Half of land use carbon emissions in Southeast Asia can be mitigated through peat swamp forest and mangrove conservation and restoration

Domain: earth_science
Published: 2025-01-28
Editorial outcome: major_revision

Abstract
Southeast Asia (SEA) contributes approximately one-third of global land-use change carbon emissions, a substantial yet highly uncertain part of which is from anthropogenically-modified peat swamp forests (PSFs) and mangroves. Here, we report that between 2001–2022 land-use change impacting PSFs and mangroves in SEA generate approximately 691.8 ± 97.2 teragrams of CO2 equivalent emissions annually (TgCO2eyr−1) or 48% of region’s land-use change emissions, and carbon removal through secondary regrowth of −16.3 ± 2.0 TgCO2eyr−1. Indonesia (73%), Malaysia (14%), Myanmar (7%), and Vietnam (2%) combined accounted for over 90% of regional emissions from these sources. Consequently, great potential exists for emissions reduction through PSFs and mangroves conservation. Moreover, restoring degraded PSFs and mangroves could provide an additional annual mitigation potential of 94.4 ± 7.4 TgCO2eyr−1. Although peatlands and mangroves occupy only 5.4% of SEA land area, restoring and protecting these carbon-dense ecosystems can contribute substantially to climate change mitigation, while maintaining valuable ecosystem services, livelihoods and biodiversity.

Introduction
Peatlands and mangroves collectively act as natural carbon sinks owing to atmospheric carbon uptake by photosynthesizing vegetation exceeding ecosystem respiration, with the excess accumulating under anoxic conditions as partially decomposed organic matter. Globally, peatlands store ~604 ± 130 gigatonnes of carbon (GtC)1–3, equivalent to 31% of the terrestrial carbon pool4, despite only accounting for 3% of the terrestrial surface area3. Tropical peatlands cover at least 44 million hectares (Mha) and store an estimated 105GtC5. These peatlands, found primarily at lower elevations but occasionally at higher ones, often support forest vegetation known as peat swamp forest (PSF).

Mangroves, a form of coastal forested wetland found in tropical, subtropical, and warm temperate regions, extend across 14.7Mha6, and may store between 3.1 and 12.3 GtC7 although there is considerable uncertainty associated with this estimate. When combined, tropical peatlands and mangroves store around 25% of global tropical forest carbon stocks8, while occupying <3% of land at low latitudes. However, large areas of PSFs and mangroves have been converted to alternative land-uses9,10, and are now functioning as net sources rather than sinks of greenhouse gases (GHGs)11–13. Emissions from peatlands and mangroves are largely from the oxidation of irrecoverable soil organic carbon—ancient (>100 years) carbon that cannot be replaced on timescales relevant to climate change mitigation strategies14. Reducing emissions through conservation and restoration of peatlands and mangroves can thus provide a cost-effective contribution to the mitigation of anthropogenic GHG emissions, thereby assisting countries with these important wetland ecosystems to meet their Nationally Determined Contributions (NDCs) as part of the Paris Agreement15.

The eleven Southeast Asian countries (Brunei, Cambodia, Timor Leste, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand, and Vietnam) cover just 4–5% of the world’s land area, but host 39% and 33% of the global extent of, respectively, tropical peatlands and mangroves6,16. PSF has been commonly drained and converted into industrial tree and agricultural plantations13 with the forest area further reduced by fire17. Aquaculture ponds and agriculture have largely replaced mangroves18, with remnant areas degraded through timber extraction and other land uses. Habitat losses from deforestation, land-use change, drainage, and biomass burning are responsible for a rapid depletion of carbon stocks and substantial emissions of GHGs including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)11,13,19–21. Previous assessments of CO2 emissions following deforestation and land conversion were conducted at both global and local levels for peatland13,20,22–24 and mangroves11,21. However, these assessments often lacked detailed information on interannual variations in emissions. This gap limits our understanding of the actionable mitigation potential through the conservation and restoration of these ecosystems. In addition, the climate change mitigation effects of restoring these ecosystems when in a degraded state or converted to other land use are still not fully understood, despite rising interest in developing restoration for carbon credit projects in these ecosystems25. Understanding the climate change mitigation potential of PSFs and mangroves conservation and restoration in Southeast Asia has grown in importance as nations increasingly include these carbon-rich ecosystems in plans to achieve emissions reduction targets outlined in their NDCs26–29.

Here, we present estimates of region-wide and national-level carbon dioxide equivalent (CO2e) emissions (combined CO2 and CH4) associated with deforestation—defined as tree cover loss, fires, and land-use change affecting PSFs and mangroves, along with potential climate change mitigation effects (avoided emissions and carbon removal) from conservation and restoration of these important ecosystems in Southeast Asia for the past two decades (2001–2022). Annual rates of deforestation of PSFs and mangroves were determined for the period of interest, along with changes in the extent of replacement land covers (Fig. 1). Former PSFs that had been converted to other land uses prior to 2001 were also identified and the associated annual CO2e emissions from peat decomposition were estimated. Unlike earlier assessments (refs. 11,13,20–22), we provide a detailed breakdown of estimated emissions by source, including biomass carbon loss from deforestation, peat decomposition, peat drainage and burning, and mangrove soil excavation or drainage. Additionally, we assess the mitigation potential of revegetation and rewetting in converted and drained PSFs and mangroves. Estimates of the mitigation potential of conserving and restoring PSFs and mangroves and their relative comparison with annual average of national and regional land-use emissions across Southeast Asia countries are provided. These estimates are timely as they come ahead of the next round of submissions of the NDCs in 2025 and the Global Stocktake in 2028 to The United Nations Framework Convention on Climate Change (UNFCCC), in which a substantial proportion of promised reduced emissions are intended to come from the land-use change sector (Figs. S1 and S2).

Results
Trends of land-use change following deforestation
From 2001 to 2022, ~41% (239,723hayear−1) of PSFs and 7.4% (16,635hayear−1) of mangroves in Southeast Asia were lost relative to their extents in 2000 (Figs. 2, S3, and S4). By 2017, the annual rate of loss of PSFs had declined to 108,458hayear−1, while that of mangroves remained high at 18,400hayear−1, exceeding the 2001–2022 average (Figs. S3 and S4). The reduction in the rate of loss of PSF was associated with a decrease in the annual area affected by biomass burning (185,834hayear−1) compared to previous years (375,117hayear−1 on average from 2001 to 2015) (Fig. S5). The extent of peatland burn each year between 2001 and 2022 was highly variable (Fig. S5), ranging from 37,263 ha in 2017 to 853,474ha in 2006 (annual average 314,891 ± 263,938hayear−1). This variability is best explained by sub-decadal drivers of rainfall variations in the region, notably the climatically dry phases of the El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD)30, which predisposed drained peatlands in Southeast Asia to burning.

Estimated loss of PSFs by 2022 for the whole Southeast Asia region is ~12.1 Mha or 1 Mha larger than reported by Miettinen et al.10, who estimated that 11.1 Mha of PSFs in Peninsular Malaysia, Sumatra, and Borneo had been lost by 2015. To standardize the timeline for fair comparison with this earlier assessment, we estimated that ~11.2 Mha of PSF had been lost by 2015, highlighting the extent of expansion of new land uses on PSF continued after 2015, albeit at a lower rate (0.13 Mhayear−1) (Fig. S3). Conversion to tree plantations was among most common land-use change impacting PSFs in the region from 2001 to 2022 after other land use and degraded PSF classes, resulting in a loss of almost 2.27 Mha. By comparison, oil palm plantations replaced around 1.36 Mha of PSFs over the same period (Fig. S3). Tree and oil palm plantations and their associated drainage canals (0.30 Mha) accounted for ~74% (3.93 Mha or 0.17 Mhayear−1) of the total deforested PSF area between 2001 and 2022, with Indonesia and Malaysia the most severely impacted. Around 2.01 and 0.86 Mha of tree and oil palm plantations, respectively, replaced PSFs in Indonesia, compared with 0.25 and 0.49 Mha in Malaysia.

Losses of mangroves in Southeast Asia between 2001 and 2022 were ~0.36 Mha (7.4%) (Fig. S4) and were mainly driven by economic demand for commodities associated with aquaculture and wood products, accounting for 62% of total loss areas between 2001 and 2022. During the analysis period, ~0.064 Mha of mangroves were converted into aquaculture ponds in Southeast Asia. Further, around 33% of mangroves cleared between 2001 and 2022 had recovered their original vegetation cover by 2022 (Fig. S4).

Carbon emissions by sources
Deforestation and land-use change impacting PSFs and mangroves in Southeast Asia generated an estimated 691.8 ± 97.2 teragram (Tg) of CO2e (CO2 and CH4) emissions annually between 2001 and 2022 (Figs. 2 and 3), with ~98% (684.3 ± 97.4 TgCO2eyear−1) from PSFs and 2% from mangroves (13.53 ± 2.34 TgCO2eyear−1). Emissions from PSF comprise those from peat decomposition (ca 46%), biomass loss following deforestation (23%), peat burning (20%), and peat loss and on-site emissions following construction of artificial peat drainage canals (9%) (Fig. 2). Our estimated average annual regional emissions are higher compared to refs. 24,13,20,31, whose estimate range from 208 to 560 TgCO2eyear−1, although these estimates only considered Sumatra, Peninsular Malaysia, and Borneo and excluded peat fires (Table S1). However, when uncertainty is incorporated, our estimates are within the range of those recently provided by the UNEP Global Peatland Assessment23. Further, our estimates are 100% greater than those provided in ref. 19 for PSFs and mangroves in Southeast Asia over the same period (2001–2022). These differences are likely due to a lack of consideration in ref. 19 of emissions from fires, the construction of aquaculture ponds and drainage canal network, and aquatic sources. However, similar trends in national emissions are evident (Table S1), except for the case of Myanmar where determination of the extent and nature of peatland disturbance has been difficult (see ‘Methods’ for further explanation of data limitation).

Relatively high (above average) annual CO2e emissions were mostly associated with periods of prolonged drought when the dry phases of ENSO and IOD coincided (e.g., 2006, 2009, 2015, and 2019). In contrast, low annual CO2e emissions were observed during climatically relatively wet years (e.g., 2007–2008, 2010, 2020–2022), which may provide biophysical support for rewetting of drained peatlands and mangroves (Figs. 3 and S6). This pattern suggests that interdecadal climate variability has an important influence on annual emissions in the region, particularly those linked to major peat-burning events32. Major biomass fires and their associated emissions were not restricted to periods of ENSO-related, prolonged droughts, however, e.g., during the 2013–2014 fire season (Fig. 3)33. Furthermore, annual CO2e emissions from peat decomposition and drainage increased steadily between 2001 and 2022 (Fig. 3) with expansions in the extent of converted PSFs and the associated surface area of drainage canals (Figs. S3 and S7). By contrast, annual CO2e emissions from biomass loss linked to deforestation decreased over the same period following reduced rates of PSF deforestation, particularly after 2019 (Fig. 3).

Indonesia was the main contributor to regional CO2e emissions from deforestation and conversion of PSFs and mangroves (73% or 504.7 ± 95.4 TgCO2eyear−1), followed by Malaysia (14% or 96.4 ± 8.5 TgCO2eyear−1) (Figs. 2 and 3), with the two countries accounting for 87% and 72% of the total areas of PSFs and mangroves in Southeast Asia, respectively. Our estimated annual emissions for Indonesia are substantially higher than reported in the country’s Second Forest Reference Level (FRL)34 (209 TgCO2eyear−1), possibly because of: (1) differences in the period of assessment (Indonesia’s FRL reported emissions over a shorter period between 2006/2007 and 2019/2020 compared to our analysis period between 2001 and 2022), (2) the exclusion of emissions from land use converted prior to commencement of period of interest, and (3) unaccounted soil carbon stock losses linked to the construction of drainage canals, aquaculture ponds and subsequent increased export of dissolved organic carbon. Our estimates for Indonesia are ~160 TgCO2eyear−1 lower than those provided by the UNEP Global Peatland Assessment23, while they are similar to those for Malaysia. The disagreement in estimated CO2e emissions is due to differences in the area of peatland (Table S2) and the exclusion of emissions from peat fires in the UNEP Global Peatland Assessment. For example, the extent of peatland in Indonesia according to the UNEP Global Peatland Assessment23 was reported 20.9 Mha (and 39.8 Mha retrieved from pixel calculation of the original tiff data, see Table S2) or 6.2 Mha larger than our study, while their year(s) of activity data were unspecified. The area of peatland in Indonesia used in our analysis (14.8 Mha which is based on ref. 16) is comparable with the latest national peatland distribution assessment (13.4 Mha)35. Further, our analysis suggests that annual CO2e emissions from the other nine countries in the region were substantially larger than previous estimates24 (Table S1). This inconsistency is likely due to limited information on peatland distribution and definition in these countries23,36, which leads to high inconsistency and uncertainty between available datasets16,37, and overlapped distribution of both peatland and mangrove (Table S3 and ref. 38). Our findings suggest that improving the accuracy of GHG emission estimates from land-use changes on peatlands will require improved accuracy of emission factors and information on peatland distribution and extent (activity data)38.

Deforestation and land-use change affecting PSFs accounted for the vast majority of annual total CO2e emissions (Figs. 2 and 3) owing to mangroves being less extensive, storing less carbon per unit area (400–2000 ton Cha−1) than peatlands (1500–5000 ton Cha−1), and experiencing lower rates of deforestation over the last two decades39–41. The results suggest that peatland conservation may have larger immediate carbon benefits compared with mangroves. However, more people are likely to benefit directly from the ecosystem services provided by intact mangroves than peatlands42, including for climate change adaptation and food security. Additionally, mangroves are threatened by multiple stressors in addition to land-use change, particularly accelerated sea level rise43, and tropical cyclones44. Impacts of sea level rise and other climate change stressors on mangrove and coastal peatland soil carbon stocks are currently uncertain and likely to vary spatially45–48. While the preservation of soil carbon stocks is enhanced under prolonged periods of waterlogged conditions49, other factors such as changes in water quality and erosion50 and the effects of nutrient enrichment51 could alter the balance between carbon accumulation and loss and thus emissions.

Mitigation potentials by regrowth and restoration scenario
Secondary regrowth or recovered PSFs and mangroves on previously deforested areas between 2001 and 2022 were estimated to cover, respectively, 0.49 Mha and 0.12 Mha (see Figs. 4a and S8, and ‘Methods’). Biomass regeneration sequestered an additional 15.9 ± 2.0 TgCO2eyear−1 of mitigation potentials (Fig. 4b). This estimate is based on annual biomass carbon stock enhancement rates of 11.9 ± 3.0 MgCO2eha−1year−152 and 14.6 ± 9.2 MgCO2eha−1year−153 for PSF and mangrove, respectively. We observed that the secondary regrowth of PSF, as identified using the tropical moist forests (TMFs) dataset54, was not detected in areas of recent deforestation (2017–2022). Consequently, carbon biomass sequestration for new regrowth during this period could not be quantified. Estimates of enhanced carbon stocks via biomass regrowth would only offset 2.3% of annual CO2e emissions from deforestation and land-use change of PSF and mangroves in Southeast Asia, suggesting that avoiding clearance of PSF offers greater mitigation potential compared to regrowth in the short-term, because of the extended time taken by regenerating forest to reach maturity, and thus achieve maximum carbon accumulation53,55 (Fig. 4b). Nevertheless, actual carbon uptake from regrowth and restoration likely exceeds our estimate because the majority of carbon stocks in wetlands are soil-based rather than above-biomass. However, we noted limited availability of data on soil carbon sequestration from regrowth and restoration of PSF as well as little evidence of soil carbon stocks buildup following mangrove restoration, particularly after 5 years of planting56. We suggest that future research aimed at improving carbon uptake accounting from PSFs and mangrove restoration should be focused on assessing soil carbon fluxes particularly their soil organic matter burial rates.

We estimated that implementation of restoration after 2022 involving revegetation of ~2.64 Mha degraded and deforested PSF (Fig. 4c), rewetting of ~5.34 Mha tree and oil palm plantations on drained peatlands, and recovery of mangroves on ~0.07 Mha bare land and aquaculture ponds has a potential climate change mitigation of ~94.4 ± 7.4 TgCO2eyear−1 (Fig. 4d and Table 1). Of this total, rewetting of drained plantations on peatland accounts for about 66%, with emission reductions from the revegetation of degraded and deforested PSFs and mangroves the remaining 33% and 1%, respectively. Rewetting drained and managed peatlands is the most effective means of achieving emissions reductions in the short term instead of revegetation, given the lag between reforestation and maximum carbon accumulation (Fig. 4b). While demand for plantation-based commodities remains, potential leakage can perhaps best be minimized through a combination of switching to taxa that are better able to withstand high water tables (e.g., paludiculture crops)57, increased productivity in unaffected areas and reduced wastage. Further research is needed to determine the optimal depth of the water table for maintaining low GHG emissions without jeopardizing the production of plantation commodities58. Overall, the implementation of restoration often faces complex biophysical, economic, and governance barriers31,57,59–62 and may lead to enhanced emissions elsewhere63. Results presented here suggest that the fulfillment of emission reduction targets for the land sector in Southeast Asia will be strongly dependent on the conservation in a more-or-less intact state of PSFs and mangroves.

Potential contribution to emissions reduction targets
As a proportion of national-level land-use change emissions, the climate change mitigation potential through avoided deforestation and conversion of PSFs and mangroves is far more significant for some countries in the region than others (Table 1). Avoiding deforestation and conversion of PSFs and mangroves to other land uses is equal to 88%, 64%, 60%, and 39% of the national land-use emissions of, respectively, Malaysia, Indonesia, Brunei Darussalam and Myanmar.

The climate change mitigation potential is less for the other countries in the region—The Philippines (26%), Cambodia (18%), Vietnam (13%), Thailand (10%), Laos (9%), Singapore (2%), and Timor Leste (0.04%). Indonesia alone is responsible for the largest (73%) annual CO2e emissions from PSFs and mangroves and therefore the greatest potential for reducing emissions from land-use change in the region (1422 TgCO2eyear−1, or 35%). In its NDCs, Indonesia plans to reduce its emissions from Forestry and Other Land-use (FOLU) by 2030 up to 729 TgCO2e, depending on the level of external support29. Our estimated annual emissions from PSFs and mangroves in Indonesia are ~504.7 ± 95.4 TgCO2e, and thus similar to the country’s target of emission reductions from FOLU without external financial support (500 TgCO2e). The conservation of PSFs and mangroves alone could thus potentially meet the country’s unconditional reduction target for FOLU emissions28. This potential could be increased further by 15% (76.7 ± 7.3 TgCO2e) through the restoration of PSFs and mangroves that have been degraded or converted to other land uses (Table 1).

Conserving and restoring PSFs and mangroves in Southeast Asia, despite their relatively limited extent (5.4% and 0.2% of land in the region and globally, respectively), could potentially contribute ~770 ± 97 TgCO2eyear−1 of reduced emissions to climate change mitigation. These numbers include the opportunity for restoration of currently degraded PSFs and mangroves in Southeast Asia that could contribute ~94 ± 7 TgCO2e annually. If realized, such efforts could reduce GHG emissions from FOLU by 54% in Southeast Asia region, and 16% globally (Fig. 5). Peatland and mangrove conservation and restoration thus should be considered an important part of meeting NDC targets, despite their limited area. The Association of Southeast Asian Nations (ASEAN) Peatland Management Strategy 2023–2030 specifically promotes the sustainable management of peatlands and aligns with the national-level conservation and restoration targets as agreed under the Convention of Biodiversity (COP15) ref. 36. Effective implementation of these targets may still not be sufficient for Southeast Asia countries to compensate for their land-based emissions (Table 1), however. Far more ambitious national-level conservation and restoration targets than those already agreed may be required in order for Southeast Asian countries to meet their obligations under the Paris Agreement.

Our findings highlight the importance of conserving and restoring carbon-rich peatland and mangrove ecosystems as a natural climate change solution for countries with substantial coverage and losses of these ecosystems (Table 1). Further, this study provides timely evidence of the scale and magnitude of actionable and pragmatic climate change mitigation strategies provided by conservation interventions aimed at both restoring and protecting PSFs and mangroves in Southeast Asia ahead of the next round of NDC submissions in 2025 (Fig. 5). PSF and mangroves in the region clearly have an important role to play in meeting net zero emissions by 2050 under the Paris Agreement. For countries other than Indonesia that have not specified their emission reduction targets for the land sector in their submitted NDC, the findings presented here have important implications for policy development and implementation, at national, regional, and global levels.

Methods
Activity data
We compiled and used multiple published spatial datasets for activity data (area of PSFs and mangroves deforested and converted to other land covers), using basemaps for peatland16 and mangroves64. For deforestation of PSFs and land-use change, we overlaid the peatland basemap and datasets of annual tropical moist forest (TMF) change between 2001 and 2022 and main class land cover types in 2022 (e.g., deforested and degraded PSF, regrowth of PSF, tree plantation, water surface, and other land covers) provided by ref. 54. Deforestation of PSFs was thus defined in a similar way to the original dataset, which was tree canopy loss at 30 × 30 m resolution. Peatland land cover prior to 2001 was identified by overlaying the peatland base map with any peatland area deforested prior to 2001 as identified by ref. 54. Therefore, historical and annual areas of each land-use were tracked from the deforestation data—a common approach applied to identify historical land-use change65. Subset examples of land use transition validation by using satellite imageries are provided in Fig. S9. Additional datasets were included and overlaid with deforested and converted PSFs, including a map of industrial and smallholder oil palm plantations66, and drainage canal network map67. Annual burned area data obtained from NASA MODIS MCD64A1 product68 was overlaid with peatland extent to generate estimated peat burned area.

For mangroves, we assessed deforestation area by overlaying the mangrove area baseline in 200064, and annual tree cover loss between 2001 and 202269. Deforested mangrove distribution and area were then identified when tree cover loss pixels overlapped with mangrove baseline data. Land-use changes in deforested mangroves over the assessment period were identified through a dataset of drivers of mangrove loss provided by ref. 70. Six main land covers are considered; bare land, mangrove, coastal wetlands, aquaculture pond, other land cover, and water body. However, this dataset does not cover Singapore, Brunei Darussalam, and Timor Leste. The extent of mangrove loss in these three countries was low. Combined losses of mangroves in Singapore, Brunei Darussalam, and Timor Leste account for less than 0.1% of total losses of mangroves in Southeast Asia between 2001 and 2022. The oil palm dataset66 was combined with information on mangrove deforestation from the first analysis to identify and quantify the extent of mangroves converted to oil palm. We applied a similar approach for PSF to track historical annual land cover change in mangroves including mangrove regrowth by using known deforestation year data. All of the spatial data analyses were carried out using 30 × 30 m pixel-based calculations in Google Earth Engine. Except for the baseline vector maps of peatlands and mangroves, all other spatial data, including generated activity data, used in our analysis had a resolution of 30 m.

Carbon emission factors
Tier 1 emission factors from the 2013 IPCC Wetlands Supplement71 and 2019 Refinement to the 2006 IPCC Guidelines72 were applied to both PSFs and mangroves to calculate CO2 and CH4 emissions arising from deforestation, peat fires, drainage development and decomposition of peat, and from the construction and operation of fish/shrimp ponds. While Tier 1 emission factors are global, most of the compiled data specifically for peatlands are from the Southeast Asia region71. In those cases where valid Tier 1 emission factors were unavailable or not relevant, emission factors from earlier studies conducted in Southeast Asia were applied (see further detail on emission factors and assumptions that underpinned the analysis in Table S4). The current study focused on the following sources of GHG emissions: (1) loss of biomass carbon from deforestation involving both PSFs and mangroves under the assumption that hydrology and tidal connectivity remained intact, (2) peat decomposition from soil CO2 and CH4 effluxes and dissolved organic carbon, (3) CO2 and CH4 emissions generated by peatland fires, (4) excavated peat soil carbon stock loss and on-site aquatic CO2 and CH4 effluxes from the drainage canal surface network, (5) on-site aquatic CH4 effluxes from aquaculture ponds, and (6) losses of soil carbon stock during the conversion of mangroves to aquaculture ponds. Owing to a paucity of relevant literature, we did not include N2O emissions, although these may be substantial from some land uses73.

Determination of carbon removals and emissions reduction potential
For the estimation of carbon removals and emissions reduction potentials from forest regrowth, carbon stock enhancement was used for the forest regrowth and revegetation categories, as well as estimates of potential reduced emissions reduction following rewetting in tree and oil palm plantations on peat (see Tables S4 and S5 for detailed carbon removals and assumptions regarding the peat water table and emissions reduction potentials used in this study). Carbon removal factors following regrowth and revegetation of PSFs and mangroves were adopted from global biomass carbon stock enhancement synthesis studies developed by refs. 52 and 53 (Table S4). Potential reductions of CO2 and CH4 emissions following optimal retweeting scenarios on converted PSF for tree plantation and oil palm were derived from the relationship between water table depth (WTD) and CO2 and CH4 emissions developed by ref. 58. We considered rewetting scenarios only feasible for tree and oil palm plantations due to insufficient water table data and emissions factor for rewetting degraded peatlands as well as high land management uncertainty of idle degraded peatlands74. The optimal rewetting scenario was determined by increasing WTD to 30 cm below the surface. According to global data synthesis (ref. 58), the lowest tradeoff between CO2 and CH4 emissions is when WTD is 30 cm below the peat surface. Emissions of CO2 and CH4 were calculated using, respectively linear and non-linear relationships. Raising WTD by 10 cm led to a reduction in CO2 emissions of a maximum of 3 MgCO2ha−1year−1, while CH4 emissions are nearly zero when WTD is kept below 30 cm from the surface.

Determination of carbon emissions and emission reductions
Estimates of annual CO2e (CO2 + CH4) emissions generated by each of the six potential sources considered here were the product of activity data (annual deforestation and land-use change area) and emission and removal factors. Both carbon stock difference and carbon flux change approaches were used, depending on the mechanisms of carbon release (e.g., one-time vs. continuous carbon losses). For example, a carbon stock difference approach was applied to estimating CO2e emissions from deforestation and soil carbon stock loss following peat drainage canal development, and the construction of aquaculture ponds. Moreover, soil CO2 and CH4 fluxes were used for calculating emissions from peat decomposition and aquatic carbon emissions. For the calculation of emissions, we applied CH4 global warming potential at the 100-year timescale (GWP100) of 28 as a basis for comparing the relative warming impact between CH4 and CO2 as suggested by the Fifth IPCC Assessment Report75. We acknowledge that the conventional GWP100, despite being broadly used, has theoretical limitations when applied to natural systems (i.e., single pulse emissions while ecosystem emissions are typically continuous) and that the 100-year time scale is purely arbitrary. Moreover, we recognize that other metrics such as the Sustained Global Warming Potential76 and GWP* may be preferred when quantifying the radiative forcing of ecosystems over time77. We thus consider our estimated emissions to be conservative because of our use of the GWP100 index, which is in line with international GHG reporting standards under the UNFCCC75. Estimated CO2e emissions by sources and countries are presented in the dataset for this paper78.

Biomass carbon loss following deforestation. Estimates of annual CO2e emissions following deforestation were determined from the direct loss of aboveground biomass carbon stocks for both PSFs and mangroves. The aboveground biomass carbon stocks baseline for PSF was obtained from Southeast Asia data synthesis by ref. 79, while we applied Tier 1 IPCC values for mangroves71 (Table S4).

Peat decomposition. We considered both peat CO2 and CH4 effluxes and used Tier 1 emission factors for calculating annual levels of peat decomposition for different land uses (Table S4). Estimated annual peat decomposition was the product of CO2 and CH4 emissions on the area of converted PSF, and therefore the magnitude of annual peat decomposition increased over the analysis period. The extent of deforested and converted PSF prior to 2001 was determined and used to generate estimates of annual emissions from peat decomposition. We assumed that converted and drained peatlands, regardless of the time since conversion, are continuously generating CO2 and CH4 as a function of decomposition. This assumption limits the ability to include peat depletion time where shallow peat may disappear faster than deep peat over a certain period, and thus will likely contribute to an overestimation of GHG emissions.

Peat fire emissions. Emissions following peat fires were calculated by considering the mass of organic soil fuel, gasses (CO2 and CH4), and the relevant combustion factor (see Table S4 for detailed values) along with the area of peatland burnt that year determined from the NASA MODIS MCD64A1 burnt area monthly product68. In this study, we used the latest combustion factor data developed by ref. 80, where it is assumed that peat fires only generate 54% of combustion products. Consequently, the emissions from peat fires in this study may be lower than Tier 1 IPCC data where peat fires are assumed to generate 100% combustion products.

Peat drainage. For calculating CO2 and CH4 emissions from peat drainage, we considered three carbon loss pathways: (1) loss of peat soil carbon stock following excavation of peat for canal development, (2) dissolved organic carbon export, and (3) on-site CO2 and CH4 emissions from the drainage canal surface area. For loss of soil carbon stock linked to the construction of a drainage network, the uppermost 1 m of peat was assumed to have been excavated and all excavated peats were rapidly decomposed on exposure to the atmosphere.

Emissions from conversion of mangroves to aquaculture ponds. Conversion of mangroves to aquaculture is associated with enhanced emissions of carbon owing to substantial soil carbon loss following soil disturbance and removal during pond development81. In this study, we assumed that ~54% of total soil carbon stocks have been lost during the construction of aquaculture ponds53. We adopted Tier 1 2013 IPCC Wetlands supplement for aggregated organic and mineral soil carbon stocks as we believe this value is conservative to cover large hydrogeomorphic variability of mangroves in Southeast Asia. Moreover, we included on-site CH4 emissions from the aquaculture ponds through the use of a Tier 1 IPCC 2019 Refinement emission factor72.

Uncertainty analysis
We estimated the uncertainty of CO2 and CH4 emissions presented in Figs. 2 and 3 by combining the uncertainty of activity data with the relevant emission and removal factors. Spatial uncertainty of 11% was applied for land use change on PSFs (ref. 54), while no spatial uncertainty estimates are available for the burned area, drainage canal network, and land-use change on mangroves. Therefore, final emission uncertainty estimates for these activity data relied on the uncertainty of the emission factor. An error propagation method82 was applied to generate uncertainty estimates, and this method estimates the uncertainty at 95% confidence intervals.

Limitation of this study
Our regional and national-scale analysis of CO2e emissions from PSFs and mangroves in Southeast Asia improved earlier assessments by providing higher spatial and temporal resolutions (Table S1). However, several limitations remain, mainly concerning the availability, reliability, and consistency of data. First, we recognize uncertainties and inconsistencies characterizing national-level peatland distribution and area statistics (Table S6), problems that were also highlighted by the UNEP Global Peatland Assessment23. Our analysis relied upon ref. 16, in which the estimates of peatland area for Indonesia and Malaysia are relatively conservative when compared with those provided in refs. 37, 23, 24 while comparing well with earlier assessments 10, 35. We are aware that the high estimate of peatland area reported for Myanmar, Philippines, Vietnam, and Cambodia in the dataset of ref. 16 used in this study warrants future refinement through ground validation, as these mapped peatlands may correspond to areas with organic-rich soils of fluvial origin. We acknowledge that for these countries, estimates of peatland area in ref. 16 are between the higher estimates in ref. 37 and the lower ones in ref. 23 (Table S2). We thus believe that estimates provided in ref. 16 offer a middle ground in terms of peatland area data that are currently publicly available, noting that spatial datasets referred to in refs. 10, 20, 24, 31 are not publicly accessible and thus could not be used in the present study. Second, we used the only available drainage canal network data derived from satellite image analysis by ref. 67. These data were not sufficiently and finely resolved to include the narrowest canals located in degraded peatlands. Further, the data cover only Sumatra, Peninsular Malaysia, and Borneo. Thus aquatic emissions from peatlands in other parts of the region (e.g., Papua) are not considered in our study. Third, processing secondary datasets that differ in their format and spatial resolution may generate another layer of spatial uncertainty. Fourth, we have been unable to implement an independent ground-check of activity data. Moreover, there is an ongoing need to generate improved emission factors in order to reduce uncertainties in assessments of soil carbon stocks loss following the excavation of peatlands (for drainage canal network) and mangrove soils (for aquaculture pond development). It is crucial to improve these aspects in future assessments.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Sasmito et al. present a study of carbon emissions from degraded and converted mangroves and peatlands in Southeast Asia and estimate the potential emission reductions from restoration and conservation. The derived results support substantial possibilities for CO2 emission reductions that would make important portions of NDCs in Southeast Asia. The study is based on the integration of various, previously published spatial datasets and the application of IPCC emission factors to arrive at country specific carbon emissions. It shows the continuous growth of massive carbon emissions from degradation and conversion of peatlands and mangroves. The results are relevant for the countries concerned in this study (policy making) and of interest to the research community working on nature-based solutions. It expands on previous regional studies by integrating mangrove and peatland forests and their carbon losses and by including countries previously not covered. However, some inconsistencies between data/results and previous studies have been identified that are outlined below and that need to be fully addressed.\n\nGeneral comments:\nThe study leverages on available data sets and it appears that land-use change and carbon fluxes are not verified with ground truthing for countries that are not the focus of peatland and mangrove research (Indonesia, Malaysia). This challenges some of the results such as large C emissions from peatlands in Myanmar or the absence of peatland emissions from Brunei. To support the important findings on emission reduction potentials verification with field data (as from other publications) would be extremely useful, especially for Timor Leste, The Philippines, Thailand, Cambodia, Myanmar, Laos and the Vietnam. The study needs to be supported with full data sets in SI or in a public database so that results are reproducible. This should include inventories of mangrove and peat soil areas, burnt areas, revegetation, drainage canals and deforestation rates for each country. The SI materials should provide country-wise maps so that land-use changes are clearly documented (at least for the beginning and end of the study period) and shape or KML files made available for download.\n\nThe rates of peatland and mangrove losses are only meaningful if they can be compared against the natural baseline of existing peatland and mangrove areas per country. Policymakers, stakeholder, and researchers need to know where these ecosystems occur and what areas are available to develop respective policies and carbon market projects as well as for NDC accounting. However, except for a column of numbers in Fig. 2a which must represent combined mangrove and peat areas per country no specific baseline data for these ecosystems is presented.\n\nAlthough Fig. 1a is of relatively low resolution it shows very large areas of peatland for Myanmar, Laos, Cambodia, Vietnam and The Philippines. These large areas are not in line with national inventories presented in the ASEAN peatland management strategy 2023-2030 (ASEAN 2023) and the latest global peatland map in UNEP’s Global Peatland Assessment (UNEP 2022), which both show much smaller areas of peatland for these countries. One wonders where all this peatland is supposed to be in the mountains of northern Myanmar and Laos, the monsoon forest areas of Cambodia or the Philippine island of Mindanao (where two peatland areas have been documented in the literature). UNEP (2022). Global Peatlands Assessment – The State of the World’s Peatlands: Evidence for action toward the conservation, restoration, and sustainable management of peatlands. Main Report. Global Peatlands Initiative. United Nations Environment Programme, Nairobi.\nhttps://asean.org/wp-content/uploads/2023/12/APMS-7-Nov-web.pdf\nhttp://www.aseanpeat.net/\n\nThe assumed large peatland areas support results of very high rates of annual PSF losses (and related CO2 emissions), which are most likely overestimations. For example, summing the presented annual average losses (2001-2022, Tab. 1) of nearly 4900 ha (Myanmar), 2100 ha (The Philippines), 420 ha (Laos), and 650 ha (Cambodia) yields total PSF losses that exceed the documented total peatland areas of these countries as given by the ASEAN (2023) inventory. E.g. Myanmar has only 11,200 ha peatland but would have suffered from over 100,000 ha PSF losses, Laos has only 1500 ha peatland but is estimated to have lost almost 9000 ha. These large inconsistencies need to be resolved and numbers corrected downward to reflect the actual area of documented peatland in these countries.\n\nUNEP (2022) presents updated estimates of annual peatland emissions for Asian countries and except for Indonesia and Malaysia estimates annual emissions of between 0 and 5 Mt CO2-e for all other countries considered in this study. The reported large emissions from Myanmar, The Philippines, Vietnam and partly Cambodia are not in line with the UNEP report. Also, Joosten (2009) provides CO2 emission estimates for these countries and should be consulted. All reported annual CO2 emissions (although also including small proportions from mangroves) exceed the emission estimates by Joosten (2009) and UNEP (2022) for these countries. Emission estimates should be supported by independent studies and conservativeness applied, otherwise wrong conclusions are made about massive emission reductions from NBS, when in fact no such emissions or large carbon stocks exist.\n\nJoosten, H. (2009) The Global Peatland CO2 Picture: peatland status and drainage related emissions in all countries of the world. Wetlands International, Ede.\n\nThe drainage canal map by Dadap et al (2021) only provides canal networks for Sumatra, Borneo and Peninsular Malaysia. It remains unknown how the authors could determine drainage density and associated oxidative peat-carbon emissions for Indonesian Papua, The Philippines and the countries of Mainland Southeast Asia for which, to my knowledge, such datasets are not published. Moreover, Dadap et al. 2021 acknowledge that their map is not of sufficient resolution to identify all canals with less than 5 m width, which is a particular problem in forested areas. The modelled drainage impact and associated CO2 emissions from peatlands of Sumatra, Borneo and Peninsular Malaysia are therefore most likely underestimated. These limitations need to be acknowledged while improvements of the method to arrive at more robust emission data have to be discussed.\n\nIt is not clear how vegetation regrowth has been derived. I’m not aware of publications that show reforestation on this large spatial scale (e.g. 50,000 ha regrowth peatlands in Indonesia in 2015 according to Fig. 4 and S7)? Where was this? Any maps? Evidence for regrowth needs to be supported with either relevant literature, ground truthing data or aerial imagery of sufficient resolution.\n\nThe manuscript provides no estimates on peat fire emissions from Brunei and states that no data were available. However, Brunei has been affected by peat fires in a similar frequency as Malaysia and Indonesia and multiple fire events have been documented with Landsat (Addly et al. 2022). Burn scars can also be traced in Google Earth. The overall GHG accounting for Brunei is not correct. It is claimed that most mitigation potential exists for mangroves and less for peatlands. However, the overall mangrove coverage in Brunei is only 18,000 ha (determined in 1980s by Anderson & Marsden 1984) and has hardly changed since the 80s. On the other hand, peatland covers ca. 100,000 ha and recurring peat fires (Addly et al. 2022) and persistent drainage (Cobb et al. 2020) must cause large carbon emissions. For example, Joosten (2009) estimates 0.6 Tg CO2 emissions from drained peatlands for 2008 (two orders of magnitude higher than the presented total annual emissions from Brunei which are argued to be dominated by mangrove degradation). Better estimates of land-use changes and fire impacts need to be provided in a revised version to arrive at a correct mitigation potential. See:\nAnderson & Marsden (Forestry Consultants) Ltd. (1984) Brunei Forest Resources and Strategic Planning Study, Final Report.\nAddly, A.A. et al. (2022) Is the residual ash method applicable to tropical peatlands? A case study from Brunei Darussalam. Mires & Peat, (28).\nCobb, A.R. et al. (2020) Carbon storage capacity of tropical peatlands in natural and artificial drainage networks. Environmental Research Letters, 15, p.114009.\n\nThe discussion needs to be expanded beyond the presentation of data and opportunities for NDCs. It requires more integration of published literature, especially older, foundational literature. Most cited papers are from the past 5 years or so, giving the impression these insights are all new. The authors need to consult more literature on regional GHG emission assessments and the feasibility of restoration measures as suggested below. Please provide a more detailed discussion on the constraints on restoration/reforestation, which likely impact projected emission reduction estimates.\n\nMoreover, regional plans must be considered, for example, it should be discussed how the ASEAN peatland management strategy goals of restoring 30% of degraded peatlands and protection of 30% of ASEAN peatlands by 2030 would contribute to NDCs. Are these goals sufficient, feasible or require revision?\n\nSpecific comments:\nl. 68-70: This statement misses other peatland CO2 emission estimates at jurisdictional levels. Estimates of country-wise emissions were first presented by Joosten (2009) and more recently by UNEP (2022). These important references were apparently not consulted and should form an important basis for comparison.\nl.113-115: Which areas are compared? Miettinen et al. 2016 only cover western Indonesia and Malaysia whereas more countries are included in this study. What is the difference for 2015 for the same area as Miettinen et al. 2016? Please compare the same areas to make this a stronger statement.\nl. 115: Results not only suggest but show/prove further expansion (although at slower rates after 2015). This is clear from the Fig. S3.\nl. 124: This estimate of PSF loss is likely an underestimation. Given that in insular Southeast Asia peatlands are naturally forested (Anderson 1983) the area of peat soil indicates the original cover of peat swamp forest in this area. Based on this assumption Dommain et al. 2016 suggest that approximately 10 Mha PSF were lost from for Sumatra, Borneo and Peninsular Malaysia by 2010 alone. Please clarify for which area and timeframes these losses apply and correct/compare data.\nAnderson, J.A.R. (1983) The tropical peat swamps of western Malesia. In: Gore, A.J.P. (ed.) Mires: Swamps, Bogs, Fen and Moor. Ecosystems of the World 4B, Elsevier, Amsterdam, 181–199.\nDommain, R. et al. (2016) Ecosystem services, degradation and restoration of peat swamps in the Southeast Asian tropics. In Bonn, A. et al. Peatland restoration and ecosystem services: Science, policy and practice, Cambridge University Press, pp.253-288.\nl. 122: What is the source for the 1.41 Mha losses? Which region is concerned? All area considered in this study?\nl. 149-150: The annualized peatland emissions compare well with the 520 Pg CO2 estimated for 2008 by Dommain et al. (2016) and 535 Pg C O2 estimated for 2015 by Miettinen et al. (2017) both for western Indonesia and Malaysia and excluding peat fire emissions. Discussion of regional emission estimates with available literature would improve the paper.\nl. 179-183: An important point. But how can permanence of their C pools/sequestration be ensured with rapid sea level rise that threatens survival of mangroves? The authors should discuss this issue to remain conservative in their emission reduction estimates. Which management strategies could preserve mangroves from sea-level rise which would otherwise kill them off? Can the mangrove contribution to NDCs by 2050 (and beyond to remain at net zero) when sea-levels will be higher than they are now be compared with that of 2022?\nl. 197: Can the authors provide any field-based evidence for such rates of biomass growth. Disturbed peatlands suffer from poor forest regeneration and disturbed peatland is widely covered with stable fern dominated shrublands. See:\nGiesen, W. (2004) Causes of Peat Swamp Forest Degradation in Berbak NP, Indonesia, and Recommendation for Restoration. Euroconsult, Arnhem, 127 pp.\nVan Eijk, P. & P. H. Leenman (2004) Regeneration of Fire Degraded Peatswamp Forest in Berbak National Park and Implementation in Replanting Programmes. Alterra, Wageningen, 83 pp.\nWösten, J.H.M. et al. (2006) Interrelationships between hydrology and ecology in fire degraded tropical peat swamp forests. International Journal of Water Resources Development, 22, 157–174.\nl. 204-206: It is critical to address leakage effects that would be able to offset emission reductions from rewetting plantations sites. As it is highly unlikely that tree crop plantations can be maintained under high water tables it is legitimate to ask where these plantations would move to and what the associated GHG emissions on new sites would amount to. These potential leakage effects need to be accounted for in quantifications of national emission reduction potentials.\nl. 213: Also Giesen (2004) and Dommain et al. (2016) provide detailed insights into challenges of restoration.\nl. 217/Fig 4C. Why is rewetting not considered an option for degraded peatlands? They are typically dissected by drainage or logging canals which cause peat oxidation and fires. Reversing these impacts should provide further emission reductions.\nL. 275-277: Is it correct that no ground truthing has been conducted to constrain the claims of the peatland/mangrove area coverage and land use changes and the study is purely based on remote sensing?\nl. 287-300: It remains vague how natural regeneration (regrowth) was calculated. It seems the authors somehow balanced annual deforestation rates? Please provide the precise method as well as visual proof for regeneration.\nl. 321-322: The adoption of C removal from global syntheses studies may not be in line with the regionally observed poor forest regeneration on peat, which is hampered by fluctuating water tables, limited availability of seed dispersers, recurrent fires and the stability of alternative plant communities dominated ferns (refs. below). It would be appropriate to present alternative C removal rates for fern/shrub communities dominating degraded peatlands. But these might not be available.\nGraham, L.L.B. & Page, S.E. (2012) Artificial Bird Perches for the Regeneration of Degraded Tropical Peat Swamp Forest: A Restoration Tool with Limited Potential. Restor. Ecol. 20, 631–637.\nKobayashi, S. (1999) Initial phase of secondary succession in the exploited peat swamp forest (Shorea albida) at sungai Damit, Belait in Brunei Darussalam. In Proceedings of the International Symposium on Tropical Peatlands. Bogor, Indonesia, 22–23 November 1999. pp. 205–214.\nBecek, K. et al. (2022) Shorea albida Sym. does not regenerate in the Badas peat swamp forest, Brunei Darussalam–An assessment using remote sensing technology. Forest Ecology and Management, 504, p.119816.\nl. 340-343: There are alternative metrices for calculating the climate impact. In fact, newer metrics have replaced GWP as the latter assumes single pulse emission which don’t occur in ecosystens. See Neubauer, S.C. & Megonigal, J.P. (2015) Moving beyond global warming potentials to quantify the climatic role of ecosystems. Ecosystems, 18, pp.1000-1013.\nThe net climate effect of CH4 and CO2 peatland emissions can be best quantified with a radiative forcing model: Günther, A. et al. (2020) Prompt rewetting of drained peatlands reduces climate warming despite methane emissions. Nature Communications, 11, p.1644.\nl. 357-359: This assumption ignores the peat depletion times that will differ depending on available peat depth. Were any peat depth maps consulted to confirm that shallow peats have not been fully oxidized/burnt within the considered time frame? In Indonesia some areas of shallow peatland have already disappeared from sustained deep drainage.\nSI: I’m confused by the unit (10^3 ha) in the y-axes of SI Figs. 3 to 7. Are these numbers really in 1000 ha? If so they are not in line with Tab.1 and would suggest for example mangrove losses of over 15,000 ha in 2019 and 2020 in Singapore. Please clarify and correct if necessary.", "reviewer_2": "Please see attached word document below", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers and we combined our reports (below). This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\n\nThis paper by Sasmito et al. is welcomed as we strongly support the premise that land use emissions in Southeast Asia can and should be mitigated through peat swamp forest and mangrove conservation and restoration. We believe that its publication will encourage and endorse government policies and practices with this aim, particularly given the upcoming key international climate milestones: Nationally Determined Contributions (2025) and Global Stocktake (2028) under the United Nations Framework Convention on Climate Change.\n\nConservation and restoration of these carbon-rich ecosystems is vital for the local people and ecosystems and for our planet. Not only would conservation and restoration of peatlands and mangroves definitely enhance carbon sequestration and reduce GHG emissions enormously, the value of the provision of ecosystem services would be vast. For example, millions of people in SE Asia and beyond rely on mangroves for their livelihoods and even more importantly for defences against sea level rise and storm surges. This protection also safeguards carbon stocks in other coastal ecosystems (including coastal peatlands) from flooding and erosion, so the mitigation of carbon emissions is potentially far greater than the data presented here suggests.\n\nThe authors clearly went to great lengths to respond to the earlier reviewers particularly with respect to their methods and we commend them for this. Despite the enormous difficulties in determining land cover change and emissions without ground truthing, we have an acceptable level of confidence in their data for the purposes of carbon accounting. Consequently, our comments and suggestions are all quite minor. Obviously, ground truthing with carbon measurements of peat cores (depths and densities) and mangrove soil cores, dissolved organic carbon and gaseous emissions measurements etc., particularly throughout restoration projects would provide stronger data, and hopefully this paper will generate more of this research in the future.\n\nComments:\n1. Like one of the reviewers, we were surprised at the broad extent of land throughout SE Asia (e.g. in Myanmar) that apparently was once peatlands (mostly marked as ‘Other land cover’ Figure 1). This is not a total surprise however, as peat is often overlooked. For example we have recently found extensive areas of subtropical peat along the coast of Queensland with peat up to 8 m or more deep both as wire-rush peatlands, and lake basin peat deposits (despite a Queensland government soil scientist saying that ‘peat in Queensland is negligible’). However, is it possible that some of these areas were once freshwater swamps with highly organic mineral soils, rather than peatlands? The amount of carbon stored in freshwater swamps is substantially lower than in peat swamps, although the carbon turnover is very high. Another difference is that peat swamps are rainwater fed and only have water flowing out of them, not into them (tropical peat swamp forests grow on a dome of peat), whereas freshwater swamps typically have river inflows bringing nutrients, so this could be seen on a map. Despite this, given that most of the peatlands are in Indonesia and Malaysia, this would likely not impact the overall carbon budgets provided in the paper.\n2. It would be very helpful to have another figure showing the areas of remaining peatlands and mangroves to compare with Figure 1. Or even marked on Figure 1 as another item in each of the keys? The online maps provided (https://ee-sdsasmito.projects.earthengine.app/view/sea-peatmangrove) are a wonderful resource and they would be even better if there were maps with the areas of existing peatlands and mangroves as well. So maybe this would be sufficient if a new figure can’t be included.\n3. Why does the key for Figure 1 b show ‘Mangroves’, but the key for Figure 1a shows ‘regrowth PSF’? The Figure legend notes that these are ‘Regrowth mangroves’ – so are they regrowing following destruction of old growth forests? If so, to be consistent it would help to state this on the key (and in the databases). Or are they new mangroves also - growing seawards via succession?\n4. Line 47: Tropical peatlands cover at least 44 million hectares (Mha), store an estimated 105 GtCref. 7 and at lower elevations support woody vegetation (known as peat swamp forest, PSF). Firstly, the term ‘woody vegetation’ does not do the forests justice as the trees can be 50 m tall and are far from woody shrubs. Lots of temperate peatlands are mostly sphagnum and sedges but they may also have woody shrubs as well, and they are nothing like PSF. So maybe use the term ‘trees’ or ‘forests’ instead of ‘woody vegetation’? Further, forested peat exists at many higher elevations in the tropics too e.g. Cameron Highlands cloud forests in Malaysia have quite a deep peat substrate. We suggest: Tropical peatlands cover at least 44 million hectares (Mha), store an estimated 105 GtCref. 7 and at lower elevations, and sometimes higher, support forests (known as peat swamp forest, PSF).\n5. Line #86 (i.e. loss of biomass carbon stocks following deforestation, peat drainage and burning, and soil excavation) It is worth mentioning further potential gaseous emissions and dissolved organic carbon losses due to other land use changes in mangroves, such as agriculture, where soils are often drained exposing them to oxic conditions. We suggest (i.e. loss of biomass carbon stocks following deforestation, peat drainage and burning, and mangrove soil drainage and excavation)\n6. Line # 163. Low emissions during relatively wet years provides strong support for rewetting of drained peatlands and mangroves, and we think this is worth mentioning.\n7. Line # 231-233: Authors acknowledge that a major portion of the ecosystem carbon stock (ECS) of peatlands and mangroves is in their soils. Specially for mangroves, soil C stock (1st metre) can be >70% of ECS. Therefore, C uptakes estimated here could be significantly lower than the actual potential for carbon storage within restored peatland/mangrove ecosystem. It can be important to expand the discussion on this, especially by providing recommendations to minimise uncertainties and avoid underestimations. For instance, especially in mangrove settings, having the sediment accumulation scenarios can be crucial for predicting carbon storage as restoration progresses. If the mangrove soil has been excavated, the elevation deficit created may also provide additional accommodation space for soil carbon accumulation after re-introduction of tidal waters.\n8. Line # 233-234: We can only partially agree with “No data on soil carbon sequestration from regrowth and restoration of PSF and mangroves are available”. Maybe this is true under global and regional contexts. But some data exits on soil carbon accumulation for restoring mangrove wetlands at local scales. Providing recommendations on how these local scale soil carbon accumulation data can be extrapolated along regional or global scales could be important.\nE.g.,\n• Krauss, K.W., Cormier, N., Osland, M.J., Kirwan, M.L., Stagg, C.L., Nestlerode, J.A., Russell, M.J., From, A.S., Spivak, A.C., Dantin, D.D. and Harvey, J.E., 2017. Created mangrove wetlands store belowground carbon and surface elevation change enables them to adjust to sea-level rise. Scientific Reports, 7(1), p.1030.\n• Oh, R.R.Y., Friess, D.A. and Brown, B.M., 2017. The role of surface elevation in the rehabilitation of abandoned aquaculture ponds to mangrove forests, Sulawesi, Indonesia. Ecological Engineering, 100, pp.325-334.\n• Carnell, P.E., Palacios, M.M., Waryszak, P., Trevathan-Tackett, S.M., Masqué, P. and Macreadie, P.I., 2022. Blue carbon drawdown by restored mangrove forests improves with age. Journal of Environmental Management, 306, p.114301.\n9. Line # 402: We think some explanation is need for considering only biomass carbon stock as a source of emissions under deforestation land use category (Table S4). Was it assumed that hydrology and tidal connectivity remained intact, thereby maintaining original soil anoxia and decomposition mechanisms?\n10. References: Formatting errors common. E.g. Journal names need caps, and need to be consistently abbreviated.\n\nWe trust this paper will generate further interest in undertaking experiments and surveys to determine the impact of restoration of mangroves and peatlands on GHG emissions and also provision of ecosystem services. Various community-based restoration projects exist in SE Asia and the Pacific islands, particularly for mangroves, but also for peatlands. Many of these receive very little monitoring of any kind, if any, let alone the type of scientific monitoring needed to truly determine their success and to establish the protocols for best planting and management. Often, they get started with government or industry assistance or overseas aid, and then after the planting nothing else is done. There is no monitoring and insufficient care to ensure the seedlings survive and thrive. So not only should rehabilitation of mangroves and peatlands be promoted throughout SE Asia, there needs to be appropriate guidance, education and community support to safeguard and monitor its success.\n\nCatherine Yule\nIroshaka Cooray (ECR)", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We thank Reviewer 1 for the constructive comments. Addressing their comments has strengthened our manuscript. Below we outline how we have addressed their general and specific comments (see blue-coloured text). Specifically in this revision, we have rerun several analyses to:\n1. improved data comparability with existing studies;\n2. re-calculated our estimate of peatland area and associated carbon emissions for Brunei Darussalam;\n3. identified and mapped locations where mangroves and peatlands appear to overlap, and re-calculated their associated emissions;\n4. provided satellite image composites for land cover map verification;\n5. provided study limitations sub-section to the Methods section;\n6. stored dataset resulted from this study in public repository;\n7. provided codes to generate land use change map;\n8. changed the title.\nFurther point by point responses are provided below."}, {"round": 2, "reviewer_comments": {"reviewer_2": "This paper puts into the spotlight CO2e fluxes from peat swamp forests and mangroves, which are crucial and threatened carbon sinks. The study is timely given the upcoming updates to NDCs and the Global stocktake. I commend the authors for carrying out a revised analysis of their model to consider the double accounting on some lands as well as the additional references added around this. Given the relatively small area of overlap, (3%) between mangroves and peatlands according to the satellite data, and the uncertainties of the satellite datasets, the revised approach used by the authors to estimate emissions is, in my opinion, perfectly suitable. The authors have made great adjustments to the manuscript and addressed my concerns from the first round of review.\n\nNice revision to Figure S6 and Figure 2.\n\nI appreciate the authors efforts to include a comparison of other datasets and putting the data into perspective of existing literature. However, my main concern is now some of the large differences between this study’s estimate produced and that of previous studies. While the authors make some suggestions to explain these differences, the ones provided do not seem sufficient to really account for up to 100% difference that is reported for some studies. While it may seem nuanced to focus on these differences, within the policy domain and the IPCC there is increasing push for clarity (see specific points below) and to limit confusion, giving credibility to emissions estimates within LULUCF which vary so greatly already between estimates.\n\nPoints to consider:\nAbstract – line 30: you mention that 691TgCO2eyr is produced due to land use change on peat swamp forest and mangroves. How does this number compare to the one-third of land use change emissions you mention? One option would be to add the absolute number to line 25, or contextualise 691Tg e.g. X% of Souteast Asian land use change C emissions.\nLine 50- suggest to add “but with considerable uncertainty” to the end of “and may store between 3.1 – 12.3 GtC.”\nLine 53 – acronym GHG: looks like it is the first time you use it – so put in “Greenhouse Gas (GHG)”\nLine 116 – is the estimated loss of PSF by 2022 for the whole region or also the 3 regions used by Miettinen?\nLine 134 – refer back to Figure S4.\nFigure 2 – legend does not include “Emplacement of drainage canals”, yet this is mentioned in the text (Line 152) and referred back to Figure 2.\nLine 147 to 162: Given the stark differences between some of the studies shown in Table S1, it would be crucial to consider the implications of the divergence of evidence. For example, on Line 159 – you write that by including the emissions from construction of aquaculture ponds and drainage canals networks and aquatic sources this explains why your emissions estimates are higher than ref 21. Given that earlier you write emissions from canals account for 9% of emissions, I wonder how much this can really explain this 100% difference in the results. Perhaps it is worth to include a sentence on future research that could look into bringing clarity between existing estimates.\nNote on Table S1 – to make the data more comparable with Miettinen and others who only look at peatlands only, would it make sense to split up the first column (i.e. This Study) into peatlands and mangroves separately with a sum of the two as an additional column?\nLine 198 – Similar to my point above- given this divergence of data, what are the key implications of this and next steps.\nLine 226 – what do the two rates refer to i.e. 11.9 and 14.6MgCO2ehayr, regrowth rates for PSF and mangroves respectively?\nLine 232 to 234 – this seems like speculation as soil can also continue to be a source of carbon even after restoration. Do you have reference that has shown the recovery of soil C in restoration of PSF and mangroves? If not, perhaps rephrase as a research gap – e.g. no research yet on the impact of soil C in recovering PSF and mangroves that explores the soil C fluxes (whether positive or negative).\nLine 239 – can you put this number of 94TgCo2e/yr into relation to past emissions.\nLine 261 – two “in” after one another.\nFigure 4 – sorry to not have asked this earlier but it is very noticeable that after 2016 regrowth is much lower (4a). This is an interesting result on its own I think, and it would be good to highlight why in the text (if you know).\nFinally, I cannot open the link to ref 80 - the figshare document - will the results of Co2 fluxes be available spatially explicitly as well? This would be a very noteworthy and helpful result for users.", "reviewer_4": "We are happy with the authors’ responses to our suggestions.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We thank Reviewer 2 for her/his valuable time to reading our first-round revision and providing constructive comments to the previously revised manuscript. We have addressed all comments directly below."}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have addressed all my comments and I support publication in Nature Communications.\nBest wishes,\nViola Heinrich", "reviewer_4": "We are happy with the authors’ responses to our suggestions.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter