Major natural sources include wetlands, termites and release from onshore and offshore geological sources. Recently, living vegetation has also been suggested as an important natural source of CH4. Of the globally significant sources of CH4 to the atmosphere, natural sources are currently outweighed by anthropogenic sources. Together they emit some 582Tg CH4 each year, with ~200Tg arising from natural sources (Denman et al, 2007). Given the estimated global CH4 sink of 581Tg per year, the current increase in atmospheric CH4 concentrations should, therefore, be only 1Tg CH4 per year. But even with ongoing efforts to reduce anthropogenic emissions, and so arrest the trend of increasing CH4 concentrations in the atmosphere, future enhancement of natural CH4 emissions due to climate change threatens to negate some, or all, of these attempts at mitigation.
Wetland CH4 emissions (excluding rice cultivation) are estimated to total between 100 and 231Tg per year (Denman et al, 2007) - equivalent to around one quarter of global CH4 emissions. The large range of these estimates reflects the uncertainty as to the underlying determinants of net CH4 flux in wetland ecosystems, this uncertainty being further compounded by the effects of enhanced global warming. We know that the three key determinants of CH4 emission from wetlands are temperature (Christensen et al, 2003), water table depth (MacDonald et al, 1998) and substrate availability (Christensen et al, 2003), but the degree of sensitivity of emissions to changes in these determinants remains poorly resolved. Of the three, temperature is most often found to be the dominant factor. For example, over a number of northern wetland sites, soil temperature variations accounted for 84 per cent of the observed variance in CH4 emissions, with emissions showing a strong positive response to increased temperature (Christensen et al, 2003). The impacts of climate change in the 21st century on these emissions could therefore be substantial. A doubling in CO2 concentrations (3.4°C warming) is predicted to result in a 78 per cent increase in wetland CH4 emissions (Shindell et al, 2004). Gedney et al (2004) estimated that this climate feedback mechanism would amplify total anthropogenic radiative forcing by between 3.5 and 5 per cent by 2100. Wetlands then, are critical to the current and future global CH4 budget.
If we are to successfully mitigate and adapt to climate change in the 21st century it is vital that we improve our understanding of this feedback mechanism. In Chapter 3 'Wetlands', Torben Christensen reviews the scientific basis of wetland CH4 fluxes, emissions estimates and projected responses to climate change. He concludes that new generations of ecosystem models will allow the incorporation of such feedbacks into climate projections, but that significant gaps remain in our understanding of how tropical wetland CH4 emissions will respond to changes in precipitation and high-latitude wetland emissions to changes in temperature.
The natural emission of CH4 from so-called 'geological' sources has often focused on CH4 hydrates (also called clathrates) - ice-like mixtures of CH4 and water found in ocean sediments - that are thought to be responsible for between 4 and 5Tg of CH4 emission to the atmosphere each year. These CH4 hydrates and the potential of climatic warming to destabilize them has received significant attention in recent years (for example Westbrook et al, 2009). However, in Chapter 4 'Geological Methane', Giuseppe Etiope argues that estimates of emissions from hydrates remain highly speculative and that the overall geological source of CH4 to the atmosphere is much bigger and more diverse than is commonly reported. He highlights the large losses of CH4 from seeps, mud volcanoes and geothermal/volcanic areas that cumulatively could be responsible for between 40 and 60Tg CH4 each year, and on a par with the largest of the anthropogenic CH4 sources and second only to wetlands as a natural CH4 source. Etiope reviews the evidence for significant CH4 losses from onshore and offshore seeps, differentiates between 'natural' emissions of CH4 associated with coal and oil deposits and those that result from fossil fuel extraction by humans, and assesses how geological CH4 is classified. Commonly, it is categorized as 'fossil methane' if it is more than 50,000 years old and so radiocarbon free. Finally, he assesses the determinants of these geological CH4 sources and their dependence on seismic activity, tectonics and magmatism, concluding that the atmospheric greenhouse gas budget of the planet is far from independent of the earth's geophysical processes.
Though some termite species produce no CH4 at all and those that do rarely exceed more than half a microgram per termite day, the shear mass of termites globally has given rise to some very large estimates (as much as 310Tg per year) of their contribution to global CH4 emissions. In Chapter 5 'Termites', David Bignell examines the evidence base and the trend towards smaller global estimates of CH4 from termites as understanding and measurements have improved. He reviews the differences in CH4 production rates between species and the reasons for these, assessing the methodologies used for these measurements and highlighting the importance of soil-mediated CH4 oxidation in determining the net flux of CH4 from termite colonies. Bignell also examines the issues surrounding the upscaling of CH4 fluxes and the importance of changes in land use, whether in response to human activity or climate, in determining termite CH4 emissions. In conclusion he suggests that the importance of termites as a global CH4 source has probably been overstated in the past, with a more accurate estimate of annual emissions from this source being well below 10Tg and so placing the termite CH4 source as a relatively minor component of the global CH4 budget.
The substantial lowering of this estimate suggests that the strength of other CH4 sources is actually greater than previously thought. As we saw for the geological CH4 source, much of this 'missing' source can be accounted for by onshore and offshore seeps. However, a novel CH4 source discovered in 2006 may also help to bridge any global CH4 budget gap and it is to this source -that of vegetation - that we now turn.
As described in more detail in Chapter 2, the bulk of non-fossil CH4 emitted to the atmosphere each year is microbially mediated. Methane production (methanogenesis) in wetland soils, for example, involves the microbial mineralization of organic carbon under the anaerobic conditions common to waterlogged soils. In the absence of oxygen, the organic carbon (usually simple carbon compounds such as acetate or CO2) is used as an alternative terminal electron acceptor and so provides a source of energy for the methanogens. The atmospheric signal of such microbial methanogenesis is such that enhanced CH4 emission in the tropics during and after periods of heavy rain and waterlogging of soils can be clearly discerned by satellite. An anomaly in this relationship has been observed over some areas of the planet, in particular over the Amazon Basin, where CH4 concentrations in the atmospheric column appear to be much higher than would be expected given the prevailing conditions in the soil below. Frank Keppler and his team were the first to suggest that such anomalies may be a result of the above-ground vegetation itself producing CH4 under aerobic conditions, and so adding to the overall concentration of CH4 in the atmosphere. They provided an initial estimate of the strength of this CH4 source being between 10 and 40 per cent of global emissions. In Chapter 6 'Vegetation', Andy McLeod and Frank Keppler review the evidence for this novel CH4 source and the developing postulations as to its mechanism. In particular, they highlight the potential role of UV radiation and reactive oxygen species in determining CH4 emissions from vegetation. They examine the very limited number of estimates for the global magnitude of this CH4 source from their own groups and others, and suggest that, even with the large uncertainty that exists in these estimates, the net climate-forcing benefits of the establishment of new forests and enhanced CO2 sequestration would far exceed any negative effects due to additional CH4 emissions from the trees.
Biomass burning accounts for between 14 and 88Tg of CH4 each year. Methane emissions arising from biomass burning are a result of incomplete combustion and encompass a wide range of sources, including woodlands, peatlands, savanna and agricultural waste. Burning of peat and agricultural waste may produce especially high CH4 emissions due to the generally high water content and low oxygen availability common to the combustion of these fuel sources. Differentiating between 'natural' and 'anthropogenic' biomass burning is inherently difficult given the coincidence in time and space of many of these events and the difficulty in separating their atmospheric signals. As such, Chapter 7 'Biomass Burning' by Joel Levine, addresses both causes and here is taken as a source that spans both natural and anthropogenic portions of global CH4 fluxes. He reviews the regional patterns and sources of biomass burning, and the methods used to estimate emissions, suggesting that the bulk of biomass burning and resulting CH4 emissions globally are anthropogenic in origin. Levine also points to the importance of biomass burning outside of the tropics, highlighting the interaction between reduced precipitation rates due to climate change and enhanced biomass burning in boreal forests. Finally he discusses how changes in climate and land use in the future may alter biomass burning and CH4 emissions from this source globally. With the changes in climate projected for the 21st century, Levine warns that CH4 (and CO2) emissions from biomass burning are likely to increase globally, providing a potentially very important positive feedback mechanism.
The frequently waterlogged soils common to many rice fields can provide the anoxic, carbon-rich conditions required for high rates of microbial methanogenesis (see Chapter 2). Most rice paddies are submerged for around a third of the time, though practices vary widely around the world based on rice variety, culture and water availability. As with termites, the estimate of CH4 from rice cultivation has seen a trend of downward revision in recent years as understanding of its determinants, field measurement and modelling have improved estimates. Nevertheless, with a projected 9 billion people to feed globally by 2050, rice cultivation is likely to comprise a significant proportion of the world's agricultural land and, without intervention, to remain as an important source of CH4 globally.
In Chapter 8 'Rice Cultivation', Franz Conen, Keith Smith and Kazuyuki Yagi review the estimates of CH4 emissions from this source, with recent estimates generally being between 25 and 50Tg CH4 per year. They underline the importance of increasing demand on future emissions and provide an overview of the microbially mediated production and oxidation of CH4 in rice paddy soils. Various cultivation strategies and locations are then examined and their relative importance in terms of CH4 emissions assessed. Continuously flooded/irrigated rice emerges as the strongest CH4 source per unit area, with drought-prone, rain-fed rice having much lower or sometimes zero CH4 emissions per unit area. Conen et al then examine the ways in which CH4 emissions per unit yield can be altered through changes in land and water management, rice variety and application of fertilizers and residues. Finally, they review the global assessments of CH4 emissions from rice cultivation, the methodologies employed and the potential for reducing emissions from this source in the future.
Ruminant livestock, such as cattle, sheep, goats and deer, primarily produce CH4 as a by-product of feed fermentation in their rumens. The bulk (>90 per cent) of the CH4 is then emitted through belching - some dairy cattle emitting several hundred litres of CH4 in this way each day. In 2005, CH4 emissions from ruminant livestock were estimated to be around 72Tg per year. As with rice agriculture, CH4 emissions from ruminant livestock are highly dependent on demand pressures and with a global trend of increasing consumption of both meat and dairy products emissions are expected to rise to around 100Tg CH4 per year by 2010. In Chapter 9 'Ruminants', Frank Kelliher and Harry Clark review the estimates of global and national CH4 emissions from this source, the ways in which they are calculated and the uncertainties inherent in such estimates. They then examine the role of feedstock type and quality in determining ruminant CH4 emissions and go on to describe the various strategies available to reduce these emissions in the short, medium and longer term. Such strategies include the reduction in demand for ruminant meat and dairy products, changes in livestock diet and production efficiency, and the use of vaccines.
Microbial methanogenesis in livestock manure and wastewater can produce significant amounts of CH4 due to the high availability of substrates (acetate, CO2 and H2) and the anoxic conditions that tend to prevail. Globally, agricultural waste and wastewater are together responsible for the emissions of between 14 and 25Tg CH4 each year. As with direct CH4 emission from ruminants, manure-derived CH4 emissions are coupled to livestock demand pressures, with increases in demand for meat and dairy products tending to increase manure production and related CH4 emissions. Similarly, the rapidly increasing human population is itself increasing pressure on sewage and wastewater treatment capacity around the world and has the potential to greatly enhance CH4 emissions from this source. Emissions from livestock manure are often included in total livestock CH4 emissions source estimates, but in considering mitigation is it useful to separate these sources. In Chapter 10 'Manure and Wastewater', Miriam van Eekert, Hendrik Jan van Dooren, Marjo Lexmond and Grietje Zeeman review the key processes responsible for manure and wastewater CH4 emissions and the methods used to estimate them. They then focus on a range of established and putative mitigation options, including anaerobic digestion, manure and sludge handling, and livestock diet manipulations. For both manure and wastewater, anaerobic digestion is shown to have great potential through the effective interception of CH4 and its use as an alternative energy source with which to replace conventional fossil fuel-derived energy sources.
Landfill sites can provide ideal conditions for methanogenesis, with the plentiful supply of substrate held under anoxic conditions making some landfill sites very powerful point sources of CH4 production and, if uncontrolled, emission. As sewage sludge and agricultural waste may also be incorporated into landfills, the CH4 source strength for these two categories may overlap somewhat. However, for much of the world, it is CH4 derived from anaerobic decomposition of municipal rather than agricultural waste that dominates. Early estimates of global CH4 emissions from landfill were of the order of 70Tg per year, but successful implementation of mitigation strategies has seen a reduction in emissions from this source in many developed nations. In Chapter 11 'Landfills', Jean Bogner and Kurt Spokas review the landfill CH4 source, its determinants and its measurement. They examine and update progress on mitigating landfill CH4 emissions using CH4 collection and the enhancement of CH4 oxidation rates in landfill cover soils. They conclude that, although landfill CH4 emissions constitute only a small part (~1.3 per cent) of total anthropogenic GHG emissions globally, improved CH4 recovery and cover soil oxidation has the potential to further reduce emissions from this source, with the former providing a useful energy source with which to offset fossil fuel use.
Much of the ~75Tg of CH4 emission attributable to fossil energy use each year is derived from release during fossil fuel extraction, storage, processing and transportation. Some CH4 is also emitted during incomplete fossil fuel combustion. At 30-46Tg of CH4 per year, coal mining and extraction constitutes one of the largest individual source activities of anthropogenic CH4. The CH4 is formed as part of the geological process of coal formation and large deposits can then remain trapped within or close to the coal seam until released by mining operations. Methane concentrations between 5 and 15 per cent in the air of coal mines represent an explosion hazard and so ventilation is commonly employed to rid deep mines of this CH4. In Chapter 12 'Fossil Energy and Ventilation Air Methane', Richard Mattus and Ake Kallstrand briefly review the sources of fossil energy CH4 before focusing on strategies to reduce CH4 emissions from coal mine ventilation air.
Finally, in Chapter 13 'Options for Methane Control', André van Amstel identifies and reviews a suite of 27 different CH4 emission mitigation strategies that are proven and that can be deployed immediately. He examines their relative costs and effectiveness, both regionally and globally, between 1990 and 2100 and concludes that many of these strategies can be successfully implemented at little or no net cost in the coming decades.
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