A just-published analysis of data received from a satellite in 2004 has shown that at least during that year, livestock in the U.S. emitted more methane into the atmosphere than did the oil and gas industry. In their article published in Journal of Geophysical Research: Atmospheres, a team of researchers from Harvard University, California Institute of Technology and the University of California studying the data note that such emissions were far higher than was reported by the U.S. Environmental Protection Agency (EPA).
Specifically, the researchers found satellite data showed livestock emitted 13 million tons of methane over the summer in 2004 (the EPA reported 9.7 million tons). They found the satellite data also showed that the combined emissions of the oil and gas industry amounted to 7 million tons (the EPA reported 9.9 million tons).
Unfortunately the sensor on the satellite was unable to show methane amounts after 2004, thus more data is not available. That will, however, change soon as a new satellite with sophisticated atmospheric gas monitoring sensors aboard is set to launch next year. More information on the role that methane plays in changing our climate can be found here.
Methane has a pretty short half-life before it oxidises to CO2 and water. Either UV catalyzed, or it gets up high enough to react with ozone.Regardless of how much is emitted (excluding something like an ocean clathrate catastrophe) it is effectively gone in twelve years.