More Gas in the Greenhouse

Methane, a natural and man-made atmospheric gas, and nitrogen trifluoride (NF₃), a gas used during the manufacture of flat-panel monitors, are two greenhouse gases on the rise in the atmosphere, according to researchers at Scripps Institution of Oceanography at UC San Diego.

Scripps geochemists Ray Weiss, Jens Mühle, and colleagues made unprecedented measurements of NF₃, a gas present in air at fractional parts-per-trillion levels, and found that amounts of the gas have risen about 20-fold over the past three decades, with a present rate of increase of 11 percent per year. The pair also co-authored a paper led by MIT researchers Matthew Rigby and Ron Prinn that documented a surprising rise in atmospheric methane levels after nearly a decade of stability.

The journal Geophysical Research Letters published the NF₃ paper in its Oct. 31 issue and will publish results of the methane study on NOV 14.  Both papers are part of the NASA-sponsored Advanced Global Atmospheric Gases Experiment (AGAGE) research program.

NF₃ is used as a cleaning agent during the making of microcircuits, liquid crystal displays, and thin-film photovoltaic cells. It had been considered such a safe alternative to other gases that its regulation as a global warming contributor was considered unnecessary. Weiss, Mühle, and colleagues’ new research suggests, however, that it escapes into the atmosphere at rates greater than manufacturers had previously estimated. According to new industry estimates of global NF₃ usage that have become available after publication of the October paper, the amount of NF₃ escaping into the atmosphere is about nine percent of that which is used.

"From a climate perspective, there is a need to add NF₃ to the suite of greenhouse gases whose production is inventoried and whose emissions are regulated under the Kyoto Protocol, thus providing meaningful incentives for its wise use,” Weiss said.

Methane comes from a variety of sources ranging from wetlands to landfills. The MIT-led team found that after eight years of relative stability, atmospheric methane abundances began to rise simultaneously at monitoring stations around the world in 2006. Some researchers have postulated that a rise in atmospheric methane in response to climate warming is due to the thawing of Arctic permafrost. The permafrost releases methane formed in past millennia by the decomposition of ancient plants. But if this were the sole cause, then scientists would expect Northern Hemispheric atmospheric methane rise to be dectected before a rise in the Southern Hemisphere.  The MIT-led research team points out, however, that if there were a simultaneous reduction in the rate at which methane is destroyed by atmospheric chemical reactions, then the thawing of permafrost could still play a significant role in the observed changes.

“This could be the beginning of something significant, but it is still too early to know if it is sustained” said Weiss. “The situation bears close watching.”

Methane and NF₃ contribute a fraction of the global warming effect of carbon dioxide, which a variety of human activities have made into the world’s leading greenhouse gas. Methane is the second most common greenhouse gas, contributing about one-third of the warming effect of CO₂, while NF₃ contributes at present only 0.04 percent. Both gases, while present in the atmosphere in smaller quantities however, are more efficient warming agents than carbon dioxide. A given mass of atmospheric methane is more than 20 times more powerful than CO₂ while NF₃ is 17,000 times more potent.

— Robert Monroe