Co-benefits of forest carbon projects in Southeast Asia

Forest carbon projects can deliver multiple benefits to society. Within Southeast Asia, 58% of forests threatened by loss could be protected as financially viable carbon projects, which would avoid 835 MtCO2e of emissions per year from deforestation, support dietary needs for an equivalent of 323,739 people annually from pollinator-dependent agriculture, retain 78% of the volume of nitrogen pollutants in watersheds yearly and safeguard 25 Mha of Key Biodiversity Areas. Forest conservation contributes to climate change mitigation and delivers a host of other benefits to society, but such benefits are usually assessed qualitatively at the project level. This study provides a quantitative assessment of multiple benefits from forest carbon projects across Southeast Asia.

Here we assessed the co-benefits of establishing carbon projects that focus on avoided deforestation across Southeast Asia. First, we mapped the locations of standing forests that could be protected as financially viable carbon projects based on net present values (NPVs) and considering additionality over a 30-year time frame 2 (see the Methods for the details). We then modelled the extent to which carbon projects would (1) mitigate climate change from the avoided emissions from deforestation 2 , (2) support crop pollination services for pollinator-dependent agriculture 8,9 , (3) maintain water quality regulation services for downstream rivers and lakes by retaining nitrogen in watersheds 8,9 , and (4) safeguard Key Biodiversity Areas (KBAs) 10 .
We find that 114 million ha of forests in Southeast Asia could be protected as viable carbon projects (NPV > 0) on the basis of our conservative starting carbon pricing scenario of US$5.80 per tCO 2 e (refs. 2,3,11 ). Protecting forests through carbon projects would thereby avoid 835 ± 348 MtCO 2 e of emissions from deforestation across the region per year ( Fig. 1a and Supplementary Table 2). Forests in the Indonesian provinces of Riau and West Kalimantan have the greatest climate mitigation potential at up to 49 tCO 2 e ha −1 yr −1 .
Forest carbon projects in proximity to agricultural lands also provide important foraging and nesting habitats for wild pollinators 4,8,9 . These pollinators not only ensure the ecosystem health of adjoining forest patches but also support pollinator-dependent agricultural production and nutritional services within the immediate vicinity. We find that this benefit can serve the dietary needs of an equivalent of 323,739 ± 18,725 people across the region every year, on the basis of pollinated micronutrient production and dietary intake requirements (Fig. 1b and Supplementary Table 3). This service is particularly important in the Malaysian state of Sabah, where pollination service supported by each hectare of protected forest provides enough micronutrient production to fully meet the needs of up to 42 people, with more people potentially benefiting from having their nutritional needs even partially supported by pollination.
Forests are also known to absorb nutrients such as nitrogen from the environment for biomass growth and metabolism. This uptake would in turn reduce the amount of nutrients that flow into freshwater habitats within the area's watersheds and thereby improve the quality of water flowing downstream, reducing the need for added treatment of potable water 8,9 . On the basis of an InVEST Nutrient Delivery Ratio model 8,9 , we find that 2.86 ± 0.03 Mt of nitrogen

Co-benefits of forest carbon projects in Southeast Asia
Tasya Vadya Sarira 1,2,3 ✉ , Yiwen Zeng 2,3,4 ✉ , Rachel Neugarten 5,6 , Rebecca Chaplin-Kramer 7,8 and Lian Pin Koh 2,3 ✉ pollutants (representing an estimated 78% of potential nitrogen pollutants across Southeast Asia) per year would be avoided from the establishment of carbon projects ( Fig. 1c and Supplementary Table 3). This is particularly important for people who rely on the Mekong River, where nutrient loads from surrounding agriculture may impact livelihoods and access to clean drinking water.
KBAs are sites that contribute greatly to the global persistence of biodiversity 10 . Protecting forests through carbon projects would service. c, Freshwater service. d, KBAs. e, Spatial overlay of any level of co-benefits attained within profitable forest carbon areas. Yellow represents areas that are profitable for carbon, areas in blue provide one co-benefit in addition to carbon, areas in green provide two other co-benefits and areas in pink provide three co-benefits in addition to carbon.
thus conserve 25 ± 3 Mha of KBAs in Southeast Asia, which represents half of all terrestrial forest KBAs in the region ( Fig. 1d and Supplementary Table 3). We also identify hotspots where the establishment of carbon projects could deliver multiple co-benefits. We find that there are an estimated 6.6 Mha of forests in Southeast Asia that deliver some level of all four assessed benefits (Fig. 1e). Most of these hotspots are located in Thailand (1.7 Mha) and Indonesia (1.6 Mha). We also find that approximately 107 Mha of forests in the region would deliver at least one co-benefit in addition to climate change mitigation.
Our findings are based on a conservative starting carbon price of US$5.80 per tCO 2 e (refs. 2,3,11 ). If carbon prices increased in the future, we would expect an increase in the regional extent of forests that could be protected as financially viable carbon projects. This in turn presumes an increase in the quantity of co-benefits that could be delivered to society 7 .
We thus performed an additional analysis to assess the effects of carbon pricing on the delivery of co-benefits from forest carbon projects in the region (Fig. 2). We find that an increase in carbon price to US$25 per tCO 2 e-the average price of carbon adopted by western nations 11 -would result in corresponding increases in climate mitigation potential (from 835 ± 348 MtCO 2 e yr −1 to 875 ± 364 MtCO 2 e yr −1 ; 5% increase), crop pollination (from 323,739 ± 18,725 to 372,390 ± 17,225 equivalent people fed; 15% increase), water quality regulation (from 2.86 ± 0.03 Mt to 3.76 ± 0.02 Mt of nitrogen retained; 24% increase) and biodiversity conservation (25 ± 3 Mha to 35 ± 3 Mha of KBAs protected; 29% increase). Further increases in carbon price would result in diminishing returns in benefits ( Fig. 2 and Supplementary Tables 4 and 5).
Consequently, such increases in carbon prices could promote the financial viability of carbon projects, allowing them to compete with other potentially lucrative land-use alternatives (such as palm oil production 12 ). Alternatively, mechanisms such as payments for ecosystem services and other conservation strategies could complement the establishment of carbon projects to further incentivize landholders to invest in protection and potentially increase the likelihood of the permanence of protections 2,12 .
Importantly, the realization of co-benefits from forest carbon projects is essential to the alignment of climate policies such as the Paris Agreement with key global policy frameworks such as the Post-2020 Global Biodiversity Framework and the United Nations Sustainable Development Goals (SDGs). For example, forest carbon projects directly address the conservation of terrestrial ecosystems, enabling countries to better meet the targets of SDG 15.1 as well as Goal A of the Global Biodiversity Framework 13 . They also allow for the synergistic achievement of other goals and targets across the SDGs such as food security (SDG 2), clean water (SDG 6) and biodiversity, as well as other co-benefits not assessed in this study, such as terrestrial surface cooling (SDG 13 on climate action) 13 . Quantifying forest services further exemplifies the interconnections and importance of forest ecosystems for biodiversity and people. Particularly for communities in Southeast Asia engaged in subsistence and/or smallholder agriculture, forests support their production of food and contribute to their livelihoods, as well as provide clean water for drinking and household use across the region 4,14 .
Naturally, forests in carbon projects can also provide many other socio-economic benefits such as recreation and cultural, gender and economic empowerment for local communities 15 . While these benefits are typically measured qualitatively and are important in addressing human development goals, quantifying these benefits would require a more nuanced understanding of interrelations between forest services and the realized benefits to people, as well as the socio-political ecology at the local scale 15 . Other types of carbon projects-namely, those focusing on reforestation and improved land management-can also contribute to mitigating climate change and provide a variety of co-benefits, though their potentials may also be limited by specific economic and social constraints 16 .
The investment in the protection of forests, their natural capital and their ongoing provision of services through carbon projects enables a financially viable and sustainable means of addressing other socio-economic and environmental issues beyond climate change. By assessing this potential in Southeast Asia, we demonstrate the potential of carbon finance to meet global climate and human development ambitions.

Statistics
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Data analysis
Climate mitigation potential and return-on-investment were processed and calculated using R version 3.6.0, using the 'raster' package. The analyses of ecosystem service models and subsequent integration of rasters were performed in Python 2.7.15. Map visualizations were formed in ArcGIS 10.6.1.
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Data
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March 2021
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Study description
While there is growing demand for high-quality, nature-based carbon credits by public and private sectors to meet their climate goals through certified carbon projects, the co-benefits from carbon projects are typically documented at the project level, rather than being systematically considered during earlier stages of policy and decision-making processes at national and regional levels. We assessed the co-benefits of forest conservation through high-quality, nature-based carbon projects by first mapping forests across Southeast Asia that could be protected for financially viable carbon projects. Then, we modeled the extent to which forest protection would (i) contribute to climate change mitigation, (ii) support crop pollination services for pollinator-dependent agriculture, (iii) regulate water quality and (iv) protect biodiversity.