, which is about 22% of global methane emissions from human endeavors. So imagine my surprise upon learning that methane from geologic sources is not as well known as from cows.

Researchers Giuseppe Etiope and Alexei Milkov have quantified one of the missing methane sources - mud volcanoes. Worldwide, there are more than 900 mud volcanoes in 26 countries, with 300 more on shallow ocean shelves and many others uncounted. These volcanoes can be less than 3-ft to more than 2,000-ft high.

As reported in the June issue of Geology , the two scientists measured the methane output of a mud volcano in eastern Azerbaijan. It turns out that these mud volcanoes produce up to 6 to 10 million tonnes of methane per year into the atmosphere. Worldwide, they emit up to 25% of the annual geologic methane input to the atmosphere.

Why geologic sources of methane have not been included in the foremost climate change models? It's "basically a lack of multidisciplinary climate studies," says Etiope. Their contribution could have enormous implications for climate change models.

Etiope is a geologist at the National Institute of Geophysics and Volcanology in Rome. Milkov is a geologist with BP in Houston.