High greenhouse gas emissions mitigation benefits from mangrove rehabilitation in Sulawesi, Indonesia

Mangroves have generated considerable interest in forest carbon discourse given their disproportionately high carbon storage and sequestration potential compared to terrestrial habitats. Capitalising on this potential, however, remains largely unrealised due in part to an incomplete understanding of the full greenhouse gas mitigation benefits provided, particularly through changes in landuse where aquaculture ponds are converted back to mangroves. Here we address such uncertainties by estimating the overall net greenhouse offset potential from rehabilitating mangroves in Tiwoho, North Sulawesi (Indonesia). Estimates of both baseline emissions and sequestration as mangrove forests regrow after rehabilitation were assessed, with results contrasted against published data on established mangroves and terrestrial forest carbon projects registered with the Verified Carbon Standard. Additionally, net emissions removals and reductions from Tiwoho is scaled-up to the national level in order to estimate the contribution rehabilitation of disused ponds can make to climate change mitigation. We also monetarise potential returns generated to date and frame this against project implementation and opportunity costs. Our data indicates that, on average, Tiwoho's rehabilitating sites could potentially mitigate upwards of -27.6 +/- 1.7 Mg CO2-e per hectare per year. This is upwards of 3 times that of other terrestrial forestry carbon projects and, assuming a voluntary carbon market price of USD $8.1 per Mg CO2-e per hectare for afforestation/reforestation projects, equates to recuperation of 76.2% of initial project implementation costs over approximately 10 years with an annual return of 7.6%. This return rate rivals that of most asset classes targeted by private sector investors. Scaling results to encompass the net emissions removals and reduction potential from rehabilitating the similar to 600, 000 hectares of disused ponds in Indonesia may offset upwards of -16.56 +/- 1.02 million Mg CO2-e per hectare per year. Additionally, mitigation benefits from rehabilitation may equate to 6 times that of terrestrial projects if situated in ponds displaying both high baseline emissions and high rates of re-growth whilst accounting for sequestration from benthic algae and the oceanic burial of dissolved inorganic carbon. These findings strengthen the evidence base needed for public and private sector support for mangrove rehabilitation within the forest carbon sector.