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Globally, deforestation and conversion of mangroves has been shown to contribute 0.08–0.48 Pg CO₂e yr⁻¹, or 10% of the total global emissions from tropical deforestation, even though mangroves account for only about 0.7% of the world’s tropical forest area². C losses from mangrove conversion can be high not only because of losses from aboveground C pools but also belowground pools. Potential C losses from mangroves converted to shrimp ponds in the Dominican Republic were 661–1,135 MgC ha⁻¹ (ref. 7).

In 1980, there were 4.2 Mha of mangrove forests along Indonesia’s 95,000 km of coastline³. Over just 20 years mangrove forest cover had declined about 26%, to an estimated 3.1 Mha (ref. 8). In 2005, mangrove forest cover had further decreased to 2.9 Mha (ref. 3). On the basis of FAO data, cumulatively Indonesia has lost 30% of its mangrove forests between 1980 and 2005; this is equivalent to an annual deforestation rate of 1.24%. Recent estimates of Indonesia’s mangrove cover suggest a total loss of 40% in the past three decades⁴. Aquaculture development was the main cause⁵, after it expanded rapidly in 1997–2005 and resulted in an officially recorded active pond area of about 0.65 Mha (ref. 9). It was also reported that the revenue from shrimp export approached US$ 1.5 billion in 2013; almost 40% of the total revenues arising from the Indonesian fishery sector¹⁰.

As most countries do not have sufficient information to include mangroves in their national reporting to the United Nations, it is important to generate country- or region-specific data on C stocks and emission factors from various land-use activities in mangroves. In the latest National Communication¹¹ to the United Nations Framework Convention on Climate Change (UNFCCC), Indonesia did not specifically include mangroves, because the IPCC Guidelines for wetlands greenhouse gas (GHG) inventories became available only in 2013 (ref. 12). Indonesia’s mangroves are subject to tremendous development pressures despite the fact that sustainable mangrove management could contribute substantially to meeting the proposed national GHG emissions reduction target of 26–41% by 2020. If conservation actions were taken, emissions from mangrove conversion would be reduced¹³. However, to be a part of a land-based GHG emission reduction activity, information on C storage and its dynamics is necessary.

We assessed ecosystem C stocks of 39 mangroves located in eight sites spanning longitudes of 105°–140° E (Supplementary Fig. 1 and Supplementary Table 1). The mangrove C stocks were partitioned by pools, including aboveground live and dead trees, belowground roots, downed wood, and soils stratified into meaningful depth layers¹⁴. Coupled with deforestation estimates this allowed us to use a stock change approach¹⁵ to estimate emissions from land use, as well as mitigation potentials.

We found that the average C of the plant/biomass pools was 211 ± 135 MgC ha⁻¹, with the lowest values found for plots located in Cilacap, Java (9 ± 10 MgC ha⁻¹) and the highest values found for plots located in Bintuni, West Papua (367 ± 80 MgC ha⁻¹; Supplementary Table 2). The average values reported here were similar to those of the primary mangrove forests dominated by Rhizophora apiculata in Malaysia (216 MgC ha⁻¹) and Bruguiera gymnorrhiza in Indonesia¹⁶ (205 MgC ha⁻¹). Among the sampled mangroves, we found significant variations in soil bulk density (BD) and soil C content, and therefore soil C density and soil C pools (Fig. 1 and Supplementary Table 2). Differences in C stocks among sites were analysed using analysis of variance (Supplementary Table 3).

Soil bulk density (a), soil carbon content (b) and soil carbon density (c) of mangroves by depth from eight regions of Indonesia. Horizontal bars signify the standard error.

The Indonesian mangrove deforestation rate from 1980 to 2005 was 52,000 ha yr⁻¹ (ref. 3). Although covering less than 2% of the forest area²², this was 6% of the 0.84 Mha yr⁻¹ annual forest loss reported for Indonesia⁶. With a total C storage of 3.14 PgC, Indonesian mangroves have significant potential to contribute to climate change mitigation efforts. Assuming that the fate of soil carbon following land-use change in Indonesia were to be similar to that measured in abandoned shrimp ponds in the Dominican Republic⁷, emissions from land use were estimated to be 0.07–0.21 Pg CO₂e yr⁻¹, or an average estimate of 0.19 Pg CO₂e yr⁻¹. Total emissions from the Indonesian land-use sector were estimated to be 0.7 Pg CO₂e yr⁻¹ (ref. 11). This suggests that avoiding mangrove deforestation would reduce emissions by an amount equal to 10–31% of estimated annual land-use emissions at present.

Having regional and global estimates of C stocks and emission factors helped the progress of managing coastal blue carbon^{2, 13, 23}. However, generating country-specific estimates derived from the extensive measurements demonstrated here is an improvement for the next important step for informed decisions on climate change mitigation, restoration of degraded mangroves, and fishery practices.

To put this into the global perspective, on the basis of our estimates, Indonesian mangrove loss contributed 42% of the global emissions from the destruction of coastal ecosystems, including marshes, mangroves and sea grasses, which have been estimated to release 0.15–1.02 Pg CO₂e annually²³. It was also estimated that the global blue carbon emissions were equivalent to 3–19% of all GHG emissions from global deforestation, and resulted in economic damages of US$ 6–42 billion per year at a price of US$ 41 per ton of CO₂ (ref. 23). Following earlier study, the economically viable abatement cost was less than US$ 10 per ton of CO₂ (ref. 24), Indonesia could potentially gain substantial social benefits from avoiding mangrove conversion.

Provided that the cost structure, including that of project development costs, are well defined, the net benefits from avoiding mangrove deforestation and conversion may be established. Nevertheless, climate change mitigation by preventing the existing C from being released can be bundled with adaptation measures for coastal protection of rising sea levels. Mitigation and adaptation strategies also promote the benefits of ecosystem services by improving community nutrition and livelihoods from near-shore capture fisheries.

Indonesia’s shrimp industries, which are mainly large scale, generate revenue of US$ 1.5 billion annually^{9, 10}. The increased production is associated with an expansion in 1997–2005, which then levelled off at an average area of around 650,000 ha (Supplementary Table 5). To provide a global context of mangrove C dynamics (ecosystem C stocks, deforestation rates and total C stocks) of this study and the major driver of mangrove conversion, we compiled published data from nine other countries (Table 1). Along this line we plotted the calculated total national mangrove C stocks, as shown in Fig. 3a, and compared the decreasing trends of mangrove area (Fig. 3b) with shrimp production²⁵ (Fig. 3c) and emissions of CO₂ (Fig. 3d).

Full table

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