Global potential and limits of mangrove blue carbon for climate change mitigation

This data package includes the two 1-km resolution global maps of tropical mangrove forests between ~31°N and 39°S produced from the study: 1) investible mangrove blue carbon (in tCO2e ha-1y-1) and 2) profitable mangrove blue carbon (in tCO2e ha-1y-1). It also includes a sample R script to reproduce these layers and the relative country-level project development and maintenance cost estimates. Investible mangrove blue carbon: To model and produce a spatially explicit map of investible mangrove blue carbon, we first estimated the total volume of CO2e across three pools in mangrove forest areas—aboveground carbon, belowground carbon and soil organic carbon: Aboveground carbon: We used a recent global mangrove aboveground biomass model by Simard et al. 2019 to estimate the volume of aboveground carbon. We applied a stoichiometric factor of 0.475 to convert biomass estimates to carbon stock values. We also performed an uncertainty analyses to account for variability in this stoichiometric factor. We then used a conversion factor of 3.67 to convert carbon stock values to CO2e volume. Belowground carbon: We then used the aboveground biomass from Simard et al. 2019to estimate the belowground (root) biomass, following the allometric equation from Hutchison et al. 2014: Belowground biomass = 0.073 •Aboveground biomass1.32. Our belowground biomass estimations fall within the range of previously derived ratios of aboveground:belowground (root) biomass ratios. We applied the same stoichiometric factor (0.475) and conversion factor (3.67) to estimate the volume of CO2e associated with belowground biomass. Soil organic carbon: Additionally, to fully consider ecosystem mangrove carbon stock, we also utilized mangrove soil carbon stocks obtained from Sanderman et al. 2018, applying a conversion factor (3.67) to estimate the volume of CO2e. To these biomass carbon estimates, we then applied key criteria that enables certification of carbon credits under the rules of the UNFCCC, Kyoto Protocol, and the various voluntary certification standards such as the Verified Carbon Standard (VCS). Importantly, our analyses were guided by the requirements stipulated by VCS—the most widely used voluntary greenhouse gas program globally: Additionality: A major component of certification is ‘additionality’ or the amount of carbon stocks that would have been lost without the intervention of forest protection of the proposed project. To estimate additionality, we assume future rates of mangrove forest loss to follow existing patterns between the years 2000–2016. This data was obtained from Goldberg et al. 2020. This was calculated as the annualized rate of mangrove loss within each ~1 km cell. We then applied this estimated annual deforestation rate to the volume of CO2e associated with mangrove forest (calculated above), to derive the volume of CO2e that would be certifiable and thus investible under the VCS. Decay rates: We also considered the annual decay rate specific to mangrove forests [29]. This was based on two carbon pools—the belowground (root) biomass, with a decay rate of 0.20, and soil organic carbon, with a decay rate of 0.10. These values are based on median estimates from Lovelock et al. 2017, and we also performed an uncertainty analyses to account for variability in these decay rates. Buffer credits: Lastly, we also applied the VCS requirement to set aside buffer credits of 20% net change carbon stocks in each area to account for risk of non-permanence. Profitable mangrove blue carbon: To estimate the relative profitability of these mangrove blue carbon sites, we utilized the map of investible mangrove blue carbon to calculate the net present values (NPV) based on several simplifying assumptions obtained from previous studies’ published data. We first used the cost of project establishment at US$232 ha-1, based on a wide range of costs that are key to the development of a project such as project design, governance and planning, and enforcement. We also used an annual maintenance cost of US$25 ha-1, which can include aspects such as monitoring, finance and administration. Given the potential for establishment and maintenance cost to vary between countries, then weighted this cost by countries’ per capita gross domestic product (GDP) to estimate the relative cost per country. We then assumed a constant carbon price of US$5 t-1CO2e for the first five years, roughly matching the average carbon price of all avoided deforestation projects recorded by Forest Trends’ Ecosystem Marketplace reports between 2006–2018. After the first five years, we assumed a 5% price appreciation for subsequent years over a 30-years project timeframe. Based on these criteria, we calculated the NPV as well as the accumulated profits over the next 30 years, based on a 10% risk-adjusted discount rate. Using the spatially explicit NPV estimates, we excluded areas that were not financially sustainable (negative NPV), and calculated the extent, climate mitigation potential and return-on-investment within the remaining, profitable, areas.. Further details for these datasets are presented in Zeng et. al. For questions or issues on the spatial data layers, please contact Yiwen Zeng (zengyiwen@nus.edu.sg).&nbsp