A new NOAA study shows that methane emissions from the United States did not grow significantly from 2000 to 2013 and are not likely to have been an important driver of the increase in atmospheric methane levels observed worldwide after 2007, as other studies have suggested.

The paper, published in the Journal of Geophysical Research: Atmospheres on March 24, provides additional insight into a question that has puzzled scientists for the past decade: What has been causing the increase in global methane levels since 2007?

To examine whether U.S. oil and gas development could be playing a role, NOAA scientist Lori Bruhwiler and an international team of scientists analyzed methane levels in air samples collected by NOAA aircraft around the U.S. They did not find evidence of large increases in methane emissions.

“Our results show that U.S. methane emissions have likely grown at a very slow rate,” said Bruhwiler, the lead author of the paper. “Other scientists have proposed that large increases from the U.S. are a significant contributor to the global increase, and we just couldn’t find evidence of this from our measurements.”

Methane levels on the rise 

Globally, methane levels in the atmosphere grew from the 1980s, when measurements began, to about 1999. Methane levels flattened between 1999 and 2007, and then resumed their growth at a time that coincided with an historic surge in U.S. oil and gas development activity. Several published studies have sought to draw a link between the increased the U.S. oil and gas activity and increases in global methane levels.

The question of whether U.S. fossil-fuel-sector methane emissions are on the rise is important because of the potential that new extraction technologies used here could be exported to exploit unconventional oil and gas reserves around the world.

Natural gas, which is composed primarily of methane, is regarded by some as a transition fuel between coal and renewable energy sources because natural gas, when burned,  produces about half the carbon dioxide emissions of coal, according to the U.S. Energy Information Agency.

A potent greenhouse gas 

Methane is also the second largest human-caused contributor to global warming after carbon dioxide. Though not as abundant as CO2 in Earth’s atmosphere, methane is much more potent - with 28 times the warming influence of CO2 over 100 years. Because of its global warming potential, methane leakage must be limited for there to be a climate benefit in switching from coal to natural gas.

Other recent studies – including two based on the analysis of satellite estimates of atmospheric methane -- have suggested that U.S. methane emissions rose by up to 30 percent from 2002-2014. Bruhwiler and her team of  researchers performed a thorough analysis of the satellite data used in the previous studies and could not identify evidence of a large increase in emissions from that data. Bruhwiler’s analysis of data from aircraft sampling across the U.S. also showed no trend of large growth in emissions.

NOAA scientists have contributed to several other recent studies that point to a biological, rather than fossil fuel, source for increasing global methane levels. A paper published last year found that the growth in atmospheric methane was likely due to large increases in emissions from microbial sources such as wetlands, livestock, waste and rice agriculture, especially in the tropics.

Bruhwiler’s co-author Sourish Basu, a CIRES scientist working at NOAA, said the new study was an effort to assess whether current tools and observing systems would allow scientists to evaluate the effectiveness of efforts to limit methane leaks from fossil fuel development.

“As an atmospheric scientist, my question is this: can we tell the difference between successful and unsuccessful efforts,” Basu said. “What is required to detect changes in emissions? We plan to keep refining our techniques to answer such questions.”