Changes in fossil fuel use in recent decades contributed to methane and ethane levels in the atmosphere dropping, according to a research report by University of California at Irvine (UCI) professors that was published Thursday in the journal Nature. Since 2007, however, methane levels have begun to climb again.

The report's senior author, a UCI chemistry professor, concluded that increased capture of natural gas from oilfields probably accounted for up to 70% of the substantial leveling off seen in atmospheric methane at the end of the 20th Century. "We can now say with confidence that, based on our data, the trend is largely a result of changes in fossil fuel use," said Donald Blake, a UCI chemistry professor who co-authored the report with several other researchers.

The more recent increases in methane levels have renewed the urgency to understand what happened during the 20- to 25-year decline, and that is what Blake and his team have tried to determine more precisely. Other colleagues from UCI last year published separate conclusions -- one identifying less natural gas emissions in the oilfields, and the second one concluding that changing fertilizer and water prices in agricultural fields was the major cause of the long decline.

Methane has 20 times the global warming potential of carbon dioxide (CO2), although CO2 is filling the atmosphere in far larger amounts, Blake said. After decades of increases due to worldwide industrial and agricultural activity, the tapering of methane from the 1980s through 2005 was remarkable, and he added that scientists have long wrestled with the cause for this decline.

After methane, ethane is the most abundant hydrocarbon in the remote atmosphere, the UCI researchers said in summarizing their years of global atmospheric ethane level recordings.

"We show that global ethane emission rates decreased from 14.3 to 11.3 teragrams/year, or by 21%, from 1984 to 2010," they wrote. "We attribute this to decreasing fugitive emissions from ethane's fossil fuel source -- most probably decreased venting and flaring of natural gas in oilfields -- rather than a decline in its other major sources -- biofuel use and biomass burning."

Ethane's major emission sources are shared with methane, the report said. It also acknowledged that the jury is still out on what was the major cause of the declining levels: reduced fossil fuels or less biofuel/biomass emissions.

"Our findings suggest that reduced fugitive fossil fuel emissions account for at least 10-21 teragrams/year [30-70%] of the decrease in methane's global emissions, significantly contributing to methane's slowing atmospheric growth rate since the mid-1980s."

The researchers claim to have conducted the world's longest continuous methane and ethane sampling. They have concluded that their data shows the major factor for the declines is "most likely the trapping and sale of natural gas for use as a fuel source."

In other words, the development and growth of the natural gas industry as an offshoot of oil production helped reverse the amount of naturally occurring methane emitted into the atmosphere. This "sharply reduced" the skyward venting and flaring of methane -- the main component of natural gas -- from oilfields, the researchers said.

The researchers said the reversal is important because methane is a potent greenhouse gas (GHG), "second in importance only to carbon dioxide." No real progress can be made regarding climate change "without tackling CO2, but bringing methane under control would certainly help," said UCI researcher Isobel Simpson.

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