Rätsel gelöst: Mangel an Hydroxyl-Radikalen in der Atmosphäre lässt Methangehalt stark ansteigen, obwohl Emissionen zurückgegangen sind

Methan ist ein wichtiges Treibhausgas. Zwischen 1983-2000 stieg die Konzentration stark an. Dann verharrte die Kurve für einige Jahre auf einem konstanten Plateau und setzte ihren Anstieg erst 2007 wieder fort (Abb. 1). Eine seltsame Entwicklung, ganz anders als das stetig steigende CO2.

Abb. 1: Verlauf der atmosphärischen Methankonzentration während der letzten 35 Jahre. Quelle: NOAA


Was steckt hinter diesem Schlangenlinien-Verlauf? Sind es wieder die bösen fossilen Brennstoffe? Oder ist der Schwarze Peter bei den Blähungen der Kühe zu finden? Löcher in Gaspipelines? Eine Studie der University of Bristol ist dem Mysterium nachgegangen. Eine Forscherguppe um Matt Rigby vermutet, dass die Methanemissionen in den letzten Jahren gar nicht abrupt angestiegen sind, sondern einfach Hydroxyl-Radikal fehlen, die das Methan normalerweise abbauen. Hier die Pressemitteilung: vom 17. April 2017:

Banned industrial solvent sheds new light on methane mystery

Since 2007, scientists have been searching to find the cause of a sudden and unexpected global rise in atmospheric methane, a potent greenhouse gas, following almost a decade in which concentrations had stayed relatively constant.

Recent studies have explored a range of possible causes. Suggestions have included a rise in oil and natural gas extraction, increased emissions from tropical wetlands or increases in emissions from growing East Asian economies. However, a new paper by an international team of scientists in the Proceedings of the National Academy of Sciences (PNAS) investigates an alternative possibility: a rise and fall in the concentration of the substance that destroys methane in the atmosphere, the hydroxyl radical. Lead author, Dr Matt Rigby from the University of Bristol’s School of Chemistry and Cabot Institute, said: „A change in the hydroxyl radical concentration would be a neat explanation for the changes in methane that we’ve seen. „It would mean that emissions may not have increased suddenly in 2007, but rather, risen more gradually over the last couple of decades.“

Since the global concentration of the hydroxyl radical cannot be measured directly, the team’s findings were made by studying the rate at which the solvent methyl chloroform, which is also destroyed by hydroxyl, was removed from the atmosphere. Professor Ron Prinn from the Massachusetts Institute of Technology, who co-authored the paper and leads the Advanced Global Atmospheric Gases Experiment (AGAGE), an international project that measures greenhouse gas concentrations, said: „We have been monitoring trends in the methyl chloroform for nearly 40 years because of its role in depleting stratospheric ozone. „Because methyl chloroform is now banned under the Montreal Protocol for the Protection of the Stratospheric Ozone Layer, we’ve seen its concentration drop very rapidly. „We can examine how this rate of decline changes from one year to the next to infer the hydroxyl radical concentration.“

Dr Steve Montzka from the National Oceanic and Atmospheric Administration (NOAA), who also co-authored the paper, and operates an independent measurement network for methylchloroform, added: „This paper re-examines some of the assumptions that had previously been made in studies of hydroxyl radical and methyl chloroform and shows how they influence our understanding of methane’s atmospheric sink. „To me, one of the main findings is that our objective analyses of two sets of observations tells essentially the same story, even as it becomes more and more difficult to measure methyl chloroform given that its concentration is approaching zero.“ Dr Rigby added that there was still uncertainty remaining. He explained: „Whilst there are strong hints in our study that hydroxyl radical changes could be playing a significant role in the fluctuations in methane growth, our uncertainties are very large. „In future, we need to think about new ways to reduce this uncertainty, if we are to truly understand changes in atmospheric methane.“

The study also lead to a more certain, but unexpected finding: that emissions of methyl chloroform had not dropped to zero. Dr Rigby said: „Because its production is now banned globally, we were expecting to see no emissions of this substance at all. However, we have very strong evidence that emissions are continuing.“ The team are preparing a follow-up study that would determine where these emissions are originating. Meanwhile, they are continuing to monitor methane in the atmosphere, and are waiting to see whether its current rate of increase will continue.

Paper: ‚The role of atmospheric oxidation in recent methane growth‘ by M. Rigby et al in Proceedings of the National Academy of Sciences

Am selben Tag erschien auch eine Pressemitteilung des California Institute of Technology zum selben Thema:

‚Detergent‘ Molecules May Be Driving Fluctuations in Atmospheric Methane Concentrations

New study suggests hydroxyl radicals may be behind unexplained recent increase in methane levels

During the early 2000s, environmental scientists studying methane emissions noticed something unexpected: the global concentrations of atmospheric methane (CH4)—which had increased for decades, driven by methane emissions from fossil fuels and agriculture—inexplicably leveled off. The methane levels remained stable for a few years, then started rising again in 2007. Previous studies have suggested a variety of potential culprits behind the renewed rise: increasing emissions from high-latitude wetlands, increasing fossil fuel emissions, or the growth of agriculture in Asia.

However, new modeling by researchers at Caltech and Harvard University suggests that methane emissions might not have increased dramatically in 2007 after all. Instead, the most likely explanation has less to do with methane emissions and more to do with changes in the availability of the hydroxyl (OH) radical, which breaks down methane in the atmosphere. As such, the amount of hydroxyl in the atmosphere governs the amount of methane. If global levels of hydroxyl decrease, global methane concentrations will increase—even if methane emissions remain constant, the researchers say.

Methane is the second most prevalent greenhouse gas, after carbon dioxide. However, the colorless, odorless gas can be difficult to track and derives from a wide range of sources, from decomposing biological material to leaks in natural gas pipelines. When atmospheric concentrations of methane increase, it may not be correct to chalk it up solely to an increase in methane emissions, says Caltech’s Christian Frankenberg, co-corresponding author of a study on the decadal trends of methane concentrations that was published the week of April 17 in the early online edition of the Proceedings of the National Academy of Sciences. Frankenberg is an associate professor of environmental science and engineering at Caltech and a research scientist at the Jet Propulsion Laboratory, which is administered by Caltech for NASA. His collaborators on the paper are Paul Wennberg, the R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering at Caltech, and Alexander Turner and Daniel Jacob of Harvard. „Think of the atmosphere like a kitchen sink with the faucet running,“ Frankenberg explains. „When the water level inside the sink rises, that can mean that you’ve opened up the faucet more. Or it can mean that the drain is blocking up. You have to look at both.“

In this analogy, hydroxyl represents part of the draining mechanism in the sink. Hydroxyl is the neutral form of the negatively charged hydroxide molecule (OH). It is described as a „radical“ because it is highly reactive and, as such, acts like a detergent in the atmosphere, breaking down methane into oxygen and water vapor.  Tracking decadal trends in both methane and hydroxyl, Frankenberg and his colleagues noted that fluctuations in hydroxyl concentrations correlated strongly with fluctuations in methane. However, the authors do not yet have a mechanistic explanation for the last decade’s global changes in hydroxyl concentrations. Future studies are needed to investigate this further, Frankenberg says. The researchers also would like to see the trends they detected verified with a more detailed study of both methane sources and sinks. „The tropics are the tricky part,“ Frankenberg says. „They’re very complex in terms of methane emissions and destruction.“ Methane has the shortest lifetime in the tropics due to the large amounts of water vapor and radiation there. But because tropical areas are often remote and cloud-covered (thwarting satellite observation), they remain understudied, Frankenberg says. The PNAS study is titled „Ambiguity in the causes for decadal trends in atmospheric methane and hydroxyl.“ Alexander Turner, graduate student at Harvard University, is the lead author. The co-authors are Christian Frankenberg and Paul Wennberg from Caltech, and Daniel Jacob from Harvard. This research was funded by the Department of Energy and a NASA Carbon Monitoring System grant.

Von den deutschen Medien berichteten Scinexx und WetterOnline über die Entdeckung.