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This is a copy of paper submitted to Ecosystems. Please do not quote, copy, share,
print, multiply or distribute, except within the STRP group on Wetlands and
Climate Change.
Current and future CO2 emissions from drained peatlands in
Southeast Asia
Shortened title (<45 characters):
CO2 emissions from drained peat in Southeast Asia
Authors:
Aljosja Hooijer (WL | Delft Hydraulics, PO Box 177, 2600 MH Delft, The Netherlands;
[email protected], Tel +31 15 2858770)
Henk Wösten (Alterra, Wageningen University and Research Centre, PO Box 47, 6700 AA
Wageningen, The Netherlands)
Marcel Silvius (Wetlands International, PO Box 471, 6700 AL Wageningen, The Netherlands)
Susan Page (Department of Geography, University of Leicester, University Road, Leicester, LE1 7RH,
United Kingdom)
Jaap Kwadijk (WL | Delft Hydraulics, PO Box 177, 2600 MH Delft, The Netherlands)
Josep G. Canadell (Global Carbon Project, CSIRO Marine and Atmospheric Research, GPO Box 3023,
Canberra, ACT 2601, Australia)
Abstract (250 words max)
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Forested tropical peatlands in Southeast Asia store at least 42,000 Million metric tonnes of soil carbon.
Human activity and climate change threatens the stability of this large pool which has been rapidly
decreasing over the last few decades due to deforestation, drainage and fire. In this paper we investigate
the emission due to drainage for agricultural and silvicultural development which has dominated, and is
expected to dominate, the perturbation of the carbon balance in this part of the world. Present and
future emissions from drained peatlands were quantified using data on peat extent and depth, present
and projected land use, water management practice and decomposition rates. Of the 27.1 million
hectares of peatland in Southeast Asia, 12.9 million hectares are currently deforested and mostly
drained; this area is rapidly increasing. Current carbon dioxide (CO2) emissions caused by
decomposition of drained peatlands alone, not including the effect of peatland fires, are 632 Mt y-1
(range: 355-855 Mt y-1
). Based on a ‘business as usual’ scenario of land use change, the magnitude of
emissions may peak at 745 Mt y-1
in coming decades unless land management practices and peatland
development plans are significantly changed, and will continue throughout the 21st century. Peatland
drainage in Southeast Asia is a globally significant source of CO2 emissions and a major obstacle to
meeting the aim of stabilizing global greenhouse gas emissions. It is therefore recommended that
international action is taken to help Southeast Asian countries to better conserve their peat resources
through both forest conservation and water management improvements aiming to restore high water
tables.
Key words
peatlands, peat decomposition, water management, drainage, subsidence, CO2 emission, climate change
INTRODUCTION
Peat deposits consist of plant remains (some 10% by weight) and water, accumulated in permanently
waterlogged and acidic conditions. Peatlands are, more than any other ecosystem, the result of a fine
balance between hydrology, ecology and landscape morphology (Page and others 1999). A change in
one of these three system components will inevitably lead to a change in the other components and in
peat accumulation rate. It follows that human intervention will have a major impact on the peatland
hydrological system, and that water management must be carefully adapted to minimize this impact
(Hooijer 2005a; Wösten and others 2006a).
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Peatlands cover 27.1 Million hectares in Southeast Asia according to the data used in this study
(Wetlands International 2003, 2004; FAO 2004). Over 22.5 Million hectares (83%) of this are in
Indonesia, where peatlands make up 12% of the land area, with a further 2 Million hectares in Malaysia
and 2.6 Million hectares in Papua New Guinea. Peat thicknesses range from less than 1 to up to 20
metres (Page and others 2002); a substantial fraction of peatlands is over 4 metres thick (at least 17% in
Indonesia). These numbers yield a total carbon store in Southeast Asian peatlands of at least 42,000
Million tonnes (assuming a carbon content of 60 kg m-3).
Peatlands in Southeast Asia are now rapidly being deforested, drained and burnt for development of
agriculture (including oil palm and timber plantations) and for logging. These developments cause
major changes in the peatland hydrology, and as a result the carbon stored is now being released to the
Earth’s atmosphere through two mechanisms:
• Drainage of peatlands leads to aeration of the peat soil and hence to aerobic decomposition of peat
material that results in a sustained release of CO2 as illustrated in Figure 1.
• Fires in degraded and often drained peatlands result in further and abrupt release of CO2.
(FIGURE 1 NEAR HERE)
We analyzed present and ‘business as usual’ future CO2 emissions in Southeast Asia caused by
peatland drainage, not including the effects of peatland fires. Although the link between peatland
development and CO2 emissions is well-known from a vast body of work in Northern Hemisphere
peatlands, and research has been published into CO2 emissions due to deforestation in tropical areas
(Santilli and others 2005) and from fires in Southeast Asian peatlands specifically (Page and others
2002), little is known on the significance of sustained atmospheric C sources due to drainage in the
deep tropical peatlands.
From national and regional perspectives, policy makers are insufficiently aware of the global
implications of peatland drainage. This makes it difficult to establish fundamental connections between
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the agendas of development and climate change that would support actions consistent with long term
sustainable development (Gullison and others 2007).
In this paper we provide a comprehensive regional analysis of CO2 emissions from drained peatlands in
Southeast Asia pertaining to lowland peatlands in Indonesia, Malaysia, Papua New Guinea and Brunei.
The goal of the study is twofold: to improve understanding of the global spatial attribution of sources
of atmospheric carbon by providing estimates of carbon emissions from Southeast Asia’s peatlands,
and to provide information to national and regional policy makers and local peatland managers working
towards a better integration of the development and climate change agendas.
DATA AND CALCULATION METHODS
In order to estimate current and future CO2 emissions from drained peatlands, the following
information was obtained: (A) where and how thick the peatlands are, (B) where and how they are
drained, (C) what further drainage developments can be expected, (D) how much CO2 emission is
caused by drainage to a certain depth, and (E) how much peat carbon is available for oxidation i.e. how
long emission will continue before the peat carbon stock is depleted. The required information is
addressed in the following steps (for further details on methodologies and data sources see Hooijer and
others 2006).
(A) Peatland distribution and thickness. A peatland distribution map (Figure 2) was derived from
data provided by Wetlands International for the Indonesian islands of Sumatra and Kalimantan
(Wetlands International 2003, 2004). For the remaining areas, the Digital Soil Map of the World from
the Food and Agriculture Organization (2004) was used to determine peat percentage in soil classes.
Peat thickness data for Sumatra, Kalimantan and Papua (Indonesia) were obtained from Wetlands
International. Average peat thicknesses for Malaysia, Brunei and Papua New Guinea were
conservatively estimated on the basis of thicknesses in Indonesia (Figure 3). For the purpose of this
study, we excluded smaller peatland areas found in other Southeast Asian countries which are less
studied and represent only a small fraction of the total area and carbon volume. Peatlands over 300m
above sea level were also excluded for the same reasons.
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(FIGURE 2, 3 NEAR HERE)
(B) Distribution of drained peatlands in the year 2000. A drained area map for the year 2000 was
derived from a global land cover map, GLC 2000 (Bartholomé and Belward 2005). This dataset has a
1km resolution and is based on a classification of SPOT VEGETATION satellite images for the year
2000. The 16 land cover categories of GLC 2000 were divided into four drainage classes: ‘certainly
drained if peatland’ (cropland), ‘probably drained’ (mosaics of cropland and other land uses), ‘possibly
drained’ (shrubland and burnt areas) and ‘probably not drained’ (natural vegetation). Cells were
accordingly assigned to drained area classes (by fraction of area) as shown in Table 2. Drainage depths
for different land uses were estimated from field measurements (Table 2).
Areas of peatland within each drainage class were determined by the following geographic analysis
units using the Arc-GIS package: Provinces (in Indonesia), States and Countries (outside Indonesia).
The results were then organized by these geographic units and further calculations were performed
using this information (Table 1 provides a summary),
(TABLE 1, 2 NEAR HERE)
(C) Historical and future trends in peatland drainage. Historical trends of land use, and therefore of
drained area, were derived from changes in forest area between 1985 (FWI/GFW 2002) and 2000
(GLC 2000 data), as shown in Table 1. Overall deforestation rate in peatlands over this period was
found to be 1.3%/yr. An independent analysis of forest cover change between 2000 and 2006 for most
of Southeast Asia (reported in Hooijer and others 2006) shows that deforestation in peatlands continues
at an unchanged rate since 2000. In a ‘business as usual’ scenario, estimates of drained peatland area in
2006 and in future decades are therefore based on unchanged projection of trends over 1985-2000, by
Province (in Indonesia), State and Country (outside Indonesia). Rates of land use change within the
deforested area, as the deforested area increases, were determined using relations derived from relative
areas on peatland of ‘cropland’, ‘mosaic cropland + shrubland’ and ‘shrubland’ within deforested areas
in Indonesian Provinces in the year 2000 (Figure 4).
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(FIGURE 4 NEAR HERE)
(D) Relation between drainage depth and CO2 emissions. The relation between drainage depth and
CO2 emissions was based on findings by Wösten and Ritzema (2001), who propose that drainage in
agricultural peatlands in Malaysia results in a subsidence rate of 1 cm y-1
and CO2 emissions of 13
tonnes ha y-1
for every 10 cm of water table drawdown, on the basis of data on subsidence and soil
characteristics in drained peatlands. After comparison with findings of a number of gas flux studies in
drained peatlands in Southeast Asia (e.g., Ali and others 2006; Hadi and others 2006; Jauhiainen and
others 2005), it was estimated that every 10 cm water table drawdown results in 9.1 t CO2 ha y-1
(Hooijer and others 2006). This relation needs further development, as in reality it is non-linear and
dependent on land cover as well as drainage depth, but it is considered the best estimate now available
for drainage depths between 0.5 m and 1m, which is the most common drainage depth range in the
study region. It should be noted that CO2 emissions from drained peatlands, on a unit area basis and at
the same drainage depth, are far higher in the tropics than in temperate and boreal areas, because the
rate of aerobic decomposition is strongly influenced by temperature (Wösten and others 1997).
(E) Carbon content. Carbon content of Southeast Asian peat was assumed to be 60 kg m-3
(Wösten
and Ritzema 2001). This figure was applied to all areas.
RESULTS
Using the data and relations described above, the CO2 emission from all geographic units was
calculated as follows:
CO2 emission = LU_Area * D_Area * D_Depth * CO2_1m [t/y]
Where:
LU_Area = peatland area with specific land use [ha]
D_Area = drained area within peatland area with specific land use [fraction]
D_Depth = drainage depth in peatland area with specific land use [m]
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CO2_1m = CO2 emission at a drainage depth of 1m = 91 [t/ha/y]
It follows that a peatland area drained fully to 0.95 m on average (considered ‘most likely’ in
plantations and other large-scale ‘cropland’ areas; Table 2) will emit 86 t CO2 ha y-1
. A peatland area
drained to 0.6 m depth (typical in small-scale agricultural areas, i.e. ‘mosaic cropland and shrubland’)
for most of its area (88%, Table 2) will emit 48 t CO2 ha y-1
. An area drained to 0.33 m for half its area
(considered likely for ‘shrubland’, i.e., recently deforested areas, and burnt and degraded agricultural
areas; see Table 2) will emit 15 t CO2 ha y-1. Following this calculation method, ‘minimum’, ‘most
likely’ and ‘maximum’ emission rates for 3 land use types (‘cropland’, ‘mosaic cropland and
shrubland’, and ‘shrubland’) were calculated by varying drained area and drainage depth in each land
use type (Table 2). The overall range of emissions calculated was between 6 and 100 t CO2 ha y-1
.
Values in this range are also reported for several gas flux studies in experimental plots (Ali and others
2006; Furukawa and others 2006; Hadi and others 2006; Jauhiainen and others 2006).
Projections based on land cover data for 1985 and 2000 indicate that about 47% of peatlands in
Southeast Asia, or 12.9 Million hectares, were deforested by 2006. Projected rates of land use change
within deforested areas in Southeast Asia over the same period suggest that 17% of this land is now
affected by intensive drainage for large-scale agriculture (GLC 2000 class ‘cropland’), 67% is affected
by moderately intensive drainage for small-scale agriculture (‘mosaic cropland and shrubland’), and
16% is affected by unmanaged drainage in degraded non-agricultural areas (‘shrubland’). This results
in an estimated total drained peatland area of 11.1 Million hectares (range: 9.5 - 12.7).
By multiplying CO2 emissions per hectare of each land use type with the total area in that land use
class, we estimated that present CO2 emissions from drained peatlands are 632 Mt y-1
(range: 355 -
855). If current rates and practices of peatland development and degradation continue, CO2 emissions
will peak at 745 Mt y-1
in 2015, followed by a steady decline over many decades when increasingly
thicker peat deposits become depleted (Figure 5). In the maximum drainage scenario, projected
emissions peak at 936 Mt y-1
by 2010. After reaching a maximum, CO2 emissions will steadily decline
while ever deeper peat deposits are depleted, but they will be significant throughout this century. By
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2030, total projected emissions are 514 Mt y-1
if peatland drainage continues unmitigated; by 2070 they
would still be 236 Mt y-1
Cumulative CO2 emissions from all peatlands in Southeast Asia were calculated at 9,700 Mt (range:
5,300-13,700) by 2006, 25,900 Mt (range: 17,200-31,000) by 2030 and 37,300 Mt (range: 28,900-
39,900) by 2070. Indonesia with its vast peat resources is the single largest CO2 emitter from drained
peatlands, responsible for 82% of Southeast Asian emissions in 2006, with Sumatra being the largest
emitter closely followed by Kalimantan.
(FIGURE 5 NEAR HERE)
DISCUSSION
Current CO2 emissions from drained peatlands in Southeast Asia (excluding emissions from fires) are
estimated to be 632 Mt y-1 year (range: 355 – 855). This is equivalent to 2.4% of the 26.4 Billion metric
tonnes of CO2 y-1
of global emissions from fossil fuel combustion during the period 2000-2005 (IPCC
2007). In a ‘business as usual’ scenario that extends current drainage trends into the future, emissions
from drainage will further increase in coming decades before starting to decrease due to depletion of
shallower peat resources. Emissions from the deeper peatlands will continue throughout this century.
Further CO2 emissions from degraded and drained peatlands are associated with peatland fires, which
during non- ENSO (El Niño-Southern Oscillation) years can be of similar magnitude to those from
decomposition and several times larger during ENSO years. Page and others (2002) estimated that
emissions from peatland fires in Indonesia during the 1997 ENSO were 3000 Mt CO2 (range: 1800 –
8800), equivalent to 14% of global emissions from fossil fuel combustion. An assessment on the basis
of this figure and of annual ‘fire hotspot’ counts in Borneo over the period 1997-2006 estimated an
average emission of 1400 Mt CO2 y-1
from peatland fires in Southeast Asia (Hooijer and others 2006).
This brings the total CO2 emissions to about 2000 Mt y-1
or equivalent to 7.6% of current global CO2
emissions from the combustion of fossil fuel.
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One final component needs to be computed to understand the full impact of drainage and loss of forest
in peatlands on the net carbon balance of tropical swamp forests: the loss of carbon sink capacity.
Peatlands in their natural state store carbon at an average rate in the order of 84 g C m2 y-1 (Page and
others 2004; Rieley and others 1996). Loss of all peatland forest in Southeast Asia means a loss of CO2
uptake capacity of 84 Mt y-1
. The present loss of peatland forest in SE Asia has already reduced global
CO2 uptake capacity by at least 40 Mt y-1
.
The emissions from drained peatlands in Southeast Asia, even without including the effect of fires,
contribute more to atmospheric greenhouse gas emissions than industrialized nations like Germany or
the United Kingdom produce by the combustion of fossil fuels (804 and 558 Mt y-1
respectively, CO2
emission in 2003, CDIAC 2007). Emissions from drained peatlands are produced on what is effectively
only less than 0.1% of the global land area. Deforested and drained peatlands in Southeast Asia are
major Earth system hotspots for carbon emissions rivalled by few, if any, CO2 emission sources in
terms of emission per unit area.
Although deforestation and drainage are the current dominant factors driving carbon emissions from
peatlands, future climate change will add further pressure to peat ecosystems and most likely enhance
carbon emissions. An analysis of climate projections contributing to the IPCC Fourth Assessment show
that 7 out of 11 models predicted that by the end of this century there will be a decrease of rainfall
during the dry seasons in a number of regions of Southeast Asia (Li and others 2007). Moreover, 9 out
of 11 models predict greater interannual variation in dry season rainfall. These changes are strongest
and most consistent across the models for southern Sumatra and Borneo, where most peatland in
Indonesia occurs. Decreased rainfall during the dry season will result in lower water tables exposing
larger carbon stocks to suitable aerobic conditions for decomposition, and hence larger CO2 emissions.
A further potential effect of climate change on CO2 emissions from peatlands is supported by field
studies in southern Sumatra which shows strong positive influence of increased soil temperature on
CO2 emission (Ali and others 2006).
The carbon consequences of recent El Niño years are a good window into the future to understand the
likely positive feedbacks of climate change (lower rainfall and higher air temperatures) and continued
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land use change (deforestation and drainage). Particularly vulnerable are degraded areas where the
natural self-regulating hydrological systems have been lost and no artificial water level control system
has been implemented.
Sustainability of peatland drainage and environmental consequences
Drainage of peatlands in Southeast Asia often does not bring sustainable agricultural or economic
development. Most peatlands are located at or close to the Sea coast, with surface elevations only a few
metres above Sea level and the mineral subsoil often below the gravity drainage base, so continued
subsidence of the drained peat surface (caused by compaction, aerobic decomposition and fires) will
ultimately cause the peat area to become undrainable, often within decades of the initial excavation of
canals (Hooijer 2005b). Inundation frequency will increase and areas below the high-water level will
be affected by salt water intrusion. As the cost involved in non-gravity drainage (i.e. water pumping) is
usually too high to be economical in this region, agricultural productivity will decline. Another cause
of reduced productivity and sometimes abandonment of coastal drained peatland areas is when the
mineral subsoil is aerated and acid sulphate problems develop, as are common along many coastlines in
Southeast Asia.
Furthermore, an array of environmental and socio-economic impacts occurs as a result of deforestation
and drainage. Peat fires cause haze (smoke) problems which affect public health and economy (e.g.
transport, tourism) in much of the Southeast Asian region. Loss and degradation of conservation forest
affects biodiversity and natural timber production in the longer term. Flooding problems are reported,
but not quantified, downstream of drained and burnt peatland areas such as the former Mega Rice
Project in Indonesia (Wösten and others 2006b).
Perspectives for sustainable peatland management
Given the high CO2 emissions, per unit area and in absolute terms, and the relatively small area
involved from a global perspective, it seems that emissions from peatland deforestation and
mismanagement in Southeast Asia are both important enough to require international action and
‘compact’ enough for mitigation policies to be effective. Carbon emissions and other negative effects
resulting from unsustainable peatland management can be reduced, particularly in Indonesia, if
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international support is provided to adopt and implement a land development policy based on the
following three principles:
1. Forest conservation and drainage avoidance in remaining peat swamp forests.
2. Where possible, rehabilitation of degraded peatlands through restoration of natural hydrological
systems and of forest cover.
3. Raised water levels through improved water management in existing plantations in peatlands,
embedded in water management master plans for peatland areas.
Improved water management is the basis of conservation of peat resources in Southeast Asia. Current
management priority in most agricultural areas in peatlands is to prevent high water levels in the wet
season, and in many cases to maintain boat access along canals during the dry season. However the
excessive drainage capacity maintained in such a system, without further mitigative measures, also
leads to a drop in water table depth below the peat surface by more than 1 or often even more than 2
metres during the dry season in many drained peatland areas. Such low water levels can often be
prevented through improved water management which stabilizes water levels by adjusting drainage
capacity to meet seasonal requirements (Wösten and Ritzema 2001, Hooijer 2005b). However such
management is more costly and requires specific technical knowledge that is often not available
locally.
One important issue that was not addressed in the analysis, but which should be taken into account in
peatland water management, is the fact that drainage affects peatlands over large distances, well
beyond the 1km cell resolution used in the analysis. This means that a single road or small plantation in
peatland will cause peat surface subsidence, CO2 emission and enhanced fire risk over much larger
areas. It also implies that exclusion from development of peat deposits over 3 metres thick as is now
the law in Indonesia (although rarely enforced), is of limited practical relevance as in time most peat
deposits will be less than 3 metres thick through progressive subsidence following drainage, and would
become available for development. It seems more practical and realistic to recognize that peat ‘domes’
are self-contained hydrological units that may be kilometres or tens of kilometres accross; interventions
in part of the unit will in the long term affect the entire unit. Single peat domes are therefore best
managed under a single water management regime or at least a water management regime that aims to
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maintain the hydrological integrity of most of the unit. A decision may be made to develop a peat dome
for agriculture/silviculture, or to conserve it as an undrained forest area, but combining development
and conservation can often not be sustainable unless major and costly water management measures are
taken and maintained in buffer zones that are a minimum width of several kilometres.
Finally, an issue which deserves further attention is the fact that development of oil palm (and pulp
wood, for paper production) plantations is a major cause of peatland deforestation and drainage in
Southeast Asia. The global demand for palm oil is growing fast, and according to research by the
Indonesian Ministry of Forestry and the European Union (Sargeant 2001) much of the 300,000 ha
additional oil palm plantation development required annually will take place in Indonesia’s peatlands.
Some 25% of palm oil is already produced on peatlands and this percentage is expected to rise (Hooijer
and others 2006). A major driver of the increasing demand for palm oil is its use as a biofuel in the
European and other markets, with the aim of reducing global CO2 emissions to meet targets under the
Kyoto Protocol. We suggest that the CO2 emission from drained peatlands should be taken into account
when considering use of palm oil as a biofuel.
Future research for reduced uncertainties
In this section we discuss the main uncertainties of this analysis and identify knowledge gaps with the
aim to guide future research priorities.
• The thickness of many peatlands in Indonesia is not very well known. Peat thicknesses tend to be
greatest in the central parts of the inaccessible and often vast dome-shaped peat bodies, often tens
of kilometres across. Many of the measurements on peat thicknesses are nearer to the fringes. For
this study no data was available on thickness of peatlands in Malaysia and Papua New Guinea;
conservative estimates were used. Overall, this is likely to result in underestimation of peat
thickness and the size of the carbon stock. With the data used, the original carbon stock in
Southeast Asia is calculated at 42,000 Mt; this is at the low end of published estimates.
• Data on extent and distribution of peatlands can be improved, especially for areas outside of
Kalimantan and Sumatra.
• Carbon content of Southeast Asian peat was assumed to be 60 kg m-3
(Wösten and Ritzema,
2001), while carbon contents up to 90 kg C m-3 have been reported for various peat deposits (e.g.
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Wetlands International 2003, 2004). Assuming higher carbon content would result in higher total
carbon stocks and higher CO2 emissions from peatland drainage.
• Drainage intensity as derived from the GLC 2000 global land cover classification may not always
be accurate, for example in the case of Papua (Indonesia). Here, areas now classified as ‘mosaic
cropland + shrubland’ are known to actually be a savannah-like landscape created by traditional
land management techniques requiring regular burning (Silvius and Taufik 1990). These areas are
generally not ‘drained’ in the normal sense; agriculture often takes place on elevated islands of dug
up mud (from the submerged swamp soil). This may have lead to an overestimate of CO2
emissions from Southeast Asian peatlands, with a maximum of 16% in the unlikely case that
emissions from Papua would actually be negligible.
• The percentage of peatland drained within land use classes was estimated from field work which
naturally did not cover all peatland regions. The percentage of drained peatland may be much
larger than assessed here, as several interventions in the hydrological system are not taken into
account. These are A) log transport canals in forested areas where legal or illegal logging takes
place, and B) the impacts of plantation and roadside drainage over distances of kilometres in
forested areas. Peatland fires also have a draining effect on the remaining forested peatland as they
create depressions in the peat surface. Over 90% of peat swamp forests in Sumatra was already
affected by human interventions in the early 1990s (Silvius and Giesen 1992), so probably only
few peatlands in Sumatra, Kalimantan and Malaysia are not affected by drainage at present.
• Estimated likely drainage depths may be greater than those recommended in existing
management guidelines, but are shallower than depths often observed by the authors in practice.
Drainage depths between 1 and 2 metres are often observed in oil-palm and pulp wood plantations.
In unsuccessful and abandoned plantations such as the ex-Mega Rice Project in Central
Kalimantan (with over 1 million hectares of derelict drained peatland), drainage depths of up to 3
metres are reportedly common in the dry season. The ‘likely’ average drainage depth of 0.95m, as
assumed for plantations and other cropland areas in this assessment, is considered an
underestimate.
• Values for current (2006) land use were based on GLC 2000 data from 2000, as this is the most
up-to-date published and validated land use dataset available for all of Southeast Asia. More recent
land use data would have yielded more accurate results.
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• The ‘business as usual’ land use projection used is a continuation of past trends, so assuming
there will not be major changes on peatland conservation and management strategies. The
projection method for land uses in deforested peatland area limits the area of intensively drained
‘cropland’, including plantations, to 21% of the total peatland area. In reality, the registered area of
palm oil and timber plantation concessions alone, which require very intensive drainage, is already
23% of the peatland area in Indonesia (Hooijer and others 2006). In addition, many plantations and
other agricultural projects are developed outside this concession system. There are expectations
that most of the annual global increase in oil palm plantations, 300,000 ha y-1
for the next 15 or 20
years, will be developed in Indonesia’s peatlands (Sargeant 2001). It is therefore conceivable that
over 50% of peatlands will be intensively drained in a few decades; this would result in a CO2
emission of at least 20% higher than calculated.
• Projections have not taken into account peatland drainability and future management responses.
If peatlands become less productive because of increased flooding due to subsidence, they may be
abandoned and drainage and CO2 emissions may theoretically be reduced. Part of the carbon stock
in peatlands is below the drainage base and may never be oxidized. On the other hand, the current
experience is that abandoned peatlands continue to be drained (as drain blocking is labour
intensive and reduces access), and frequently experience fires in dry years when drainage is not an
issue. The Ex Mega Rice Project area is an example of this.
• Carbon dioxide emission resulting from a specific drainage depth is a very sensitive parameter
in the calculations. Few long-term studies of subsidence rates in drained peatlands in Southeast
Asia have been published. Short-term studies of CO2 emissions are difficult to interpret because A)
CO2 emissions from root respiration must be separated from emissions caused by decomposition,
B) short-term effects (shortly after drainage) must be separated from long-term effects, C) water
table and soil moisture regime are often insufficiently quantified, and D) an unknown part of the
carbon will not be emitted to the atmosphere but will leave the drained peatland in dissolved form
with the drainage water to end up in the Sea (where much of it will likely be oxidized and add to
atmospheric CO2 concentration after all).
• The only form of carbon emission to the atmosphere considered in this assessment is CO2
emission. Methane (CH4) emissions from both undrained and drained peatlands are found to be
modest in comparison with CO2 (Jauhiainen and others 2005; Takashi and others, this volume), but
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may still be significant from a climate perspective given that CH4 is a much stronger greenhouse
gas (23 times stronger in ‘CO2 equivalents’). CH4 emissions in drained peatlands may originate
especially where peat areas are increasingly flooded for prolonged periods, after fires or after
subsidence due to drainage, and reduced conditions are created in the inundated peat soil, but this
process has not been investigated to date.
• Several studies confirm that peatland fires occur mainly in deforested and degraded areas (Siegert
and others 2001; Page and others 2002; Spessa and others, this volume) and are linked to water
table drawdown through drainage. Although this relation has not yet been quantified, CO2
emission from Southeast Asia due to peatland fires is highly uncertain as shown in the discussion
section but can be as much as double the amount of emissions from peat decomposition. Emissions
from peatland fires are not included in the assessment presented here, which results in a significant
uncertainty in CO2 emissions related to peatland drainage.
It is concluded that most remaining uncertainties in CO2 emissions from drained peatlands in Southeast
Asia are greater to the upside than to the downside. This implies that CO2 emissions caused by drainage
in Southeast Asian peatlands are more likely to be above the middle estimate (most likely value, fire
emissions excluded) of 632 Mt y-1
than to be below this value.
It should be noted that most uncertainties pertain to the annual estimates of CO2 emission rather than
the total CO2 emission over the next 50 or 100 years. That is because in the ‘business as usual’ scenario
a lower current emission rate simply means that it will take a few decades longer for the carbon stored
in shallower peat layers to be depleted, but eventually it will still be emitted to the atmosphere.
Further work is required to reduce all the uncertainties listed above. In the short term the greatest
reduction in the overall uncertainty in CO2 emissions from drained peatlands in Southeast Asia may be
achieved through more up-to-date and accurate land use maps. In the longer term the greatest reduction
may be achieved through comprehensive field studies of CO2 emissions (and other carbon balance
components) and subsidence rates in relation to hydrological (especially water level and soil moisture)
and biological (especially vegetation cover) parameters under different water and land management
practices.
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ACKNOWLEDGEMENTS
The work reported here would not have been possible without the data and other inputs provided by a
great number of people and organizations, especially: Rinus Vis, Marcel Ververs, Rolf van Buren,
Marjolijn Haasnoot (Delft Hydraulics), David Hilbert (CSIRO), Faizal Parish (Global Environment
Centre), Fred Stolle (Global Forest Watch), Florian Siegert (Remote Sensing Solutions), Niels
Wielaard (SarVision), Jyrki Jauhiainen (University of Helsinki). This assessment is a contribution to a
synthesis effort on the vulnerabilities of tropical peatlands carried out under the auspices of the Global
Carbon Project. The original assessment was funded from internal Delft Hydraulics R&D sources,
within the PEAT-CO2 (Peatland CO2 Emission Assessment Tool) research programme.
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Tables
Table 1 Lowland peatland distribution land use and rate of forest cover loss.
Shrubland + burnt Mosaic: crop+shrub Cropland Forest cover 2000 Forest change
GLC 2000 class: 6 8 total 2 9 total 12 1 4 5 total 1985 85-'00
Lo
wla
nd
pea
tlan
d a
rea
(km
2)
Mo
saic
s &
Sh
rub
Co
ver,
sh
rub
co
mp
on
ent
do
min
an
t, m
ain
ly e
verg
reen
Sh
rub
co
ver,
ma
inly
dec
idu
ou
s, (
Dry
or
bu
rnt)
To
tal
shru
bla
nd
+ b
urn
t
Mo
saic
: T
ree
cove
r a
nd C
rop
lan
d (
incl
.ver
y
deg
rad
ed a
nd
op
en t
ree
cove
r)
Mo
saic
s o
f C
ropla
nd
/ O
ther
na
tura
l ve
g.
(shif
tin
g c
ult
iva
tio
n i
n m
ou
nta
ins)
Tota
l m
ix c
rop
lan
d +
sh
rub
(sm
all
-sca
le
agr.
)
Cu
ltiv
ate
d a
nd
ma
na
ged
, n
on
irr
iga
ted
(mix
ed)
Tre
e co
ver,
bro
ad
lea
ved
, ev
erg
reen
, cl
ose
d
and
clo
sed t
o o
pen
Tre
e co
ver,
reg
ula
rly
flo
od
ed,
Ma
ng
rove
Tre
e co
ver,
reg
ula
rly
flo
oded
, S
wa
mp
To
tal
fore
st (
incl
ud
ing
lo
gg
ed) Global
Forest
Watch /
World
Res. Inst.
Annual
change
over the
period
1985 -
2000
% area % area %area % area % area %area % area % area % area % area % area % area %/y
Total Indonesia 225234 4 2 7 3 24 27 5 27 4 30 61 81 -1.3
Kalimantan 58379 15 4 20 2 17 19 3 30 2 27 58 87 -1.9
Central Kalimantan 30951 19 2 22 2 15 18 3 33 1 24 57 90 -2.2
East Kalimantan 6655 22 19 42 0 9 9 5 29 4 11 44 85 -2.8
West Kalimantan 17569 5 1 7 2 17 19 1 28 3 43 74 92 -1.2
South Kalimantan 3204 15 3 18 6 45 51 14 14 0 4 18 41 -1.6
Sumatra 69317 0 1 1 3 34 37 10 14 2 35 52 78 -1.8
D.I. Aceh 2613 0 0 0 4 28 32 8 37 0 22 59 87 -1.8
North Sumatera 3467 0 2 2 3 39 42 20 20 1 16 36 76 -2.6
Riau 38365 0 1 1 2 24 26 7 14 3 49 66 87 -1.4
Jambi 7076 0 1 1 3 38 40 17 9 0 33 42 67 -1.7
South Sumatera 14015 0 1 2 4 57 61 12 11 1 14 26 66 -2.6
West Sumatera 2096 0 5 5 4 42 46 11 24 0 13 38 69 -2.1
Papua 75543 0 1 2 4 20 25 1 36 9 27 72 80 -0.5
Other Indonesia~ 21995 4 2 7 3 24 27 5 27 4 30 61 81 -1.3
Malaysia 20431 2 1 1 7 32 38 7 36 4 15 53 78* -1.8*
Peninsular 5990 0 1 1 4 47 50 13 37 0 0 37 78* -2.8*
Sabah 1718 8 2 10 3 28 31 17 21 21 2 43 86* -2.9*
Sarawak 12723 2 1 2 9 26 35 4 38 3 23 59 76* -1.1*
Brunei 646 3 1 4 1 9 10 2 39 6 39 84 85* -0.2*
Papua N. Guinea 25680 0 1 1 4 32 35 3 38 5 19 61 80* -1.3*
SE ASIA 271991 4 2 5 4 26 29 5 29 4 28 61 81* -1.3*
~ Land use distribution for 'Other Indonesia' assumed equal to Total Indonesia.
* 1985 forest cover outside Indonesia is estimated.
Table 2 CO2 emission calculation steps and main parameters.
minimum likely maximum
Step A: Drained area Large croplands, including plantations % 100 100 100
(within land use class) Mixed cropland / shrubland: small-scale agriculture % 75 88 100
Shrubland; recently cleared & burnt areas % 25 50 75
Step B: Drainage depth Large croplands, including plantations m 0.80 0.95 1.10
(within land use class) Mixed cropland / shrubland; small-scale agriculture m 0.40 0.60 0.80
Shrubland; recently cleared & burnt areas m 0.25 0.33 0.40
Step C: A relation of 0.91 t/ha/y CO2 emission per cm drainage depth in peatland was used in calculations.
Step D: CO2 emissions Large croplands, including plantations t/ha/y 73 86 100
(calculated from A, B, C) Mixed cropland / shrubland: small-scale agriculture t/ha/y 27 48 73
Shrubland; recently cleared & burnt areas t/ha/y 6 15 27
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Figures
Figure 1 Schematic illustration of CO2 emission from drained peatlands.
Figure 2 Forest cover on peatland in the year 2000. Note that FAO non-histosol soil classes with 20-40% peat are
not shown, hence the peat extent is greater than shown – e.g. Papua New Guinea has significant peatland cover.
PEAT-CO2 / Delft Hydraulics
Sum
atra
Kalimantan
Sarawak
PapuaIndonesia
Malaysia
Papua New Guinea
Tree cover on peatlandsforest on peatland
deforested peatland
Sources: Peat extent: Wetlands International, FAO Forest extent: GLC 2000
0 500 1000 2000
Kilometres
Clay / sand substrate
Peat dome Natural situation:• Water table close to surface• Peat accumulation from vegetation over thousands of years
Str
eam
ch
an
nel
5 to 50 km
1 to 10 m
Drainage:• Water tables lowered• Peat surface subsidence and CO2
emission starts
Continued drainage:• Decomposition of dry peat: CO2 emission• High fire risk in dry peat:
CO2 emission• Peat surface subsidence due to decomposition and shrinkage
End stage:• Most peat carbon above drainage limit released to the atmosphere,• unless conservation / mitigation measures are taken
PEAT-CO2 / Delft Hydraulics
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0
2000
4000
6000
8000
0.75 1.5 3 6 10
Peat depth class (m)
Ca
rbo
n s
tore
(M
t)
KalimantanSumatraPapuaMalaysia+BruneiPapua NG
Figure 3 Carbon stored in peatlands, by region and by thickness (source: Wetlands International). Average peat
thickness in Malaysia and Papua N.G. was estimated from peat thickness in Kalimantan and Papua (Indonesia)
respectively.
0
50000
100000
150000
200000
250000
1980 2000 2020 2040 2060 2080 2100
La
nd
use o
n p
ea
tlan
d (k
m2)
Total deforested (lowland) peatland
Large cropland areas (inc. plantations)Mixed cropland and shrubland areas
Recently cleared and burnt areas
Figure 4 Trends and projections of land use change in lowland peatland in SE Asia.
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Figure 5 Historical, current and projected CO2 emissions from peatlands, as a result of drainage (fires
excluded). The increase in emissions is caused by progressive drainage of an increased peatland area.
The following decrease is caused by peat deposits being depleted, starting with the shallowest peat
deposits that represent the largest peatland area (see Figure 3). The stepwise pattern of this decrease
is explained by the discrete peat thickness data available (0.75m, 1.5m, 3m, 6m, 10m).
0
100
200
300
400
500
600
700
800
900
1980 2000 2020 2040 2060 2080 2100
CO
2 e
mis
sio
n (M
t/y
)
Minimum due to peat decomposition
Likely due to peat decomposition
Maximum due to peat decompositionpresent