Blog: Trace Gas Dynamics in Tropical Forests


By Bertie Welch. 18th December 2013.

After carbon dioxide, methane is the second most important greenhouse gas, yet how much is produced and transferred to the atmosphere by tropical rainforests is not fully understood, because much of the research in the tropics focuses on rice paddies and wetlands, as these are significant sources of anthropogenic methane. Rice et al. (2010) suggest that globally, hardwood trees could account for emissions of ~60 Tg Ch4 yr-1.

These potentially significant methane emissions are a result of trees allowing the methane produced in deeper, anaerobic soils to bypass the methanotrophs and nitrifying bacteria in the aerobic upper soils (Milich, 1999). We know from temperate forest and mesocosm studies that trees can act as conduits for soil methane into the atmosphere.  Trees can adapt to cope with flooding-induced soil anoxia in a variety of ways: hypertrophied lenticels, adventitious roots and enlarged aerenchyma (Havens, 1994; Kozlowksi, 1997).  Studies of temperate trees such as alders have shown that they use all the above adaptations when in waterlogged soils. This results in increased methane fluxes from the trees as there is increased surface area for emission and easier transmission from soil to atmosphere (Rusch and Rennenberg, 1998; Gauci et al., 2010).

Pangala et al. (2013) showed that in a tropical peat forest in Borneo, tree stems were responsible for 60-80% of total ecosystem methane fluxes, demonstrating that trees can be a significant source of emissions. This new study in lowland evergreen tropical rainforest in Panama aims to expand on this work and investigate the extent of tree stem emissions in an area of relatively free draining soils on a fortnightly basis for 5 months, covering the dry to wet season transition starting March 2014.

There is a second aspect to the study – we don’t know how trace greenhouse gas biosphere-atmosphere exchange will be affected by an atmosphere that is being enriched with CO2. It is thought that rising atmospheric CO2 concentrations will increase primary productivity in rainforests resulting in greater amounts of litterfall. The fieldwork will be done at the Smithsonian Tropical Research Institute, Panama, on plots that are part of an ongoing litter manipulation experiment meant to simulate elevated atmospheric CO2. There are 15 plots in total: 5 control, 5 with litter removed and 5 with litter added. Sayer et al. (2011) discovered soil respiration to be significantly higher in the litter addition plots compared to the control and litter removal plots. We hypothesise that due to increased litter input (and therefore more source carbon for methanogenesis) methane emissions will be greatest in the addition plots.

Gauci, V., Gowing, D.J.G., Hornibrook, E.R.C., Davis, J.M. & Dise, N.B. (2010) Woody stem methane emission in mature wetland alder trees. Atmospheric Environment, 44, 2157-2160.

Havens, K.J. (1994) The formation of hypertrophied lenticels, adventitious water roots, and an oxidized rhizosphere by Acer rubrum seedlings over time along a hydrologic gradient. Virginia Institute of Marine Science, Gloucester Point, Va.

Kozlowski, T.T. (1997) Responses of woody plants to flooding and salinity. Tree Physiology, Monograph No. 1, 29.

Milich, L. (1999) The role of methane in global warming: where might mitigation strategies be focused. Global Environmental Change, 9, 179-201.

Pangala, S.R., Moore, S., Hornibrook, E.R.C. & Gauci, V. (2013) Trees are major conduits for methane egress from tropical forested wetlands. New Phytologist, 197, 524-531.

Rice, A.L., Butenhoff, C.L., Shearer, M.J., Teama, D., Rosenstiel, T.N. & Khalil, M.A.K. (2010) Emissions of anaerobically produced methane by trees. Geophysical Research Letters, 37, L03807.

Rusch, H. & Rennenberg, H. (1998) Black Alder (Alnus glutinosa (L.) Gaertn.) trees mediate methane and nitrous oxide emission from the soil to the atmosphere. Plant and Soil, 201, 1-7.

Sayer, E.J., Heard, M.S., Grant, H.K., Marthews, T.R. & Tanner, E.V.J. (2011) Soil carbon release enhanced by increased tropical forest litterfall. Nature Climate Change, 1, 304-307.

Image: Forest of Taman Negara National Park by Vladimir Yu. Arkhipov, Arkhivov under Creative Commons CC BY-SA 3.0.

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 *  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


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: An Ice Record of Methane Sources?


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: CCSSG Major Research Issues For Methane


By Aslam Khalil. 3rd May 2012. Dear Methane Research Colleagues The next meeting of the CCSSG is in 3 weeks. The members have been asked to summarize the important open issues in their areas of representation. For me this is methane, CO and hydrocarbons, which boils down to methane. I am writing to ask you if you have any thoughts you would like me to convey. At the moment I have the following areas that I plan to highlight (I will have maybe 15-20 minutes): 1) Temperature feedback on terrestrial emissions – the idea that the future increases of methane will likely occur from climate feedback rather than increases of direct anthropogenic emissions. These feedbacks are in three areas: Resident soil carbon pool, wetlands (including rice) and the arctic permafrost. All have different mechanisms for release and oxidation is a key player to ameliorate the impact. 2) The role of methane in greenhouse gas trading. How should it be done? 3) The uncertain role of plant emissions (although I will probably skip this and just include it on a list at the end with other possible issues that can be discussed if someone wants). If I get something from you that can be highlighted or mentioned for future discussion, I will add your name and affiliation to the list of contributors. Note that the presentation slides are usually posted by the USGRP on their website and become public information. Please send your comments to my e-mail address:

Blog: Source-driven Doubling in Methane


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: Methane and the Fierce Urgency of Now

informal shot

By Nathan Currier. 6th November 2011. Dear methane friends – I’m hoping that you might enjoy taking a look at my recent article at the Huffington Post, Methane and the Fierce Urgency of Now, which I hope will be the first of a series focusing on methane in climate policy from a variety of angles. If you like it, please post comments on their site, or just hit the “like” button, as that could help with future pieces becoming “featured” posts, getting linked to their Facebook page, etc. The idea is to help raise public awareness of the significance of methane for near-term climate policy. Further, if any of you have direct comments on it for me, or ideas re future pieces to bring methane issues before the public, I’d enjoy your feedback. Many thanks. Nathan