Falschverdächtigung: Methan aus arktischem Eismeerboden nun doch kein Klimakiller

Erinnern Sie sich noch an die von interessierten Kreisen gepriesenen Horrorszenarien, dass Gashydrate im arktischen Meeresboden im Zuge der Klimaerwärmung kollabieren und enorme Mengen an Treibhausgasen freisetzen würden? Ziemlicher Quatsch, wie jetzt eine Studie des norwegischen Center for Arctic Gas Hydrate, Climate and Environment (CAGE) herausfand. Die Methanhydrate sind viel weniger anfällig gegen die Erwärmung als gedacht. Hier die Pressemitteilung des CAGE aus dem September 2018:

Methane hydrate is not a smoking gun in the Arctic Ocean

Methane hydrate under the ocean floor was assumed to be very sensitive to increasing ocean temperatures.  But a new study in Nature Communications shows that short term warming of the Arctic ocean barely affects it.

Clathrate (hydrate) gun hypothesis stirred quite the controversy when it was posed in 2003. It stated that methane hydrates – frozen water cages containing methane gas found below the ocean floor – can melt due to increasing ocean temperatures. According to the hypothesis this melt can happen in a time span of a human life, dissociating vast amounts of hydrate and releasing methane into the atmosphere. Consequently, this would lead to a runaway process, where the methane released would add to the global budget of greenhouse gases, and further accelerate the warming of the planet.

Limited impact at an Arctic site

This dramatic hypothesis inspired science fiction and scientists alike, spurring the latter to further investigate the sensitivity of hydrates.  A new study in Nature Communications has thus found that the hydrate gun hypothesis seems increasingly unlikely, at least for a specific site in the Arctic Ocean that is highly susceptible to warming. “Short term temperature warming has limited impact on the gas hydrate stability. We show that warming can significantly affect gas hydrates in the seabed only when ocean temperature is constantly rising for several centuries,” says the lead author of the study Dr. Wei-Li Hong of CAGE and currently Geological Survey of Norway.

Hydrate mounds seeping methane for thousands of years

Hong and colleagues reported on an increase of methane flux beneath large mounds of hydrates in an area called Storfjordrenna, in the Barents Sea close to Svalbard. These gas hydrate pingos are all profusely seeping methane. But according to Hong, even though the area is shallow, and potentially susceptible to temperature change, these seeps are not intensifying because of the momentary warming. “The increase of methane flux started several hundreds to thousands of years ago, which is well before any onset of warming in the Arctic Ocean that others have speculated,” says Hong. The study was based on measurements of pore water chemistry in the sediments from the area. Pore water is water trapped in pores in soil, and can be analysed to reveal environmental changes in a given area through time. Scientists also analysed authigenic carbonate, a type of rock created through a chemical process in areas of methane release, as well as measured bottom water temperatures. Data from these analyses was then used in a model experiment.

Natural state of the system

For the past century, bottom water in the area fluctuated seasonally from 1,8 to 4,6 degrees Celsius. Even though these fluctuations occurred quite often, they only affected gas hydrates that were shallower than 1,6 meters below the sea floor. The hydrates are fed by a methane flow from deeper reservoirs. As this area was glaciated during the last ice age, this gas compacted into a hydrate layer under the pressure and cold temperatures under the ice sheet.  Hydrates can be stable in the first 60 meters of sediments. “The results of our study indicate that the immense seeping found in this area is a result of natural state of the system. Understanding how methane interacts with other important geological, chemical and biological processes in the Earth system is essential and should be the emphasis of our scientific community,” Hong states.

Reference: Hong, Wei Li, et.al., Seepage from an arctic shallow marine gas hydrate reservoir is insensitive to momentary ocean warming. Nature Communications 8, Article number: 15745 (2017). doi:10.1038/ncomms15745

Und selbst wenn das Methan aus den Gashydraten freigesetzt werden würde, hat die Natur offenbar weitere Schutzmechanismen zur Verfügung, die verhindern, dass das Gas im großen Stil in die Atmosphäre gelangt. Die University of Rochester berichtete am 17. Januar 2018, dass man eine unerwartete Pufferwirkung im Ozean gefunden habe:

Ocean waters prevent release of ancient methane

Ocean sediments are a massive storehouse for the potent greenhouse gas methane. Trapped in ocean sediments near continents lie ancient reservoirs of methane called methane hydrates. These ice-like water and methane structures encapsulate so much methane that many researchers view them as both a potential energy resource and an agent for environmental change. In response to warming ocean waters, hydrates can degrade, releasing the methane gas. Scientists have warned that release of even part of the giant reservoir could significantly exacerbate ongoing climate change.

However, methane only acts as a greenhouse gas if and when it reaches the atmosphere—a scenario that would occur only if the liberated methane traveled from the point of release at the seafloor to the surface waters and the atmosphere. With that in mind, environmental scientist Katy Sparrow ’17 (PhD) set out to study the origin of methane in the Arctic Ocean.

“While a logical suspect for arctic methane emissions is degrading hydrates, there are several other potential methane sources. Our goal was to fingerprint the source of methane in the Arctic Ocean to determine if ancient methane was being liberated from the seafloor and if it survives to be emitted to the atmosphere,” says Sparrow, who conducted the study, published in Science Advances, as part of her doctoral research at the University of Rochester.

Sparrow, her advisor, John Kessler, an associate professor of earth and environmental sciences, and other collaborators conducted fieldwork just offshore of the North Slope of Alaska, near Prudhoe Bay. Sparrow calls the spot “ground zero” for oceanic methane emissions resulting from ocean warming. In some parts of the Arctic Ocean, the shallow regions near continents may be one of the settings where methane hydrates are breaking down now due to warming processes over the past 15,000 years. In addition to methane hydrates, carbon-rich permafrost that is tens of thousands of years old—and found throughout the Arctic on land and in seafloor sediments—can produce methane once this material thaws in response to warming. With the combination of the aggressive warming occurring in the Arctic and the shallow water depths, any released methane has a short journey from emission at the seafloor to release into the atmosphere.

The researchers used radiocarbon dating to fingerprint the origin of methane from their samples. By employing a technique they developed that involves collecting methane from roughly ten thousand gallons of seawater per sample, they made a surprising discovery: ancient-sourced methane is indeed being released into the ocean; but very little survives to be emitted to the atmosphere, even at surprisingly shallow depths.

“We do observe ancient methane being emitted from the seafloor to the overlying seawater, confirming past suspicions,” Kessler says. “But, we found that this ancient methane signal largely disappears and is replaced by a different methane source the closer you get to the surface waters.” The methane at the surface is instead from recently produced organic matter or from the atmosphere.

Although the researchers did not examine in this study what prevents methane released from the seafloor from reaching the atmosphere, they suspect it is biodegraded by microorganisms in the ocean before it hits the surface waters. Mihai Leonte, a PhD candidate in Kessler’s research group, observed this process—in which microbes aggressively biodegrade methane as methane emissions increase—in a paper published last year. “Our data suggest that even if increasing amounts of methane are released from degrading hydrates as climate change proceeds, catastrophic emission to the atmosphere is not an inherent outcome,” Sparrow says.

Sparrow and Kessler’s results on the role of ancient methane sources are consistent with the findings of their Rochester colleague Vasilii Petrenko, an associate professor of earth and environmental sciences, who also radiocarbon dated methane. However, while Sparrow and Kessler dated methane found in modern-day seawater, Petrenko radiocarbon dated methane from the ancient atmosphere that was preserved in the ice of Arctic glaciers.

“Petrenko and his co-authors studied a rapid warming event from the past that serves as a modern-day analog,” Sparrow says. “They found that the emissions of methane from ancient methane sources during this warming event were minimal relative to contemporary sources like wetlands.” Kessler adds, “Our results agree with this conclusion, showing that ancient methane emissions to the atmosphere in an area that is experiencing some of the greatest warming today, is actually quite small, especially when compared to more direct emissions from human activities.” This study was primarily funded by the National Science Foundation with additional contributions from the Department of Energy.

Bereits im August 2017 erschien in Nature eine Arbeit von Petrenko et al., in der den Methanhorrorszeanarien eine klare Absage erteilt wurde:

Minimal geological methane emissions during the Younger Dryas–Preboreal abrupt warming event
Methane (CH4) is a powerful greenhouse gas and plays a key part in global atmospheric chemistry. Natural geological emissions (fossil methane vented naturally from marine and terrestrial seeps and mud volcanoes) are thought to contribute around 52 teragrams of methane per year to the global methane source, about 10 per cent of the total, but both bottom-up methods (measuring emissions)1 and top-down approaches (measuring atmospheric mole fractions and isotopes)2 for constraining these geological emissions have been associated with large uncertainties. Here we use ice core measurements to quantify the absolute amount of radiocarbon-containing methane (14CH4) in the past atmosphere and show that geological methane emissions were no higher than 15.4 teragrams per year (95 per cent confidence), averaged over the abrupt warming event that occurred between the Younger Dryas and Preboreal intervals, approximately 11,600 years ago. Assuming that past geological methane emissions were no lower than today3,4, our results indicate that current estimates of today’s natural geological methane emissions (about 52 teragrams per year)1,2 are too high and, by extension, that current estimates of anthropogenic fossil methane emissions2 are too low. Our results also improve on and confirm earlier findings5,6,7 that the rapid increase of about 50 per cent in mole fraction of atmospheric methane at the Younger Dryas–Preboreal event was driven by contemporaneous methane from sources such as wetlands; our findings constrain the contribution from old carbon reservoirs (marine methane hydrates8, permafrost9 and methane trapped under ice10) to 19 per cent or less (95 per cent confidence). To the extent that the characteristics of the most recent deglaciation and the Younger Dryas–Preboreal warming are comparable to those of the current anthropogenic warming, our measurements suggest that large future atmospheric releases of methane from old carbon sources are unlikely to occur.