Blog: An Ice Record of Methane Sources?

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By James Levine. 29th June 2012.

Methane is a key atmospheric constituent, being both a potent greenhouse gas and an influence on the tropospheric oxidising capacity – the ability of the atmosphere to rid itself of pollutants.   Future changes in the concentration of methane ( [CH4] ) thus have implications for both our climate and air quality, and an understanding of past changes in [CH4] is a prerequisite for meaningful future predictions. The study of past changes also offers valuable insights into interactions between composition and climate on timescales of anything up to 800,000 years – the length of time for which Antarctic ice provides a record of climate and the air bubbles trapped within it provide a record of [CH4].

The most striking natural features of this record are: the differences in [CH4] between glacial and interglacial periods, for instance almost doubling between the Last Glacial Maximum (LGM; 21,000 years ago) and the pre-industrial era (PI; 200 years ago); and rapid rises in [CH4], of up to half the glacial-interglacial difference in less than 100 years, accompanying northern-hemisphere warmings at the beginning of Dansgaard-Oeschger (D-O) events during the last glacial period (21,000-110,000 years ago). Whilst there has been much debate regarding the relative roles of changes in methane sources and sinks between the LGM and the PI (the dominant ones being wetlands and oxidation by the hydroxyl radical, OH, respectively), the rises in [CH4] at the beginning of D-O events have received less attention. Yet these could offer insights into the likelihood of future rapid rises in [CH4] in response to continued climate change.

Two controls on the oxidising capacity have been identified as having been influential between the LGM and the PI: emissions of non-methane volatile organic compounds (NMVOCs) from vegetation, with which methane competes for reaction with OH; and air temperatures, which affect the production of OH and the reactivity that OH shows towards methane (in addition to the kinetics of other chemical reactions). In a previous computer-modelling study, we found that the net effect of these two controls on the oxidising capacity was negligible between the LGM and the PI; their separate effects, though substantial, were roughly equal and opposite, implying the near-doubling of [CH4] during that period was essentially entirely source-driven. This conclusion, though subject to significant uncertainties, is consistent with the most recent estimates of the changes in methane sources between the LGM and the PI.

Here, we report a recent extension of that study (Levine et al., 2012) to a modelled D-O event featuring a characteristically rapid rise in [CH4] in response to a northern-hemisphere warming driven by a change in ocean circulation. We find that the influences of changes in NMVOC emissions and air temperatures across the rapid rise in [CH4] continue to offset each other, and the net effect on the oxidising capacity is again negligible. This suggests the rapid rises in [CH4] at the beginning of D-O events may have also been almost entirely source-driven, and the most striking natural features of the ice record of [CH4], over the last 110,000 years, may almost purely reflect changes in methane sources.

Levine, J. G.1, E. W. Wolff1, P. O. Hopcroft2, and P. J. Valdes2 (2012), Controls on the tropospheric oxidizing capacity during an idealized Dansgaard-Oeschger event, and their implications for the rapid rises in atmospheric methane during the last glacial period , Geophys. Res. Lett., 39, L12805, doi:10.1029/2012GL051866.

1British Antarctic Survey, High Cross, Madingley Road, Cambridge, UK
2Bristol Research Initiative for the Dynamic Global Environment, University of Bristol, Bristol, UK

Blog: Methane in the Twilight Zone

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By Nathan Currier. 11th January 2012. I thought you might be interested in this, the first of a series that will deal with the recent press coverage of the arctic methane issue, as well as other aspects of the problem…. any comments appreciated & those that aren’t too technical can go on Huffpost itself, and that will help increase traffic & thus attention to the topic….. cheers, Nathan

Blog: ESAS Story – A Postscript

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By Gail Riekie. 15th September 2010.

Earlier in the year, we reported on a paper published in the journal Science which presented results from an extensive survey of methane emissions on the East Siberian Arctic Shelf (Shakhova et al., 2010) and highlighted a methane source of previously unrecognised magnitude. (See ‘Supersaturated Siberian Seas’).

Science (vol. 329, 3 September 2010) has now published a letter from Vasilii Petrenko and several fellow methane researchers, which raises issues regarding the way that Shakhova et al. cited earlier papers in support of their statement that methane released from thawing permafrost is a “likely positive feedback to climate warming”. A response to the letter, from Shakhova and two colleagues, defending their use of the references, is also published.

The main findings of the Shakhova paper are not in contention. The debate does however highlight at least two important issues for researchers in the field of methane and climate. Firstly, as Petrenko and colleagues point out, with increasing scrutiny of climate science, absolute clarity in communicating the evidence is essential. The second, as noted by Shakhova’s reply, is the scarcity to date of large scale studies which address the increasingly important subject of climate-biogeochemistry feedback processes in the Arctic.

NERC’s recent Arctic funding announcement implicitly recognises this gap in knowledge, and in the not too distant future we hope to be reporting on innovative new projects which will reduce current uncertainties in the relationship between climate and Arctic methane release.

Natalia Shakhova, Igor Semiletov, Anatoly Salyuk, Vladimir Yusupov, Denis Kosmach, and Örjan Gustafsson. 2010. Science 1246-1250.

The Continued Rise of Atmospheric Methane Concentrations

atmosmeth

12th February 2014.

A new article in Science has discussed the recent changes in atmospheric levels of methane, as well as examining possible drivers for these changes. As highlighted on MethaneNet last October, global methane concentrations increased by 12 ppb per year in the 1980s. This rise slowed in the 1990s, and then stabilised entirely from 1999 to 2006, before concentrations began to increase again at a rate of 6 ppb per year.

As would be expected, methane growth rate varies across different regions of the globe. For instance, growth has been above global levels in the southern tropics since 2007 and the authors suggest that this may have been due to wet summers stimulating the expansion of wetland area. Additionally, they point out the large increase in Arctic methane that occurred solely during 2007, but also suggest that catastrophic emission scenarios of methane from hydrates are unlikely. Modelling of methane concentrations thus shows that tropical wetlands were responsible for driving atmospheric concentration growth in 2007, and that since then the tropics and northern mid-latitudes have been important contributors. There are also anthropogenic sources to consider. Coal mining activities have expanded across some areas of the globe, particularly China, and fracking has increased in popularity in the US.

Additional data are needed to reconcile top-down and bottom-up estimates of atmospheric methane, and more isotope studies that would allow emission sources to be identified. The authors conclude by warning that funding for the monitoring of greenhouse gases is shrinking at a time when they are sorely needed.

References

Nisbet, E.G., Dlugokencky, E.J., Bousquet, P. 2014. Science, 343, 493-495.

New Estimates of Decadal Global Methane Dynamics. MethaneNet.

Image: Ed Dlugokencky, NOAA CMDL. This plot shows methane observations from 1993 to 2007 showing the growth of methane, the seasonal variations and the difference between northern and southern hemispheres

New Arctic Methane Source?

Stefan Cook_0

16th May 2012.

A new set of airborne observations has revealed high methane concentrations in the atmosphere above the Arctic Ocean. The concentration of methane is correlated with open leads (cracks) in the sea ice and regions with fractional ice cover (Kort et al., 2012). In the recently published Nature Geoscience paper, the authors suggest that the methane derives from a previously under-recognised source, aerated surface ocean waters.

Eric Kort, who led the study, told MethaneNet that the finding was “unexpected”. He explained how the work came about. “The observations were made as part of the HIAPER Pole-to-Pole Observations (HIPPO) campaign, on which we made airborne observations from 67S to 85N. Our attention was drawn to the Arctic methane question by an observation made onboard the aircraft. We noticed that on many profiles, as we descended near the ice-ocean surface, our instruments recorded increases in methane, with no coincident increase in carbon monoxide.  This feature piqued our curiosity, and led to the analysis presented in our paper.”

The data were collected using instruments mounted onboard the NSF/NCAR Gulfstream V. During the five flights, from January 2009 to April 2010, methane, carbon monoxide, ozone and water vapour concentrations were measured at heights ranging from 0.15 – 8.5 km. Using the ancillary gas measurements, the authors were able to rule out biomass burning and oil and gas industry operations as the methane source.

Initial estimates suggest a flux rate of 0.5 – 8.0 mg CH4 d-1 m-2 from this newly revealed source. The research prompts many questions. How might future changes in sea ice cover impact the fluxes? Is there a connection with other recently reported Arctic methane sources such as degradation of subsea permafrost on the eastern Siberian shelf. How exactly is this methane produced?

Eric Kort recognizes that more information is needed, saying to MethaneNet, “hopefully our work motivates further study on methane emissions from Arctic Ocean”.

Reference:

Kort, E.A., Wofsy, S.C.,Daube, B.C., Diao, M., Elkins, J.W., Gao, R.S., Hintsta, E.J., Hurst, D.F., Jiminez, R., Moore, F.L., Spackman, J.R., and Zondlo, M.A. (2012), Atmospheric observations of Arctic Ocean methane emissions up to 82°N. Nature Geoscience, 5, 318-321.

Image of Arctic sea ice by Stefan Cook @ flickr.com

Arctic Summer Wetland Source Revealed By δ13C

Zeppelin sq

19th January 2012.

When increases in the mixing ratio of methane in the atmosphere are observed, it is useful to know which methane source is contributing to that increase. A recent paper in Geophysical Research Letters (Fisher et al., 2011) has highlighted how isotope data can help determine the source of atmospheric methane in the Arctic.

The starting point for this type of work is knowledge of the carbon isotopic signatures of the major relevant methane sources. If these d13C values are sufficiently distinct, then inverse modelling of data from atmospheric samples can yield insights into the provenance of the methane. In this study, in addition to using published figures for d13C signatures, Fisher and co-authors gathered new data, for example showing that methane from Canadian boreal pine forest fires is relatively enriched in the heavier isotope (d13C -28 ± 1 ‰). They also showed that the isotopic signature of methane from W. Spitzbergen marine clathrates (d13C -50 ± 5 ‰) is variable (d13C -50 ± 5 ‰), and that at present very little of this methane reaches the atmosphere.

Having established the source signatures, measurements of d13C-CH4 from air samples taken at the Zeppelin station in Spitzbergen in late summer/autumn 2008 and 2009, and in spring 2009, were then used to study seasonal variations in the source of that methane. At d13C close to -68 ‰, the late summer/autumn methane samples bore a striking similarity to the highly 13C depleted methane typical of wetland emissions. By contrast samples taken in spring, when the wetlands are frozen, were richer in the heavier isotope (d13C -52.6 ± 6.4 ‰). This is shown by modelling to be consistent with leakage from W Siberian gas infrastructure being the dominant source.

Rebecca Fisher, first author of the study, comments, “Isotopes are a powerful tool for constraining sources, but there are very few stations in the Arctic from which ambient air samples are collected for isotope studies. High frequency, ideally continuous, monitoring of δ13C in methane from a number of Arctic sites, onshore and offshore, will be important if future changes in Arctic sources are to be quantified.”

The conclusion that summer emissions in this part of the Arctic are in the main derived from wetlands meshes well with other studies investigating the importance of wetlands as a high latitude methane source (Tarasova et al., 2009). Given the potential for global warming to create more extensive areas of boreal wetland, and the fact that this methane source could be supplemented by methane from hydrates destabilised under increasing temperatures, the potential for dramatic rises in input of methane to the Arctic atmosphere is a cause for continuing concern.

References:

Fisher, R.E., Sriskantharajah, S., Lowry, D., Lanoisellé, M., Fowler, C.M.R., James, R.H., Hermansen, O., Lund Myhre, C., Stohl, A., Greinert, J., Nisbet-Jones, P.B.R., Mienert, J., and Nisbet, E.G. (2011). Arctic methane sources: Isotopic evidence for atmospheric inputs. Geophysical Research Letters, 38, L21803.

Tarasova, O.A., Houweling, S., Elansky, N. and Brenninkmeijer, C.A.M. (2009). Application of stable isotope analysis for improved understanding of the methane budget: Comparison of TROICA measurements with TM3 model simulations. Journal of Atmospheric Chemistry, 63(1), 49-71.

Photo: air inlet on Zeppelin station roof, taken by Dave Lowry.  

Supersaturated Siberian Seas

Winter ice

19th April 2010.

Covered in ice for 265 days of the year, and bordered by the frozen wastes of the Siberian tundra, it is hardly surprising that the shallow seas of East Siberian Arctic Shelf (ESAS) have not before now been subject to extensive monitoring for methane emissions.  However, due to the efforts of an international collaboration between researchers based in Alaska, Vladivostock and Stockholm, a comprehensive survey of methane concentrations in these waters has now been conducted.  The results, reported in Science (Shakhova et al. 2010) make compelling reading and raise new questions about future Arctic methane fluxes.

Six summer field campaigns were conducted between 2003 and 2008, and methane concentrations were measured in 5100 seawater samples taken from 1080 stations.  The researchers found that over 50% of the surface waters were supersaturated with methane. In hotspot areas, the median supersaturation was 8300%.  One over-ice winter expedition and one helicopter survey provided additional data with which to constrain estimates of the total methane flux from these Arctic seas.  Based on all the observations, the team calculated annual atmospheric methane flux from the ESAS at 7.98 Tg C-CH4. To put this in context, previous research has estimated the global methane flux from oceans as 4 – 15 Tg C-CH4 y-1 (IPCC, 2007).

The worldwide ocean methane flux figures would remain relatively small compared to terrestrial sources such as wetlands and rice paddies, even if revised upwards based on the ESAS study. The new data are however interesting  in that they demonstrate the potential for methane emissions from flooded areas underlain by large pools of carbon-rich vulnerable permafrost. The authors note that the question of whether the amount of methane being released from the ESAS sediments has changed in response to the general warming of the Arctic region is not answered by their study, and suggest that this issue merits further attention.

Natalia Shakhova, Igor Semiletov, Anatoly Salyuk, Vladimir Yusupov, Denis Kosmach, and Örjan Gustafsson. 2010. Science 1246-1250.