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2008 P21 Inter-Agency Climate Change Forum Meeting Date: December 10th 2008 Paper: IACCF 2008 P21 - Methane Emissions and their Relevance to Wetland Management and Restoration This paper was first presented to us in April 2008; it has since been updated to include emissions from coastal sources. Action by IACCF: Decide whether to develop a UK position paper on this issue. Page 1 of 10 Inter-Agency Climate Change Forum CCW paper – updated 21/8/08 CYNGOR CEFN GWLAD CYMRU COUNTRYSIDE COUNCIL FOR WALES METHANE EMISSIONS AND THEIR RELEVANCE TO WETLAND MANAGEMENT AND RESTORATION 1. Summary Anthropogenic emissions of methane in Wales have been dramatically reduced by 41.1% since 1990. However, agricultural emissions, largely from cattle and sheep, along with emissions from landfill sites remain substantial and require further action. In addition, further control of methane emissions from coal mining and leakage from the natural gas network should be sought. Natural emissions in Wales are largely from wetlands, and their management for conservation, restoration and recreation may lead to local increases. However, in the context of Wales’ overall emissions these local changes are small and will probably be offset locally by carbon sequestration in the wetlands in the long-term (100 years +) and in the short-term by the biodiversity, hydrological and landscape value that they provide. The rewetting or restoration of wetlands should not be jeopardised by concerns relating to increased methane emissions. 2. Introduction This information paper provides a brief outline of current understanding of the contribution made by methane to climate change and the relative importance of anthropogenic and natural sources, along with the scope for further reductions in them. An estimate of natural methane emissions from Welsh wetlands has been calculated, with its implication for conservation of wetlands in Wales discussed. 3. Atmospheric methane (CH4) concentrations and their role in climate change After carbon dioxide, methane is the most important of the anthropogenic greenhouse gases in terms of its contribution to climate change (IPPC, 2007). Weight for weight, methane is 21 times more effective at trapping greenhouse heat (thermal infrared radiation) than CO2 over a 100-year timescale. As methane is removed from the atmosphere more rapidly by natural processes than CO2, methane emissions have a disproportionately large global warming potential on shorter time scales. On a twentyyear timescale methane is weight for weight 56 times more powerful a greenhouse gas. Hence, changes in methane emissions have a proportionately larger effect on climate change compared to carbon dioxide. Ice cores show that over the last 650,000 years the concentration of methane has varied between approximately 400ppb during glacial periods and 700ppb in interglacials. In 2005, the global average methane concentration was recorded as 1,774ppb (IPCC, 2007). Current methane concentrations are more than double their preindustrial level and proportionally further from the normal range than carbon dioxide Page 2 of 10 Inter-Agency Climate Change Forum levels. The global warming effect (radiative forcing1) of methane emitted since 1750 is equivalent to around one-third of the warming caused by CO2 since that date. Over the last 25 years global methane levels have risen by around 30% but the annual rate of growth is highly variable. The annual growth rate has been up to 14 ppb but between 1999 and 2005 methane concentrations stabilised (Dlugokencky et al., 2003). The cause of this variation and recent stabilisation are not properly understood. Various factors have been identified: it has been suggested that volcanic eruptions such as Mt Pinatubo led to removal of atmospheric CH4 while there is isotopic evidence suggesting that wetlands and rice growing regions may contribute to the variation, along with fluctuations in biomass burning. Recent studies suggest that the stabilisation of CH4 concentrations in the atmosphere is likely to be a result of nearconstant emissions reaching equilibrium with sinks (IPPC, 2007). There is no evidence of sinks strengthening. 4. Sources of methane Most methane emissions (around 70%) are from biological sources including wetlands, wild and domesticated ruminant animals, rice agriculture, termites and forests. Fossil fuel mining and burning along with biomass burning, waste treatment and releases from geological sources including volcanic activity make up the remaining non-biogenic emissions (IPPC, 2007). Pre-industrial emissions were predominantly from natural sources (over 80%) while at present more than 60% are anthropogenic. The main anthropogenic sources globally are livestock, rice agriculture, landfill sites, waste treatment, biomass burning, coal mining and natural gas production/distribution. 5. Reducing anthropogenic methane emissions UK and Welsh emissions of methane fell by 52.3% and 41.1% respectively between 1990 and 2005 (Baggott et al., 2005). By 2005, coal mining represented only 9% of Welsh methane emissions as the decline of the coal industry resulted in a 72.9% reduction since 1990. Improvements to the natural gas distribution system reduced leakage such that losses from this sector fell by 27.5% since 1990 to represent only 5.4% of Welsh emissions in 2005. Methane from landfill sites represents 21.9% of Welsh emissions but it has been reduced by 61.6% since 1990 through capping, collection and burning of methane for energy. Currently, the main anthropogenic emissions of methane in Wales are enteric fermentation from cattle which represents 3% of the total global warming potential (GWP2) of all Welsh GHG emissions and landfill gas representing 2% of Welsh GWP (Baggott et al., 2005). Some 90% of Welsh agricultural CH4 emissions are derived from enteric fermentation in cattle and sheep (Baggott et al., 2005). This represents 54% of all anthropogenic methane emissions in Wales. At present cattle and sheep numbers largely determine methane emissions from the agricultural sector. Livestock emissions may be controlled through greater efficiency in dairy production or use of feedstuffs or husbandry techniques that reduce methane outputs from cattle. There is a need for 1 Radiative forcing is a measure of the contribution of GHGs or other drivers of climate change e.g. solar output to global warming since 1750, measured in terms of net irradiance at the top of the troposphere. 2 Global warming potential (GWP) is a measure of the relative warming effect (radiative forcing) of a unit of any greenhouse gas relative to carbon dioxide. Page 3 of 10 Inter-Agency Climate Change Forum further research to develop such husbandry methods to reduce methane emissions per unit of dairy or beef production. There is further potential to improve waste management such as the introduction of on-farm anaerobic biodigesters and power plants to use methane for local power generation. Additional capture and leakage control measures to reduce emissions from landfill and the gas distribution network are also required. 6. Methane emissions in natural ecosystems Globally, natural methane emissions represent around 32% of the total, of which 22% arise from natural wetlands – this is the single largest source of methane emissions although tropical and southern hemisphere wetlands account for more than 70% of these. The remaining natural emissions are from freshwaters (3%) and termites (7%) (Galchenko, 1989). Almost all natural methane emissions in Wales are from wetlands. The extent of natural methane emissions across Wales has not been previously quantified but using figures available from the Phase 1 Survey database for the extent of wetlands and estimates of average annual methane emissions for blanket bog/raised bog and fens a crude inventory of emissions is presented below. In a 1-year survey, methane release from the Migneint averaged 2.5 mg m-2 d-1 – equivalent to 9.1kg/ha/year (but varied between 0.15 and 6.39 mg m-2 d-1) while fens emit far more, an average of around 50 mg m-2 d-1 – equivalent to 182 kg/ha/year (but again very variable within a range 0.3 to 209.4 mg m-2 d-1) (Chris Freeman, pers. comm.). These values measured on Welsh sites are used to calculate the emissions in Table 1. However, it is important to understand we do not understand the variability in emissions between localities. Table 1. Estimated annual methane emissions for wetland habitats in Wales Habitat type Condition/Peatforming status Lowland raised bog Lowland raised bog Raised bog Modified (raised) bog Lowland raised bog Archaic peat - of presumed raised bog origin Blanket bog (lowland) Blanket bog (lowland) Blanket bog (upland) Blanket bog (upland) Blanket bog (upland) Blanket bog (upland) Topogenous fen (lowland) Topogenous fen (lowland) Blanket bog Modified bog Blanket bog Modified bog Afforested bog Archaic peat Topogenous fen Modified topogenous fen Topogenous fen (upland) Topogenous fen (upland) Topogenous fen Modified topogenous fen Soligenous fen (lowland) Soligenous fen (upland) TOTAL Soligenous fen Soligenous fen Extent (ha) Page 4 of 10 Estimated methane (tonnes) 990 annual emissions 9 7.4 820 20.4 2238 500 1200 22600 32000 9100 5000 3800 770 1300 300 2100 14800 97,518 4.5 10.9 205.6 291.2 82.8 45.5 691.6 140.1 236.6 54.6 382.2 2693.6 4876.2 Inter-Agency Climate Change Forum Key Should be currently peat forming Locally peat forming, but much at standstill Not peat forming - likely to be an active C source Total anthropogenic emissions of methane in 2003 for Wales were estimated to be 1.0 Mt C equiv while the total estimated emissions from Welsh wetlands based on the inventory in Table 1 are a little over 0.1 Mt C equiv3. This assessment suggests that wetland emissions are an order of magnitude less than anthropogenic emissions so short-term increases in methane emissions resulting from wetland management, restoration or recreation will be insignificant in terms of the overall methane inventory. Moreover, the increased risk of loss of soil carbon, particularly on peatlands, if restoration is not pursued, given that many wetlands are in unfavourable condition and not at present peat forming, should override any concerns relating to methane emissions. 7. Variability in methane emissions and climate change impacts upon them Wetland CH4 emissions vary in response to temperature and are also affected by hydrological changes. There is huge temporal variation in emissions from wetlands dependent on temperature, moisture regimes and atmospheric pressure while emissions also vary spatially due to small-scale variations in topography (e.g. between hummocks and pools), hydrology and vegetation. Methane is released from wetlands by three pathways i) diffusion through the water table and peat to the surface; ii) diffusion or active transport through vascular plants and iii) as bubbles rising to the surface – otherwise known as ebullition. Recent studies suggest that ebullition is the dominant pathway for emissions in UK peatlands (e.g. Baird et al., 2004). All models suggest that climate change will result in an increase in global wetland emissions and create a positive feedback (IPPC, 2007). Based on an assessment of the temperature and CH4 emission relationship at two wetland sites in Scotland, it has been estimated that CH4 emissions would rise by 17, 30 or 60% for a 1.5, 2.5 or 4.5C increase in temperature (Chapman & Thurlow, 1996). However, this is a result of modelling higher net ecosystem production, which provides the substrate for methane production, and if higher temperatures lead to lower ecosystem productivity or reduced precipitation and water levels then methane production will be reduced. It is by no means certain that climate change will result in increased methane emissions from wetlands, particularly in any locality experiencing seasonal decreases in rainfall – as projected for the summer in Wales. 8. 3 Wetland methane emissions in the context of their restoration and management Changes in land use from forest to grassland, arable or urban areas do not result in any significant emissions of methane and have not been quantified in the UK emissions inventory (Baggott et al., 2005). However, conversion of areas to wetland will result in increased methane emissions and the rewetting of bogs and mires can result in short-term substantial increases in the methane emissions for those individual sites. A Based on the Wetlands emission total in Table 1 and a GWP multiplier of 21 Page 5 of 10 Inter-Agency Climate Change Forum recent assessment of the total global warming potential (GWP) effect of rewetting and restoring wetlands, based on Europe’s largest wetland restoration project in Belarus, has shown an initially very high level of methane emissions and low C sequestration rate resulting in a net negative effect on climate mitigation followed by much reduced methane emissions and high carbon sequestration levels with a net positive mitigation effect while in the longer term the GWP effect of these wetlands is projected to be neutral (Joosten & Augustin, 2006). These changes must be put in context as drained wetlands have a net negative effect on GHG emissions. There is limited potential for the expansion of wetlands within Wales as their occurrence is limited by topography, hydrology and climate so no significant change in the gross scale of these natural emissions is projected, even though restoration and recreation of many wetlands is planned or already underway in Wales. For example, based on 182 kg/ha/year methane emissions recorded in Welsh fens, the Welsh BAP target for recreating an additional 100ha of fen could potentially contribute an additional 18 tonnes of methane/year – a minimal quantity in terms of the emissions inventory. This has to be set against the benefits offered by renewed peat formation, namely reduced DOC emissions into the aquatic environment, and net C sequestration. Rates of net carbon accumulation would be expected to lie in the region of 30 g C m -2 /year based on studies of rich-fens elsewhere (Yu et al., 2003) – equating to 300 tonnes of C/ha/year. The conservation, management and restoration of wetlands and peatlands should not be influenced by the relatively small net changes in overall methane emissions they may cause – especially when viewed in the wider context of preservation of existing soil carbon stores. In any case, it is illogical to consider peatland methane emissions in isolation from the total carbon fluxes. Methane emissions are vastly outweighed by the overall value of preservation of the existing, vulnerable soil carbon stores in such sites. Short-term increases in methane emissions may be the price we have to pay in order to prevent the long-term release of a far more significant amount of our overall soil carbon store. 9. Methane emissions from coastal wetlands Salt marshes and other intertidal habitats such as mudflats act as carbon sinks, but under particular circumstances can act as a source of methane emissions. However, in global terms salt marshes do not produce significant methane fluxes (Aselmann & Crutzen, 1989). Hence, they are not generally considered in the environmental biogeochemistry components of GCMs unlike terrestrial wetlands. Their emissions to the atmosphere are limited to a large extent by sulphate reduction within saltmarsh soils preventing methanogenesis and methane oxidation by chlorine at the marineatmosphere boundary layer. Nevertheless, methane emissions are much greater than in other coastal habitats such as sand dunes. The estimated global warming potential based on observed carbon dioxide, methane and nitrous oxide fluxes, was found to be around 174-fold higher in saltmarsh than nearby sandy shores in a Japanese coastal lagoon (Hirota et al., 2007). Methane emission measurements from typical saltmarsh soils and humus-rich saltmarsh soils on the German North Sea coast show contrasting results (Giani et al., 1996). In typical saltmarsh soils the sulphate concentrations of the pore-water were significant (about 10 mM) so sulphate reduction is not limited and methanogenesis would be suppressed. Methane emission rates were almost zero. However, in humusrich saltmarsh soils, methane concentrations were around a thousand times greater Page 6 of 10 Inter-Agency Climate Change Forum than in typical saltmarsh soils and the sulphate concentrations were low resulting in high rates of methane production (up to 80 mu g cm-3 day-1). Methane emission rates reached up to 190 mu g m-2 day-1 in summer, with values up to 20 mu g m-2 day-1 at other times. Saltmarsh soil can behave quite differently with typical saltmarsh soils acting as a sink for methane and the humus-rich soils as a source. Carbon cycling in intertidal mud flat sediments in the Scheldt Estuary was studied using measurements of carbon dioxide and methane emission rates (Middelburg et al., 1995). 42% of the annual amount of carbon delivered to the sediment becomes buried while only 7% is emitted as methane, the remaining 50% being released as carbon dioxide. It is clear that while such coastal habitats can be a source of methane emissions, they are primarily a major sink for carbon. Sea-level rise as a result of global warming is likely to result in the loss, or reduction in extent of many coastal wetlands and it has been suggested that this will act as a positive feedback mechanism for increasing greenhouse gas concentrations as the organic matter within coastal wetlands decomposes. However, there is no clear evidence that sea level rise and coastal erosion will result in greater fluxes of methane (or other greenhouse gases). However, it is possible for the adaptive response to sealevel rise to either increase or decrease GHG emissions. Greenhouse gas flux estimates have been calculated for a managed realignment scenario on the Humber estuary that adds 7500 ha of new intertidal habitat (Andrews et al., 2006). This suggests that despite increasing the annual sink of organic carbon in the estuary by 150%, this is offset by enhanced greenhouse gas emissions in new marshes in the low salinity upper reaches of the estuary. Net carbon storage is thus most effectively achieved by creating new coastal wetlands as saline marshes at the seaward end of estuaries – although this is just one factor influencing the location of managed realignment schemes. 10. Marine methane hydrates Large amounts of methane hydrates are stored on the oceanic seafloor (containing an estimated 4 x 106 million tonnes of methane) (IPPC, 2007). Isotopic evidence indicates that there have been several methane hydrate release events associated with warming periods in the Earth’s history but slump slides of sediments or other sudden impacts are thought to be the likely cause rather than any gradual effect of warming temperatures. Nevertheless, recent modelling suggests that a warming of seafloor waters of 3C would result in the release of 85% of current seafloor methane (Buffett & Archer, 2004). This would be a powerful positive feedback accelerating climate change that should be considered in long-term global warming scenarios and reinforces the need for urgent mitigation measures to reduce GHG emissions to avoid such elevated ocean temperatures. 11. CCW activity related to methane CCW is providing logistical and other non-financial support for research into methane emissions from mires undergoing restoration. Some of the key recent work on ebullition led by researchers at the University of Bristol has utilized Welsh sites. More recently, Professor Andrew Baird of Queen Mary University of London has initiated a project at Cors Fochno (Dyfi SSSI) to compile what is believed to be the Page 7 of 10 Inter-Agency Climate Change Forum first whole-system methane budget for a raised mire system. This information will be of considerable value for CCW, and also underlines the ongoing value of the NNR series for world-class research. We are currently compiling internal guidance for CCW Regional staff, which will incorporate the conclusions of this paper in relation to wetland management. Page 8 of 10 Inter-Agency Climate Change Forum 12. References Andrews, J.E., Burgess, D., Cave, R.R., Coombes, E.G. Jickells, T.D., Parkes, D.J. and Turner, R.K. (2006). Biogeochemical value of managed realignment, Humber estuary, UK. Science of the Total Environment, 371: 19-30. Aselmann I. and Crutzen P.J. (1989). Global distribution of natural freshwater wetlands and rice paddies, their net primary productivity, seasonality and possible methane emissions. J. Atmos. Chem. 8, 307-358. Baggott, S.L., Brown, L., Milne, R., Murrells, T.P., Passant, N. & Thistlethwaite, D.G. (2005). Greenhouse Gas Inventories, for England, Scotland, Wales and Northern Ireland: 1990 – 2003. AEA Technology, Didcot. http://www.airquality.co.uk/archive/reports/cat07/0509211321_Reghg_report_2003_Main_Te xt_Issue_1.doc Baird, A.J., Beckwith, C.W., Waldron, S. & Waddington, J.M. (2004). Ebullition of methanecontaining gas bubbles from near-surface Sphagnum peat. Geophys. Res. Lett., 31, doi:10.1029/2004GL021157. Buffett, B. & Archer, D. (2004). Global inventory of methane clathrate: sensitivity to changes in the deep ocean. Earth Planet Sci. Lett., 227, 185-199. Chapman, S.J. & Thurlow, M. (1996). The influence of climate on CO2 and CH4 emissions from organic soils. J. Agric. For. Meteorol., 79, 205-217. Dlugokencky E.J. et al., (2003). Atmospheric methane levels off: temporary pause or a steady new state? Geophys. Res. Lett., 30, doi:10.1029/2003GL018126. Galchenko, V.F., Lein, A., Ivanov, M. (1989). Biological Sinks of Methane. pp. 59-71. In: Andreae, M.O. & Schimel, D.S. Exchange of Trace Gases between terrestrial ecosystems and the atmosphere. Report of the Dalhem Workshop, Berlin, 1989. John Wiley and Son, New York. Giani, L., Dittrich, K., Martsfeld-Hartmann, A. & Peters, G. (1996). Temperate saltmarshes are a potential source of atmospheric methane. European J. of Soil Sci. 47: 175 – 182. Hirota, M., Senga, Y., Seike, Y., Nohara, S. & Kunii, H. (2007). Fluxes of carbon dioxide, methane and nitrous oxide in two contrastive fringing zones of coastal lagoon, Lake Nakaumi, Japan. Chemosphere, 68: 597-603. IPPC (2007). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Eds. S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. Averyt, M.M.B Tignor H.L. Miller. Cambridge University Press, Cambridge. Joosten, H. & Augustin, J. (2006). Peatland restoration and climate: on possible fluxes of gases and money. In: Bambalov, N.N. ed. Peat in solution of energy, agriculture and ecology problems. Tonpik, Minsk. Page 9 of 10 Inter-Agency Climate Change Forum Middelburg, J.J., Klaver, G., Nieuwenhuize, J. & Vlug, T. (1995). Carbon and nitrogen cycling in intertidal sediments near Doel, Scheldt Estuary. Hydrobiologica, 311: 57-69. Yu, Z., Vitt, D.H., Campbell, I.D. & Apps, M.J. (2003). Understanding Holocene peat accumulation pattern of continental fens in western Canada. Canadian Journal of Botany, 81; 267-282. Page 10 of 10