Levels of an important greenhouse gas have stopped growing, say U.S. scientists.

Methane levels have stayed nearly flat for the past seven years, following a rise during the two previous decades, according to researchers at the University of California, Irvine.

The findings suggest methane may no longer be as large a global warming threat as previously thought and provide evidence that methane levels can be controlled.

The research, led by professors Sherwood Rowland and Donald Blake, will be published in Thursday's online edition of Geophysical Research Letters.

"If one really tightens emissions, the amount of methane in the atmosphere 10 years from now could be less than it is today. We will gain some ground on global warming if methane is not as large a contributor in the future as it has been in the past century," Rowland, of the department of chemistry and earth system science, said in a news release.

Rowland is the co-recipient of the 1995 Nobel Prize for chemistry for discovering that chlorofluorocarbons in products such as aerosol sprays and coolants were damaging the Earth's protective ozone layer.

He and his colleagues believe the slowdown in methane growth may be in part because of leak-preventing repairs to oil and gas lines and storage facilities, which can release methane into the atmosphere.

Other explanations include slower growth or decrease in methane emissions from coal mining, rice paddies and natural gas production.

"If carbon dioxide levels were the same today as they were in 2000, the global warming discussion would leave the front page. But to stabilize this greenhouse gas, we would have to cut way back on emissions," Rowland said.

"Methane is not as significant a greenhouse gas as carbon dioxide, but its effects are important. The world needs to work hard to reduce emissions of all greenhouse gases."

Methane is the major ingredient in natural gas. It is also a powerful greenhouse gas and helps form ozone, an ingredient in smog.

Atmospheric levels of the gas have more than doubled since the Industrial Revolution in the late 1700s. About two-thirds of methane emissions can be traced to human activities such as fossil-fuel extraction.