Download cyngor cefn gwlad cymru ccw p 08 02

CYNGOR CEFN GWLAD CYMRU
COUNTRYSIDE COUNCIL FOR WALES
CCW P 08 02
February 2008
METHANE EMISSIONS AND THEIR RELEVANCE TO WETLAND
MANAGEMENT AND RESTORATION
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. Climate change guidance in preparation for Regional staff will cover this issue.
Recommendations:
Council is requested to note that methane emissions are a significant cause of climate
change and that while reductions in Welsh emissions have been successful, further
efforts to control anthropogenic emissions are required.
Council is requested to endorse the principle that natural methane emissions, within
Wales largely from wetlands, while significant should not restrict the restoration or
recreation of wetland habitats, particularly as there is evidence that in the long-term
(100 years +) these habitats will become carbon sinks.
Council is asked to note and endorse CCWs support for relevant research on Welsh
sites.
1.
Background
An information paper was requested by Council covering this topic.
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.
i)
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,
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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 inter-glacials. In
2005, the global average methane concentration was recorded as 1,774ppb (IPCC,
2007). Current methane concentrations are more than double their pre-industrial level
and proportionally further from the normal range than carbon dioxide 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 near-constant emissions
reaching equilibrium with sinks (IPPC, 2007). There is no evidence of sinks
strengthening.
ii)
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.
iii)
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
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
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 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.
iv) 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.
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.
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.
3
Based on the Wetlands emission total in Table 1 and a GWP multiplier of 21
3
Table 1. Estimated annual methane emissions for wetland habitats in Wales
Habitat type
Condition/Peatforming status
Extent (ha)
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
Estimated annual
methane emissions
(tonnes)
990
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
Key
Should be currently peat
forming
Locally peat forming, but
much at standstill
Not peat forming - likely to
be an active C source
v) 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 4.5C
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
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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.
vi) 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 shortterm substantial increases in the methane emissions for those individual sites. A 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.
vii) 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 first wholesystem 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.
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viii) 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 3C
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.
4.
Resource Implications
None
5.
Compliance
Not relevant
6.
Welsh Language Statement
Conforms with standard CCW policy.
7.
Sustainability Assessment
This paper addresses the management of biodiversity and reduction in GHG emissions
that are key aspects of sustainability.
8.
Risk Assessment
Not relevant
Dr David Parker
Director Science
This paper has been prepared by Dr Clive Walmsley and Dr Peter Jones
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ANNEX 1
References
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.
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.
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_Tex
t_Issue_1.doc
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.
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