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.

The Uncertain Climate Footprint of Wetlands Under Human Pressure

Wicken flux tower

New research published in PNAS has attempted to untangle the differences in methane, carbon dioxide, and nitrous oxide fluxes from natural and managed wetlands. As MethaneNet readers will know, wetlands play an important role in global methane dynamics but many questions remain unanswered. The new study, which primarily used FLUXNET sites, analysed data collected using both static chamber and eddy covariance methods in temperate, boreal, and arctic locations. As such, the dataset is likely to be the first large-scale synthesis of wetland flux data generated by eddy covariance.

As is frequently observed for wetlands, most of the studied sites were net sources of methane, and net sinks of carbon dioxide. For temperate sites, it was found that drainage and conversion to agriculture reduced methane fluxes but increased carbon dioxide fluxes. Rewetting or restoring such sites then creates a methane source that is not offset by the associated carbon dioxide sink. To fully quantify the impacts of management, sites with annual greenhouse gas budgets (including nitrous oxide from agricultural sites) were used to calculate radiative forcing (RF) on a 100 year time scale. These calculations also accounted for carbon offtakes, such as forestry or crop harvesting. The calculations suggested that conversion of arctic and boreal wetlands to agriculture resulted in a positive RF. Temperate wetlands converted to agriculture generally had a positive RF, though there was variation due to the type and intensity of management.

In their conclusions, the international group of authors stress the importance of long-term monitoring to fully understand ecosystem responses to both natural and anthropogenic changes. They further add that such responses may not be adequately captured by manual static chamber sampling, and that focus should be directed towards eddy covariance and quasi-continuous chamber measurements. They conclude: “Our results prove that management intensity strongly influences the net climate footprint of wetlands and in particular the conversion of natural ecosystems to agricultural land ultimately leads to strong positive RF.” These results therefore have important policy implications regarding land management and climate.

Reference: Petrescu, A.M.R., Lohila, A., Tuovinen, J-P., et al. 2015. The uncertain climate footprint of wetlands under human pressure. PNAS, doi: 10.1073/pnas.1416267112.

Photo: Mike Peacock

Blog: UV Irradiation of Aquatic Organic Carbon: An Overlooked Source of Methane?

black burn_0

By Amy Pickard. 3rd December 2013.

Amy Pickard1*, Andy McLeod1, Kate Heal1 and Kerry Dinsmore2

1 School of GeoSciences, University of Edinburgh, UK *amy.pickard@ed.ac.uk  2Centre for Ecology and Hydrology, Bush Estate, Penicuik, UK

Wetland environments, including northern peatlands, are a globally significant source of methane, releasing in the order of 100 Tg CH₄ yr-1 (Wuebbles and Hayhoe, 2002). Emissions from these systems have typically been attributed to microbial metabolism of organic carbon into methane in anaerobic conditions.

However a seminal study by Keppler et al. (2006) showed that methane was produced in aerobic conditions when plant matter was subjected to stress. UV irradiation is a known source of plant stress that was later shown to initiate methane production (McLeod et al., 2008). Further follow up studies indicated that plant pectin was a possible source for the emissions and confirmed that methane could be produced from detached leaf components (McLeod et al., 2008; Vigano et al., 2008). The key outcome of this body of research was the incorporation of aerobic methane production from UV irradiation of plant foliage into the global budget (Bloom et al., 2010). Nevertheless, to date, this pathway of aerobic methane production has only been investigated in the terrestrial environment.

The hypothesis of my research is that plant-derived material transported from the terrestrial environment to aquatic systems may release methane when exposed to UV irradiation. The NERC-funded PhD that I am currently undertaking aims to further investigate the aerobic production pathway as a potential source of methane in aquatic systems.

In order to test this hypothesis, water samples rich in dissolved organic carbon (DOC) were collected from a stream draining Auchencorth Moss (Fig. 1), an ombotrophic peatland area in south east Scotland which is one of the Centre for Ecology & Hydrology’s Carbon Catchments. Upon returning to the laboratory samples were filtered and decanted into quartz vials. They were then exposed to an ambient dose of UV irradiation for 4 hours. After irradiation, vial headspace methane concentrations were measured using gas chromatography.

These initial experiments have demonstrated that aquatic systems contain sufficient levels of plant derived material to stimulate aerobic methane production.  UV irradiation resulted in increased CH₄ production rates of 63.2 ± 16.4 nmol L-1 (mean ± standard deviation for 4 replicates) relative to unirradiated control samples. The increase in gaseous production in the headspace of irradiated vials was coupled with a decrease in DOC concentration in the water sample. This finding is in agreement with the hypothesis that dissolved plant matter acts as the aerobic methane source material and adds weight to the suggestion that UV irradiation plays an as yet overlooked role in the aquatic methane budget.

Increased losses of DOC from catchments in the northern hemisphere have been well documented (Clark et al., 2010) and are projected to accelerate with climate change (Worrall et al., 2004). This creates an interesting setting for aquatic methane research, as it follows that in catchments where more DOC is delivered to aquatic systems, methane emissions stimulated via UV irradiation of organic matter will increase concurrently. Whether this hypothesis holds true in either laboratory tests or field based experiments is yet to be determined, however our initial data present plenty of options to explore. The challenge now is to understand what environmental factors affect aerobic methane emissions and to combine both laboratory and field based experiments to determine the importance of this process in catchment level biogeochemical cycles.

Photo: The Black Burn at Auchencorth Moss. Photo courtesy of Fraser Leith

References

Bloom, A.A., Lee-Taylor, J., Madronich, S., Messenger, D.J., Palmer, P.I., Reay, D.S., McLeod, A.R., 2010. Global methane emission estimates from ultraviolet irradiation of terrestrial plant foliage. New Phytol 187, 417-425

Clark, J.M., Bottrell, S.H., Evans, C.D., Monteith, D.T., Bartlett, R., Rose, R., Newton, R.J., Chapman, P.J., 2010. The importance of the relationship between scale and process in understanding long-term DOC dynamics. Sci Total Environ 408, 2768-2775

Keppler, F., Hamilton, J.T.G., Brass, M., Rockmann, T., 2006. Methane emissions from terrestrial plants under aerobic conditions. Nature 439, 187-191.

McLeod, A.R., Fry, S.C., Loake, G.J., Messenger, D.J., Reay, D.S., Smith, K.A., Yun, B.W., 2008. Ultraviolet radiation drives methane emissions from terrestrial plant pectins. New Phytol 180, 124-132.

Vigano, I., van Weelden, H., Holzinger, R., Keppler, F., McLeod, A., Rockmann, T., 2008. Effect of UV radiation and temperature on the emission of methane from plant biomass and structural components. Biogeosciences 5, 937-947

Worrall, F., Burt, T., Adamson, J., 2004. Can climate change explain increases in DOC flux from upland peat catchments? Sci Total Environ 326, 95-112

Wuebbles, D.J., Hayhoe, K., 2002. Atmospheric methane and global change. Earth-Sci Rev 57, 177-210

Blog: Source-driven Doubling in Methane

Portrait_MN_4

By James Levine. 18th January 2012.

As a potent greenhouse gas, the amount of methane in the Earth’s atmosphere affects our climate. It also has bearing on the quality of air we breathe, as it influences the ability of the atmosphere to rid itself of pollutants (including other greenhouse gases) that could reach levels harmful to human health if allowed to accumulate. We therefore need to understand the causes of variations in the concentration of this key constituent, and an ability to account for past variations is a prerequisite for meaningful future predictions.

Air bubbles trapped in Antarctic ice (pictured) reveal large variations in the concentration of methane, [CH4], over the last 800,000 years that appear to broadly track changes in climate. The question at the heart of a recent study by Levine et al. (2011) is, why did [CH4] almost double from 360 parts per billion by volume (ppbv) at the so-called Last Glacial Maximum (LGM; around 21,000 years ago) to 700 ppbv in the relatively warm pre-industrial era (PI; about 200 years ago)? Did the amount of methane emitted from natural sources such as wetlands increase, or did the rate at which methane was removed from the atmosphere decrease, allowing those emissions to accumulate to higher concentrations? The relative contributions made by changes in methane sources and changes in methane ‘sinks’ have long been debated.

Early ‘bottom-up’ model estimates of the changes in methane sources could only explain around half the change in [CH4], appealing to a reduction in the main methane sink—oxidation by the hydroxyl radical (OH)—as the climate warmed; the warming (and increasingly wet) climate would have seen an increase in vegetation and an increase in the amount of volatile organic compounds emitted from vegetation (e.g. isoprene) that compete with methane for reaction with OH. However, the warming climate would have also been accompanied by an increase in humidity, fuelling greater OH production, and an increase in the reactivity that OH shows towards methane. So there were opposing influences on the rate of methane removal by OH.

Using a computer model of the Earth’s atmosphere, we find that the changes in emissions from vegetation would have had a significant influence on [CH4] of the sort previously proposed, but this would have been almost entirely negated by the accompanying changes in humidity and chemical reaction rates, the implication being the LGM-PI change in [CH4] was essentially entirely source driven. Meanwhile, estimates of the change in methane emissions from wetlands during this period have increased, and now include almost precisely that needed to explain the change in [CH4] without recourse to changes in the rate of methane removal. We therefore conclude that it is plausible the LGM-PI change in [CH4] was entirely source-driven, and the changes in methane sources and sinks between the LGM and the PI could be reconciled thus.

Levine, J. G.1, E. W. Wolff1, A. E. Jones1, L. C. Sime1, P. J. Valdes2, A. T. Archibald3,4, G. D. Carver3,4, N. J. Warwick3,4, and J. A. Pyle3,4 (2011), Reconciling the changes in atmospheric methane sources and sinks between the Last Glacial Maximum and the pre-industrial era, Geophys. Res. Lett., 38, L23804, doi:10.1029/2011GL049545.

1British Antarctic Survey, High Cross, Madingley Road, Cambridge, UK.
2School of Geographical Sciences, University of Bristol, Bristol, UK.
3Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge, UK.
4National Centre for Atmospheric Science, University of Cambridge, Cambridge, UK.

Blog: Ditch Blocking in a Welsh Peatland

P1020095

By Mike Peacock. 9th September 2011.

Northern peatlands are typically nutrient-poor, acidic ecosystems.  They store one third of global soil carbon, and one tenth of global freshwater.  However, over the past century they have been damaged in numerous ways; they have been burnt for agricultural management, drained for forestry and agriculture, and harvested as a fuel source.  In the UK the major change in peatland management has been through the digging of drainage ditches to lower the water table.  These ditches vary, but are typically half a metre wide and a metre deep, often in dense networks across large areas.

Decades of research into drained peatlands has created a large knowledge base on the effects of altered water tables.  Most studies report an increase in carbon dioxide emissions from respiration. Methane emissions, on the other hand, decrease.  This is due to an ingress of oxygen into the peat leading to increased methanotrophy due to a larger and more continuous oxic zone  Most studies agree that taken as a whole the biogeochemical changes following drainage lead to a decreased carbon store in the peat, and a net increase in greenhouse gas emissions.  With current concerns for climate change and carbon, this is clearly a hot topic.

Now, peatland restoration is in vogue.  In the UK the favoured method is to block the man-made drainage ditches, thus restoring the water table to approximately its original level.  A research project involving Bangor University, Leeds University, the Open University and the Centre for Ecology and Hydrology is based on one such example.  Following the collection of baseline data, hundreds of miles of ditches were blocked on the Migneint, a large blanket bog in north Wales.  The aim of the project is to examine the effects of two types of ditch blocking on greenhouse gas fluxes, as well as changes in hydrology and water chemistry.  As blocking took place in February 2011, the experimental phase is now well underway.

In addition to being drained, the study site is near the crest of a hill and so was originally moderately dry.  Methane fluxes were therefore found to be generally low from both the drained landscape, and have so far remained low from the intact blanket bog areas between the blocked ditches.  However, both types of blocking being trialled use peat dams to restrict water flow down the ditches.  Pools of varying sizes have formed behind these dams, and large methane fluxes have been recorded from their surfaces.  It appears that the removal of the oxic zone has limited the niche for aerobic methanotrophs, thus allowing more methane to reach the atmosphere.  Although these fluxes are generally fairly steady over short periods, intense pulses have also been observed, indicating methane ebullition (bubble emissions) from the peat.

Further to this, preliminary CEH/Bangor University work by Mark Cooper at an adjacent site found that blocked ditches were rapidly recolonised by Eriophorum.  These plants were large methane hotspots as they act as ‘chimneys’ to allow methane to bypass methanotrophs and enter the atmosphere.  It seems probable that the newly blocked ditches will also be colonised by Eriophorum, as this has been seen at numerous peatlands.

Hopefully the study will be able to untangle the complex effects of vegetation and hydrology on gas fluxes and water chemistry, to elucidate whether expensive ditch blocking is a cost-effective method for reducing greenhouse gas emissions, in the UK and elsewhere.

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.

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