New research suggests methane emissions in the United States may be 50 percent higher than previous Environmental Protection Agency estimations. Furthermore, nearly a quarter of all man-made emissions in the country, come from three states alone - Texas, Oklahoma and Kansas.
In a study released Monday, researchers from the Carnegie Institution, Harvard and Duke, and a group from the Lawrence Berkeley National Laboratory, found average emissions in the United States may be 1.5 times higher than previous estimates, and may be as high as 2.7 times greater in the south-central part of the country.
Methane is a natural gas sometimes used to heat stoves, and can also be used in fuel. But it's also potent for greenhouse gas, which erodes the Earth's ozone layer.
"This is the first study to quantify methane emissions at regional scales within the continental United States with enough spatial resolution to significantly criticize the official inventories," said Berkeley Lab's Marc Fischer, who contributed to the study. "Even if we made emissions from livestock several times higher than inventory estimates would suggest for the southwest, you still don't get enough to cover what's actually being observed."
Fischer said in a statement that's why it's likely that oil and gas are largely responsible for the high emissions.
To determine whether the emissions were man-made, the researchers looked at the amount of propane, a gas associated with oil and gas activities, within the concentrations of methane, and found in the south-central region of country that those proportions were very high.
The scientists measured emissions from 2007 to 2008 and found methane emissions from Texas, Oklahoma and Kansas were 2.7 times higher than what was stored in the Electronic Data-Gathering, Analysis, and Retrieval system inventory. Between those three states, the methane emissions accounted for about 24 percent of all man-made sources of methane emission in the country, the study said.
"Cows don't produce propane; oil and gas does," Fischer said.
Overall, the researchers estimate that methane emissions due to producing oil and gas could be nearly five times larger than the current EDGAR inventory and twice as large for emissions from livestock.
For this study, published in the online edition of the Proceedings of the National Academy of Sciences, the researchers used the inverse approach to gather their calculations then the EPA and the EDGAR system typically use. That method accounts for all known sources of emissions that have been measured in the past, such as samples and calculations from livestock ranches, wastewater, agriculture, landfills, coal mines, and petroleum and natural gas facilities, according to the study.
"When we measure methane gas at the atmospheric level, we're seeing the cumulative effect of emissions that are happening at the surface across a very large region," said co-author Steven Wofsy, a professor of atmospheric and environmental science at Harvard University. "That includes the sources that were part of the bottom-up inventories, but maybe also things they didn't think to measure."
But by using the inverse method, the researchers atmospheric samples from airplane flights or towers and compared those with a model of airflows over the earth, which allows them to track where the air came from.
"The bottom-up and top-down approaches give us very different answers about the level of methane gas emissions," said lead author Scot Miller, a doctoral student in earth and planetary sciences at Harvard University. "It will be important to resolve that discrepancy in order to fully understand the impact of these industries on methane emission."