Arktische Methanzeitbombe vorerst entschärft? Kein Anstieg der Methanemissionen in Alaska während der letzten 30 Jahre

Man hatte sich sehr große Sorgen gemacht: Im Zuge der Arktiserwärmung könnten große Mengen an Kohlenstoff in Form von Methan aus dem Dauerfrostboden entweichen. Methan ist ein äußerst wirksames Treibhausgas, das die globale Erwärmung dann weiter gesteigert hätte. Angesichts der möglichen Gefahr, machten sich etliche Wissenschaftlergruppen an die Erforschung der vermeintlich tickenden Methanzeitbombe. Die Ergebnisse sind hochinteressant und deuten daraufhin, dass man die Bedrohung wohl zum Glück überschätzt hat.

So gab die American Geophysical Union (AGU) am 22. Juni 2016 eine Pressemitteilung zu einem neuen Paper in den Geophysical Research Letters heraus, das Hoffnung macht. Eine Forschergruppe um Colm Sweeney hatte Luftproben in Alaska über drei Jahrzehnte hinweg ausgewertet und konnte keinen langfristigen Anstieg des Methangehalts feststellen. Aus dem Dauerfrostboden scheinen also keine gesteigerten Mengen an Methan auszugasen, obwohl sich die Region in derselben Zeit spürbar erwärmt hat. Die Wissenschaftler rätseln nun, wohin der Kohlenstoff des tauenden Permafrostbodens denn wirklich entwichen ist. Als Möglichkeiten ziehen Sie CO2-Ausgasung, Abtransport durch Flüsse oder Aufnahme des Kohlenstoffs durch die Vegetation in Betracht.

Im Folgenden die Pressemitteilung der AGU:

Study: As Alaska warms, methane emissions appear stable
Fate of carbon stored in permafrost remains subject of intense research

Analysis of nearly three decades of air samples from Alaska’s North Slope shows little change in long-term methane emissions despite significant Arctic warming over that time period, according to new research published in Geophysical Research Letters, a journal of the American Geophysical Union.

Scientists estimate that Arctic permafrost, a thick layer of frozen soil that encircles the globe, contains two and a half times as much carbon as has been emitted since the dawn of the Industrial Revolution. As the region warms, this carbon will be released from the permafrost’s icy grip. Scientists need to know where that carbon will go and what form it will take. This has become more critical since the Arctic is warming faster than other regions of Earth, with corresponding losses in sea ice coverage. Some models suggest that a portion of that carbon will be released as methane, a potent greenhouse gas that has almost 28 times the warming influence of carbon dioxide over a 100-year timescale.

In the new study, researchers from the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder, NOAA, NASA and other university partners examined 29 years of continuous, precision measurements of atmospheric methane and other gases from the NOAA Barrow Atmospheric Baseline Observatory, which is part of NOAA’s Global Greenhouse Gas Reference Network. “There has been a huge increase in Arctic warming, and while we do see spikes in methane due to short-term temperature changes, we’re not seeing a long-term change in methane levels,” said Colm Sweeney, a CIRES scientist working at NOAA’s Earth System Research Laboratory in Boulder and lead author of the new study. But that doesn’t mean thawing permafrost isn’t releasing carbon, Sweeney said. “It’s happening. It just isn’t show ing up as methane.” Arctic permafrost contains an estimated 1,000 gigatons (1,000 billion tons) of carbon. Besides being emitted as methane, carbon stored in thawing permafrost could be released into the atmosphere as carbon dioxide, carried off by meltwater into river systems, or taken up by vegetation as plant communities expand their range.

The team supplemented the continuous measurements from the Barrow observatory with measurements made by a five-year, NASA-led airborne campaign known as CARVE (Carbon in Arctic Reservoirs Vulnerability Experiment), which helped them nail down methane’s seasonal and long-term trends in the region. They saw an uptick in methane levels in late fall and winter, but no long-term signal across Alaska’s North Slope. “Bacteria that produce methane and bacteria that consume methane will both become more active as temperatures get warmer,” said Steven Wofsy of Harvard University and co-author of the study. “Our study suggests that over the past 30 years, these processes have balanced out in the study area.” The researchers conclude that observed short-term methane spikes from the Arctic will likely have little impact on global atmospheric methane levels in the long-term.

This finding is critical to science’s understanding of how the Arctic is responding to the unprecedented disruption of its climate and the degradation of permafrost. The lack of significant long-term trends indicates that processes regulating North Slope methane emissions need more study. With little observed change in methane emissions, researchers are further examining the Barrow observatory’s dataset for signs that the permafrost has been emitting carbon dioxide, by far the most significant of the greenhouse gasses, as it may be more affected by large temperature changes (and by extension, melting permafrost) in the Arctic. The Barrow dataset features in an upcoming paper on Arctic carbon dioxide levels co-authored by Sweeney. Several other research efforts are also examining this hypothesis.

Und hier der Abstract der Studie:

No significant increase in long-term CH4 emissions on North Slope of Alaska despite significant increase in air temperature
Continuous measurements of atmospheric methane (CH4) mole fractions measured by NOAA’s Global Greenhouse Gas Reference Network in Barrow, AK (BRW), show strong enhancements above background values when winds come from the land sector from July to December from 1986 to 2015, indicating that emissions from arctic tundra continue through autumn and into early winter. Twenty-nine years of measurements show little change in seasonal mean land sector CH4 enhancements, despite an increase in annual mean temperatures of 1.2 ± 0.8°C/decade (2σ). The record does reveal small increases in CH4 enhancements in November and December after 2010 due to increased late-season emissions. The lack of significant long-term trends suggests that more complex biogeochemical processes are counteracting the observed short-term (monthly) temperature sensitivity of 5.0 ± 3.6 ppb CH4/°C. Our results suggest that even the observed short-term temperature sensitivity from the Arctic will have little impact on the global atmospheric CH4 budget in the long term if future trajectories evolve with the same temperature sensitivity.

Siehe auch unseren früheren Blogartikel "Potsdamer Methan-Klimabombe erweist sich als Blindgänger: Karbonhaushalt in Tundra trotz Klimawandels stabil"