Download countryside council for wales

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.
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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
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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.
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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)
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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 4.5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 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
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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
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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 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.
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
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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.
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12.
References
Andrews, J.E., Burgess, D., Cave, R.R., Coombes, E.G. Jickells, T.D., Parkes, D.J. and
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