Rising atmospheric methane and its isotopic shifta

gbc20468-fig-0004

A paper published in Global Biogeochemical Cycles has clarified the importance of different sources contributing to the atmospheric methane burden during 2007-2014. The research used measurements taken by the USA National Oceanic and Atmospheric Administration (NOAA) Cooperative Global Air Sampling Network, and investigated changes in mole fractions and isotopic composition.

The results showed that the globally averaged mole fraction of CH4 increased by 5.7 ± 1.2 ppb yr-1 up to 2013, then by 12.5 ± 0.4 ppb in 2014. Accompanying this change was a shift in δ13CCH4 to more negative values. At the remote monitoring station at Ascension Island, this shift was −0.24 ± 0.02‰, but the same trend was observed globally.  These global changes will potentially be driven by varying processes in different regions, which the authors attempt to untangle.

In 2007 there was an increase in CH4 concentrations at high northern latitudes, which was caused by an increase in late summer emissions from wetlands, stimulated by higher temperatures. However, during 2008-2013 this Arctic increase fell below global means, indicating that no new CH4 emissions developed during this time.  In 2014, strong increases commenced again, but this time in line with global means. For the entire period, isotopic data shows a trend to more depleted δ13CCH4, indicative of wetland sources.

For Ascension Island, methane increases have been sustained for the duration of monitoring. Assuming a linear trend, isotopic change is in a negative direction and of approximately −0.03‰ yr−1. Measurements from Cape Point (South Africa) and the South Pole were similar to the Ascension ones. The authors point out that the southern Amazon is a distant source of air for Ascension Island.  They therefore posit that increased rainfall and warmth may have been responsible for increased CH4 emissions from tropical wetlands, as recent years have seen flood levels above long-term averages. The paper also emphasise the importance of emissions from ruminants in the tropics, but point to a lack of data regarding the δ13CCH4 values of such emissions.

Lead author, Professor Euan Nisbet at Royal Holloway University of London said: “Our results go against conventional thinking that the recent increase in atmospheric methane must be caused by increased emissions from natural gas, oil, and coal production. Our analysis of methane’s isotopic composition clearly points to increased emissions from microbial sources, such as wetlands or agriculture.”

To conclude, the authors stress the magnitude of the global increase in methane concentrations; 60 ppb in nine years. They write: “if methane growth continues, and is indeed driven by biogenic emissions, the present increase is already becoming exceptional, beyond the largest events in the last millennium.”

Reference. Nisbet, E.G., Dlugokeencky, E.J., Manning, M.R., et al. Rising atmospheric methane: 2007-2014 growth and isotopic shift. 2016. Global Biogeochemical Cycles,DOI: 10.1002/2016GB005406.

Image is from the above paper, under a Creative Commons Attribution 4.0 International License.

Methane, Methanogens and Permafrost

Permafrost_thaw_ponds_in_Hudson_Bay_Canada_near_Greenland

10th November 2014.

It has been known for some time that permafrost stores a significant proportion of global soil carbon, and that thawing permafrost acts as an atmospheric source of methane and carbon dioxide, thereby creating a positive feedback to climate change.  Earlier in 2014, an international team of researchers discovered a new methanogen in the class Methanomicrobia.  They named the new species Methanoflorens stordalenmirensis and proposed a new family for it: Methanoflorentaceae.

New research by the same team, at the same Swedish field site, has shed more light on the activities of M. stordalenmirensis.  The team used a natural gradient of permafrost thaw to determine how thaw-induced changes in vegetation and hydrology affect methane dynamics.  They found that average methane fluxes were zero at an intact permafrost site, but increased to 1.46 mg CH4 m−2 h−1 at a thawing Sphagnum site, and increased further to 8.75 mg CH4 m−2 h−1 at a fully thawed site dominated by Eriophorum.  There was also a significant difference in the isotopic signature of the produced methane: average δ13C of emitted methane was −79.6 ‰ at the Sphagnum site and −66.3 ‰ at the Eriophorum site.  A similar pattern was observed for porewater CH4 isotopes between the Sphagnum and Eriophorum site.

These changes in methane dynamics were accompanied by changes in the microbial communities of the sites.  For the intact site there was a low abundance of methanogens, whilst the communities at the Eriophorum site and the deeper (anaerobic) layers of the Sphagnum site contained 20-30 % methanogens.  The Sphagnum site was dominated by hydrogenotrophic methanogens, including species similar to M. stordalenmirensis, whilst the Eriophorum site featured a high abundance of acetoclastic methanogens in the genus Methanosaeta.

Further analysis suggested that M. stordalenmirensis was the best single-variable predictor of isotopic patterns, although a multi-variable model also highlighted the importance of organic matter chemistry.  Altogether, these results show how ecosystem-scale methane dynamics are driven by changes in the composition of microbial communities.  Such research can be incorporated into future models of permafrost thaw and climate change methane feedbacks.

Lead author Carmody McCalley at the University of New Hampshire said: “By taking microbial ecology into account, we can accurately set up climate models to identify how much methane comes from thawing permafrost versus other sources such as fossil-fuel burning.”

References:

Discovery of a novel methanogen prevalent in thawing permafrost. 2014. Mondav, R., Woodcroft, B.J., Kim, E-H., et al. Nature Communications, 5, doi:10.1038/ncomms4212.

Methane dynamics regulated by microbial community response to permafrost thaw. 2014. McCalley, C.K., Woodcroft, B.J., Hodgkins, S.B., et al. Nature, 514, doi:10.1038/nature13798.

Photo: Permafrost thaw ponds, Hudson Bay, by Steve Jurvetson. CC BY 2.0

Methane in the Earth System

Nisbet1 (2)

16th June 2014.

Methane in the earth system – what can we measure, understand and do about recent trends?

By Michelle Cain.

The topic of the latest Cambridge Centre for Climate Science (CCfCS) symposium was “Methane in the Earth System”, held on 5 June 2014, and supported by MethaneNet. The aim of the symposium was to bring a diverse range of speakers together to generate discussion on this broad theme. Not only did we hear from scientists about this topic, but also an engineer from the government Department for Energy and Climate Change (DECC), who was able to enlighten us on how DECC thinks about methane in terms of the UK policy environment.

Professor Euan Nisbet kicked off the afternoon with a whistle-stop tour of methane through the ages and across the globe: from methanogenesis 4 Gya (4×109 years ago) through to the present day. Entitled “Is methane the canary in the mine?”, the talk referred to the idea that methane may be like a lever that kick-starts periods of global warming. Is an increase in atmospheric methane a sign that the temperature is going to follow a similar trend? Or perhaps it’s the other way around, and increased temperature is responsible for the increasing atmospheric methane concentrations?

Observing and understanding the recent trends in atmospheric methane was the main focus of Professor Nisbet’s talk. The trends stabilised around the turn of this century, but have been rising since about 2007. The puzzle of why this has happened is yet to be solved. Measurements of isotopes of carbon in methane show that the global average isotopic ratio is getting “lighter”, meaning the sources are becoming more characteristic of wetland, and less like combustion. However anthropogenic emissions inventories show increases in recent years. If both of these trends are true, presumably the wetland source must be increasing at a greater rate than the anthropogenic source. This could be due to weather events like floods and the position of the inter-tropical convergence zone. The big question that follows on from this is whether this is a short- or a long-term trend. Professor Nisbet showed many examples of measurements, both short- and long-term, spanning the tropics to the poles, which will go some way to resolving this question in the fullness of time.

Dr Nic Gedney followed with a talk about one of the key aspects of understanding the methane trends – the modelling of methane emissions from wetlands. Both temperature and precipitation are critical for modelling wetland location and therefore methane emissions and potential feedbacks. For example, the Wetland and Wetland CH4 Inter-comparison of Models Project (Melton et al 2013) found that methane emissions increased when there was increased precipitation, and that global wetland area decreased when temperature increased. The model results revealed disagreements in both spatial and temporal extent of wetland areas and therefore methane emissions.

Dr Gedney also showed recent developments to the Joint UK Land Environment Simulator (JULES), a “process-based model of carbon, energy and water exchange between atmosphere and land surface.” The model uses drainage and topography to calculate inundation. Updating the model with a representation of organic soil and with new topography has improved the model compared to observations.

To make further improvements to the complex task of modelling methane emissions from wetlands, it will be essential to improve on observations of wetland extent. There are currently striking differences between inundation products derived from satellite observations and wetland models. However, wetlands are not always detectable until you “step into one and end up knee deep in water”, and so it’s clear that detection from space will have its limitations. Nonetheless, improvements to current measurement techniques will be a valuable step towards better understanding wetland areas.

This theme was continued in Professor John Burrows’ talk about measuring the anthropocene (in particular methane) from space, which he described as “very much work in progress”. The anthropocene is a new epoch, in which humans are changing the earth system dramatically. Examples of this include: burning fossil fuels, releasing other pollutants, causing the ozone hole and changing land use. Without measurements, it’s impossible to understand or manage how we are altering the earth system and climate.

Given that ground-based measurements will always be sparse, we need space-based techniques to get good global coverage. Ideally, a combination of low earth orbit (which has good global coverage) and geostationary orbit (high resolution, but less wide coverage) would provide the best information. Unfortunately, the current processes for getting a new satellite built are very slow, and it’s possible that the space-based measurements of methane will take a step backwards if new instruments are not developed quickly enough. CarbonSat is the next big project being considered for deployment by the European Space Agency. This would measure methane at a 2km by 2km resolution, however if successful, it won’t be launched until 2022.

Professor Burrows showed many different applications of satellite data in understanding recent methane trends. Data from the SCIAMACHY satellite have added to the quantification and identification of the recent increasing methane trend, which is mainly found in the tropics and northern mid-latitudes. These data have then been put into inverse methods to work out emissions. An inversion is a technique for calculating a best estimate of emissions by reconciling observations and a model using a cost function. Bergamaschi et al (2013) suggest that anthropogenic sources are causing the trend, and that wetlands and biomass burning dominate interannual variability. Another inversion study by Houweling et al (2014) was unable to discriminate between Asian anthropogenic and wetland sources. There are many other recent studies contributing to this debate, and this issue is far from resolved.

We heard more about inverse methods in Dr Matt Rigby’s talk, in which he explained why we should take some inverse modelling – including some of his own – with “a pinch of salt”. The key factor is how uncertainties are estimated. There are uncertainties in both the prior information and in the transport model used. Bayes’ theorem requires that these should be independent, but this is not always the case in inversions that use this assumption. Defining the size of an uncertainty may be simply an estimate, and may be “tuned” in concert with other uncertainties. Dr Rigby demonstrated an alternative approach to eliminate this particular problem: the use of a hierarchical Bayesian estimation, which estimates the uncertainties in the uncertainties. An example of this from Ganesan et al (2014), showed that a hierarchical approach tended to have a larger uncertainty, but one that is arguably closer to the true uncertainty.

Another approach for tackling the issue of uncertainty is to test a model’s sensitivity to its parameters. This can reveal large and sometimes non-linear sources of error, which are often simply untested and therefore ignored. Dr Rigby suggested that statistical emulators might be a good way to test for such systematic errors.

The concentration of atmospheric OH (the main sink for methane) is another uncertainty that affects our understanding of methane. Dr Rigby showed a time series of derived global OH concentrations based on methyl chloroform observations. This showed a low variability (<5%) in the past decade, although this is poorly constrained.

Although there is room for improvement in terms of reducing uncertainties, inverse methods are a key component to understanding methane emissions. The UK uses such techniques to verify bottom-up national emissions inventories, and is a world-leader in this kind of approach. The UK measurement network is also relatively dense, with recent projects like GAUGE adding to this.

We had a shift in perspective for the final talk of the day when we heard from Dr Philip Sargent, an engineer from DECC. Dr Sargent gave us a flavour of the concerns DECC has when it comes to methane. One aspect of DECC’s role is to weigh up which policy options emit the least greenhouse gases. For example, how does UK fracked gas compare to imports of liquid natural gas or via a pipeline? The UK must reduce carbon emissions if it is to meet the Committee on Climate Change’s 4th carbon budget in 2025.

The heart of the matter is that DECC must balance policies and spending related to methane with those related to carbon dioxide. Therefore metrics that allow comparisons to be made are essential. Dr Sargent posed an open question regarding what time horizon should be used for global warming potential (GWP). The Intergovernmental Panel on Climate Change (IPCC) says that using 100 years is an arbitrary choice. Alternatively, perhaps it is better to specify a cut-off date, and use that as the time horizon. Evidence, including uncertainties, to support the use of a particular metric and time horizon is one particular point that DECC is interested in, as it can be used for economists to base their models on. However, Dr Sargent also noted that it is preferable to continue using GWP (likely with more useful time horizons), as this term is well understood among policy makers – vital for international negotiations.

Formal proceedings ended with a discussion about the kinds of evidence the science community can usefully provide to government, which continued in more depth over the poster reception. A take home point from this final session was the importance of finding opportunities for policy makers to communicate with scientists. Sometimes talking to someone is the most effective route to providing them with the information that they require, but having that chance to talk can often be the limiting factor.

References

Bergamaschi, P., et al. (2013), Atmospheric CH4 in the first decade of the 21st century: Inverse modeling analysis using SCIAMACHY satellite retrievals and NOAA surface measurements, J. Geophys. Res. Atmos., 118, 7350–7369, doi:10.1002/jgrd.50480.

Ganesan, A. L., Rigby, M., Zammit-Mangion, A., Manning, A. J., Prinn, R. G., Fraser, P. J., Harth, C. M., Kim, K.-R., Krummel, P. B., Li, S., Mühle, J., O’Doherty, S. J., Park, S., Salameh, P. K., Steele, L. P., and Weiss, R. F.: Characterization of uncertainties in atmospheric trace gas inversions using hierarchical Bayesian methods, Atmos. Chem. Phys., 14, 3855-3864, doi:10.5194/acp-14-3855-2014, 2014.

Houweling, S., Krol, M., Bergamaschi, P., Frankenberg, C., Dlugokencky, E. J., Morino, I., Notholt, J., Sherlock, V., Wunch, D., Beck, V., Gerbig, C., Chen, H., Kort, E. A., Röckmann, T., and Aben, I.: A multi-year methane inversion using SCIAMACHY, accounting for systematic errors using TCCON measurements, Atmos. Chem. Phys., 14, 3991-4012, doi:10.5194/acp-14-3991-2014, 2014.

Melton, J. R., Wania, R., Hodson, E. L., Poulter, B., Ringeval, B., Spahni, R., Bohn, T., Avis, C. A., Beerling, D. J., Chen, G., Eliseev, A. V., Denisov, S. N., Hopcroft, P. O., Lettenmaier, D. P., Riley, W. J., Singarayer, J. S., Subin, Z. M., Tian, H., Zürcher, S., Brovkin, V., van Bodegom, P. M., Kleinen, T., Yu, Z. C., and Kaplan, J. O.: Present state of global wetland extent and wetland methane modelling: conclusions from a model inter-comparison project (WETCHIMP), Biogeosciences, 10, 753-788, doi:10.5194/bg-10-753-2013, 2013.

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.  

Clathrate Gun Shot Down?

Greenland Ice Core_0

23rd July 2010.

Arctic and Antarctic ice cores provide a rich source of evidence that both temperature and atmospheric concentrations of methane and carbon dioxide have fluctuated over the past 800,000 years. These studies have also shown that the fluctuations are cyclic, and that higher temperatures are generally associated with higher levels of methane and carbon dioxide in the atmosphere. The source(s) of the periodic increases in methane concentrations are less clear, and have been the subject of much debate. Two competing hypotheses have been proposed: (1) that the extra methane comes from increased wetlands emissions (the ‘wetland hypothesis’) and (2) that methane hydrate disintegration is responsible for the increase in methane which in turn can trigger a dramatic temperature rise (the ‘clathrate gun hypothesis’).

A new study (Bock et al., 2010) of the hydrogen isotope ratios in methane from North Greenland ice-cores dated from 33,700 – 41,000 years ago, has now tipped the balance in favour of the wetland hypothesis for the series of climate fluctuations known as the Dansgaard-Oeschger events.

The study uses the fact that the hydrogen isotope composition of methane emitted from wetlands falls in the range -300 to -400‰ δD(CH4), distinctly more depleted in deuterium than that in hydrate-sourced methane, which averages ~ -190 δD(CH4). A number of other factors (e.g. precipitation, temperature-related Rayleigh distillation, kinetic fractionation associated with the OH sink) also impact on the hydrogen isotope ratio, but these are secondary effects which only alter the δD(CH4) by a few per mill

Bock et al. observed that over the 7,300 years represented by their ice cores, the methane from the interstadial (warmer temperature) periods was ~ 10 ‰ more depleted in deuterium than that from the cooler periods (stadials), and that this change in δD(CH4) could only be explained by the wetland hypothesis. Modelling indicated the hydrogen isotope measurements in the interstadials were consistent with a six fold increase in high latitude wetland emissions, from ~5 to ~ 32 Tg CH4 year-1, an increase of ~84 to ~118 Tg CH4 year-1 from tropical wetlands, and a constant rate of emission from marine hydrates of ~ 25 Tg CH4 year-1.

Hydrates expert Professor Mark Maslin, Director of the UCL Environment Institute, agrees that the paper “provides good evidence that gas hydrates were not involved in the initial methane rise…” during the Dansgaard-Oeschger events, but believes that there is “still considerable debate about whether tropical wetland, boreal wetlands or flooded shelf due to millennial scale sea level rise are the true source of the methane rise….” The authors of the paper concede that the methane cycle in this period remains underdetermined. However, the ruling out of one important potential methane source represents a significant advance in our understanding.

References

Michael Bock, Jochen Schmitt, Lars Möller, Renato Spahni, Thomas Blunier and Hubertus Fischer (2010), Hydrogen isotopes preclude marine hydrate CH4 emissions at the onset of Dansgaard-Oeschger events, Science, v.328, 1686-1689.

Mark Maslin, Matthew Owen, Richard Betts, Simon Day, Tom Dinkley-Jones and Andrew Ridgwell (2010), Gas hydrates: past and future geohazard? Phil. Trans. R. Soc. A, v.368, 2369-2393