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The missing methane link

Methane is a greenhouse gas that remains in the atmosphere nine to 15 years and is 21 times more efficient at trapping heat than carbon dioxide. Although scientists have been able to track methane entering the atmosphere from a variety of sources, both natural and anthropogenic, they have recognized that something was missing: There was more methane being added into the atmosphere annually than could be attributed to a known source. Now, researchers working in Azerbaijan have quantified one of the missing methane emitters — mud volcanoes.

Giuseppe Etiope and Alexei Milkov measure the methane escaping from an “everlasting fire” at a mud volcano in eastern Azerbaijan. Mud volcanoes emit upwards of 6 to 10 million metric tons of methane (a powerful greenhouse gas) per year into the atmosphere. Image courtesy Giuseppe Etiope.

Around the world, these conical-shaped piles of mud and rock formed by the ejection of methane are emitting up to one-fourth of the geologic methane flux yearly, says Giuseppe Etiope, a geologist at the National Institute of Geophysics and Volcanology in Rome. Etiope estimates that geologic sources introduce 40 to 50 million metric tons of methane into the atmosphere each year, and that mud volcanoes produce at the very least 6 to 10 million metric tons of the total, as he and colleagues report in the June Geology. This contribution could have enormous implications for climate change models, which do not adequately take into account geologic emissions of methane, Etiope says.

More than 900 mud volcanoes have been counted in 26 countries, with 300 more on shallow ocean shelves and countless others deep in the ocean. Azerbaijan hosts the world’s largest mud volcanoes and the densest mud volcano population, with more than 200 onshore and an additional 160 immediately offshore in the Caspian Sea, according to the Geology Institute of Azerbaijan. The volcanoes — which range in height, from less than a meter to 700 meters, and in width, from less than a meter to several kilometers — pass gas through bubbling water pools, gas-mud vents and occasional eruptions. The gas is more than 90 percent methane and less than 10 percent carbon dioxide.

Although geologists have been studying mud volcanoes extensively for decades, this marks the first time that researchers have measured the methane contribution from such gas vents and microseepage, Etiope says. “We realized that nobody knows how much methane is emitted to the atmosphere and the ocean, and decided to find an answer to that question,” says Alexei Milkov, a geologist with BP in Houston who is a co-author on the paper.

On four large mud volcanoes in Azerbaijan, the researchers recorded the methane emissions from 24 gassy vents and bubbling pools during their non-eruptive phases. They also measured the amount of methane seeping from the ground at 87 different spots far away from the craters. These data show that over this small area (6 square kilometers), methane output is at least 1,400 metric tons per year. Extrapolation across a larger area quickly showed the researchers that the mud volcanoes were providing a significant amount of methane into the atmosphere, Milkov says.

However, he cautions, “it is still not clear” exactly how much methane these volcanoes are contributing to the atmosphere. Euan Nisbet, a geologist at Royal Holloway University in England who studies atmospheric methane, says that he also remains cautious about the actual amounts of methane that mud volcanoes release into the atmosphere annually. But, he says, “what is clear is that it will be very valuable to understand more about geological emissions, as they may be a major source [of methane] that is underestimated.”

Considering Etiope’s team’s evidence, Nisbet says, the source contributions of methane should be studied more and separated out in climate models. Currently, most climate change compilations, such as the Intergovernmental Panel on Climate Change report and the Kyoto Protocol, “neglect” geologic methane as a greenhouse gas contributor or do not break out geological sources as a separate category, he says. Geologic sources are instead “dumped in other buckets — such as leaks from the fossil fuel industry or unspecified marine emissions, for example — to balance the equations.”

Why geologic sources of methane have not been included in the foremost climate change models is “basically a lack of multidisciplinary climate studies,” Etiope says. He and Milkov both hope that if nothing else, their research will lead to more interdisciplinary studies on methane sources.

Megan Sever

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