A new snapshot of U.S. methane emissions in 2004 shows livestock — primarily cattle and pigs — were the country's worst gas emitters at the time.

The study also found that livestock expelled 40 percent more methane than had been estimated by the Environmental Protection Agency, a result in agreement with a recently published review of hundreds of emissions studies detailed in the journal Science. The animals also bested the oil and gas industry.

"We are very confident that livestock emissions were being underestimated," said lead study author Kevin Wecht, an atmospheric chemist at Harvard University in Massachusetts.

Methane is less abundant than carbon dioxide, and disappears faster in the atmosphere than carbon dioxide. However, it is more effective at trapping infrared radiation (the greenhouse effect). Methane gas comes from natural sources, such as decomposing plants in wetlands, and from human activities, including oil and gas production and animals and manure on farms. [Explore Earth's Atmosphere: Top to Bottom (Infographic)]

Most scientific studies track methane in one of two ways: either by measuring the gas in the atmosphere with instruments placed on tall towers or flown on aircraft, or by analyzing emissions directly at the source, such as at oil and gas wells.

The new analysis led by Wecht took a broader look, by using satellite monitoring of methane gas levels in the atmosphere over the United States. Wecht and his co-authors combined satellite and aircraft data to provide a comprehensive look at methane, which is a potent but short-lived greenhouse gas. The researchers only looked at 2004, because that was the year for which the best satellite coverage was available, Wecht said.

In 2004, cows, pigs and other livestock expelled more than 13 million tons (12.2 megatons) of methane, from both manure and escaping body gas, according to the study, published June 26 in the Journal of Geophysical Research: Atmospheres. The EPA estimated 9.7 million tons (8.8 teragrams) of methane from livestock in 2004.

That same year, the researchers found 7 million tons (7.2 teragrams) of methane came from oil and gas operations. The EPA attributed 9.9 million tons (9.0 teragrams) to oil and gas industries.

The new numbers put livestock well ahead of oil and gas producers in 2004. The latest greenhouse gas inventory published by the EPA still has livestock leading oil and gas in 2012 methane emissions.

Overall, the new research finds a total of 33 million tons (30.1 teragrams) of methane was released by human activities in the United States in 2004. The EPA's total for 2004 was 31 million tons (28.3 teragrams) of methane.

Natural gas numbers off?

The review published in February in the journal Science of more than 200 studies found the EPA is underestimating total U.S. methane emissions by anywhere from 25 to 75 percent.

"It's not surprising that livestock emissions were found to be higher than EPA methodologies," said Adam Brandt, an energy resources engineer at Stanford University in Palo Alto, California, and lead author of the Science study. "From the results of our analysis, we found the EPA was consistently underestimating total emissions," said Brandt, who was not involved in the new research.

Wecht said satellite tracking could provide a much-needed check on different methods for tallying methane levels. However, satellite data for past years is sparse, he added.

Satellite monitoring of methane emissions will get a much-needed boost next year, with the planned launch in 2015 of the Tropospheric Monitoring Instrument aboard a Sentinel satellite operated by the European Space Agency.

The instrument's daily snapshot of methane emissions may finally resolve some of the debate over methane emissions, Wecht said.

"A study like this will be the ideal way to try and estimate what the impact of the recent oil and gas boom has had on methane emissions," Wecht said.

Brandt said researchers would still need to measure methane outputs directly at the source. "I see huge value in [satellite studies], but you're still going to need to understand what's going on in the field."

Email Becky Oskin or follow her @beckyoskin. Follow us @livescience, Facebook & Google+. Original article on Live Science.