Methane gas emission in the U.S. is millions of tons higher than the Environmental Protection Administration (EPA) estimates, according to a new study conducted by scientists at Harvard University and seven other institutions, was published Monday in Proceedings of the National Academy of Science. In 2008, the study found the U.S. spewed 49 million tons of methane into the air—compared to the 32 million tons the EPA estimates or the 29 million tons determined by the European Commission. The scientific research showed much of the global warming-causing gas is coming from Texas, Oklahoma, and Kansas.

Natural-gas production, cattle farming, landfills, coal mining and more produce the devastating greenhouse gas, although human activities contribute 60 percent of the total.

Across the entire U.S., methane emissions are 1.5 to 1.7 times higher than has been reported by the EPA and the international Emissions Database for Global Atmospheric Research, the study said.

The difference in estimates, according to a news release from Harvard University is the methodology.

Where the EPA uses a "bottom-up" approach to determine emissions, the study used a "top-down" approach that measures the levels of methane present in the atmosphere. It then used meteorological data and analysis to trace it back to the source.

"The bottom-up and top-down approaches give us very different answers about the level of methane gas emissions," lead author Scot M. Miller, a doctoral student in Earth and Planetary Sciences through the Harvard Graduate School of Arts and Sciences, said in a statement.

“Most strikingly, our results are higher by a factor of 2.7 over the south-central United States, which we know is a key region for fossil-fuel extraction and refining.

"It will be important to resolve that discrepancy in order to fully understand the impact of these industries on methane emissions," Miller added.

Co-author of the study Anna Michalak, a scientist at the Carnegie Institution for Science in Stanford, Calif., said the paper is the "most detailed estimate to date" of total U.S. methane emissions.

Earlier this month, the United Nations reported greenhouse gas in the globe's atmosphere hit a record high in 2012.