Global Methane Budget Studies
Contributed by W.S. Reeburgh, University of California - Irvine, USA
Reprinted from IGACtivities Newsletter No. 6, September 1996.
The mixing ratio of methane (CH4) in the troposphere is increasing more rapidly than any of the other long-lived radiatively active atmospheric trace gases. CH4 has increased from a pre-industrial value near 700 ppbv to a present-day value of 1720 ppbv (IPCC, 1995). The global warming potential of CH4 results from its atmospheric lifetime of about 10 years and the fact that it is 20 to 30 times more efficient than carbon dioxide in trapping infrared radiation. A great deal of recent attention has been focused on quantifying terms in the global atmospheric CH4 budget to understand the reasons for the atmospheric concentration increase. The importance of CH4 and the amount of effort focused on understanding its budget is highlighted by the number of IGAC Activities (HESS, BIBEX, RICE, TRAGEX, BATGE, MAGE) conducting research aimed towards understanding CH4 sources, sinks, and controls.
The atmospheric CH4 budget is based on a framework of atmospheric burden, residence time, isotope, and model constraints. We know that net CH4 emission from all sources is nearly balanced by photochemical oxidation by hydroxyl radical in the troposphere, the major sink. Upland soils have been recognized recently as a smaller sink. The tropospheric concentration increase results from a source/sink imbalance of about 37 Tg CH4 per year (IPCC, 1995). Isotopic measurements are used to distinguish between fossil and modern CH4 sources (carbon-14 CH4), and to provide an additional constraint on source and sink strengths (carbon-13 CH4 ). The contemporary atmospheric increase, which links to the ice core paleo-record, has been documented by the U.S. National Oceanic and Atmospheric Administation Climate Monitoring and Diagnostic Laboratory (NOAA/CMDL) sampling program and a number of other programs. Figure 1 (Dlugokencky et al., 1994) summarizes precise measurements on time series flask samples collected at CMDL clean air stations located at a range of northern and southern latitudes. It is a clear demonstration of the dynamic nature of the global CH4 cycle. The preponderance of Northern Hemisphere sources is evident in the interhemispheric concentration gradient. The annual cycle, driven by seasonal changes in emission and photochemical oxidation, can be discerned in the changes in the mixing ratio magnitude and the shift in phase between the Northern and Southern Hemispheres. Perhaps most important, the global concentration increase of 1% per year is evident.
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In order to understand the CH4 cycle, we must understand the source and sink processes for each of the budget terms. An example of a process that has been overlooked until recently is microbially-mediated CH4 oxidation. The global atmospheric CH4 budget is based on net emission to the atmosphere, so microbially-mediated CH4 oxidation, which is included in the net emission terms and is therefore "invisible", is not specifically identified. CH4 oxidation in soils and waters occurs at interfaces or in subsurface zones and is capable of dramatically limiting or modulating emission from well-known sources. Most aerobic, moist (15-50 wt.% H20) soils are capable of consuming atmospheric CH4. Potential consumption by these soils is high, but the supply of CH4 to subsurface sites of oxidation is diffusion-limited. Most of the information on CH4 oxidation results from tracer measurements of oxidation rates, static chamber measurements at sites where net consumption occurs, studies involving specific inhibitors of CH4 oxidation, or manipulation of environments involving displacement of oxygen with nitrogen or argon. In some environments, such as landfills and wetlands with deeply lowered water tables, the zone of oxidation is too far below the surface to use these methods. Stable isotope measurements coupled with an understanding of fractionation accompanying oxidation are being explored as a means of estimating the extent of oxidation in these systems.
What is the role of microbially-mediated CH4 oxidation in the global budget? Table 1 (Reeburgh et al., 1993) estimates the importance of microbially mediated oxidation in the global CH4 budget. Estimates of CH4 oxidation from published measurements for each of the source terms in the Fung et al. (1991) budget are added to the net efflux terms to produce an estimate of gross CH4 production. The oxidation estimates are based on limited data, and although they cannot be constrained like the atmospheric budget, they are conservative. It is instructive to view terms in the CH4 budget by the relative magnitudes of production and consumption. For terms where CH4 is emitted directly to the atmosphere and microbially-mediated consumption is believed to play no role (animals, biomass burning, coal production, natural gas venting and flaring), net emission to the atmosphere equals production. Microbially-mediated oxidation ranges in importance in the other sources. Oxidation can be a small portion of emission (wetlands, landfills, distribution leaks), about equal to emission (termites, hydrates), or greatly in excess of emission (rice, oceans), highlighting the need to understand microbially mediated oxidation as a control on the CH4 budget . Overall, microbially-mediated CH4 oxidation is about 200 Tg per year larger than annual rates of emission. The responses of CH4 production and oxidation to changing climate may be profoundly different, so it is important to understand CH4 production and oxidation at a process level.
| Table 1. Net Global Atmospheric CH4 Emission, Global Consumption and Gross Global Production (Tg CH4 per year) E + C = P Net
Total Sources 500** Photochemical oxidation -450 Soil consumption -10 40 40*** Total Sinks -460** 688.3 Total Production 1188.3 * Scenario 7, Fung et al. (1991); *** Soil consumption of atmospheric CH4 added to the gross budget as an equivalent production term |
Measured CH4 fluxes are highly variable in time and space. Fortunately, good agreement has been obtained in scaling exercises between chamber, eddy correlation, and aircraft boundary layer measurements in several field campaigns. Each of the techniques provides a unique insight into CH4 dynamics. Aircraft data provide instantaneous evaluation of fluxes over very large landscapes. Eddy correlation and other micrometeorological techniques are very useful for areally intergrated, temporally continuous measurements. Chamber measurements lend themselves to process studies in complex landscapes. The very best programs will integrate across these scales. Lack of an inexpensive rapid response sensor for CH4 has hampered the collection of areally integrated seasonal flux data using eddy correlation and eddy accumulation techniques, but the value of long-term continuous data sets and the ability to link fluxes to the local physical climate outweighs the cost of the sensor. Only a few long term data sets exist and most are based on static chambers and emission season data only. Most flux estimates are still based on static chambers and scaling up is based on areal distribution of habitats based on vegetation types. More sophisticated studies of plant community structure and correlations with seasonal CH4 efflux is a promising area of study. There are still large gaps in coverage and the wetland data sets suffer from a North American/European bias. Future programs like the IGBP Northern Eurasia Study planned for Siberia (Steffen and Shvidenko, 1996) should provide much better coverage.
CH4 dynamics of natural environments should be placed within the context of the carbon balance of the ecosystem. For example, vascular plants play a major role in providing substrate for CH4 production and oxidation as well as transporting CH4 to the atmosphere, bypassing the oxidation zones in many environments. One promising observation is a relationship between net ecosystem exchange of carbon dioxide (NEE) and CH4 flux observed for inundated wetland plants. The observation that CH4 emitted from wetlands is composed of carbon-14-contemporary carbon points to the need for pulse-labeling studies and a focus on rhizosphere processes. Ecosystem-level models and soil climate models are giving promising initial results, and incorporation of results from process studies will lead to further improvements.
How sensitive is the CH4 budget to climate change? Studies along natural gradients offer the most realistic means of assessing sensitivity of terms in the global CH4 budget to changes in climate. Field manipulation experiments, (air and soil temperature, carbon dioxide, water table, vegetation) are another approach and are in progress at a number of sites. Manipulations in closed systems (like Biosphere 2, which has no photochemical sink) offer potential for understanding the role of microbially mediated oxidation in model systems under altered temperature, moisture, and carbon dioxide conditions.
Anthropogenic sources of CH4 are becoming better understood both as significant sources and as sources that lend themselves to mitigation efforts. For example, a recent survey (D. Blaha and K. Bartlett, unpublished data) has shown that landfills in New England are an important regional source, emitting about as much CH4 as local wetlands. Globally, landfills are an uncertain term in the budget. Modern landfills in developed countries are being equipped with CH4 recovery systems, and the recovered CH4 is being used for power generation and as a feedstock for synthesis of chemicals like methanol. Emissions from landfills are expected to increase in developing countries, and attention to landfill design can enhance natural oxidation in the absence of recovery systems. Studies on rice indicate that water management, as well as fertilizer and organic matter management, can have major effects on oxidation and CH4 emission. A reduction in anthropogenic emissions of only 8% is sufficient to eliminate the annual increase in tropospheric CH4 (IPCC, 1995). Not only are mitigation strategies needed, but measurement techniques and programs designed to evaluate the effectiveness of the mitigation efforts are required.
Which terms in the global CH4 budget are increasing? What steps can be taken to reduce anthropogenic emissions? How sensitive are terms in the CH4 budget to changes in moisture, temperature and carbon dioxide? At present we are unable to make a priori predictions of CH4 flux, but current studies in a wide range of environments are developing a mechanistic understanding of processes affecting CH4 fluxes. We are optimistic that incorporation of this information into models, future field programs, and mitigation strategies will lead to useful results.
References:
Dlugokencky, E. J., L. P. Steele, P. M. Lang, and K. A. Masarie, The growth rate and distribution of atmospheric CH4. J. Geophys. Res., 99, 17,021-17,043, 1994.
Fung, I., J. John, J. Lerner, E. Matthews, M. Prather, L. P. Steele and P. J. Fraser, Three-dimensional model synthesis of the global CH4 cycle, J. Geophys. Res., 96, 13,033-13,065, 1991.
Steffen, W.L., and A.Z. Shvidenko, Eds., IGBP Northern Eurasia Study: Prospectus for an Integrated Global Change Research Project, IGBP Report No. 37, 95 pp., avail. from IGBP Secretariat, Royal Swedish Academy of Sciences, Box 50005, S-104 05 Stockholm, Sweden, 1996.
IPCC (1995) Climate Change 1994. Radiative Forcing of Climate Change. Working Group I. Summary for Policymakers. Intergovernmental Panel on Climate Change, Cambridge University Press, UNEP.
Reeburgh, W. S., S. C. Whalen, and M. J. Alperin, The role of methylotrophy in the global CH4 budget. pp.1-14 In: Microbial Growth on C-1 Compounds, J. C. Murrell and D. P. Kelly, eds, Intercept, UK, 1993.
