The international scientific community is increasingly recognizing the role of natural systems in climate‐change mitigation. While forests have historically been the primary focus of such efforts, coastal wetlands – particularly seagrasses, tidal marshes, and mangroves – are now considered important and effective long‐term carbon sinks. However, some members of the coastal and marine policy and management community have been interested in expanding climate mitigation strategies to include other components within coastal and marine systems, such as coral reefs, phytoplankton, kelp forests, and marine fauna. We analyze the scientific evidence regarding whether these marine ecosystems and ecosystem components are viable long‐term carbon sinks and whether they can be managed for climate mitigation. Our findings could assist decision makers and conservation practitioners in identifying which components of coastal and marine ecosystems should be prioritized in current climate mitigation strategies and policies.

Avoiding catastrophic climate change requires rapid decarbonization and improved ecosystem stewardship. To achieve the latter, ecosystems should be prioritized by responsiveness to direct, localized action and the magnitude and recoverability of their carbon stores. Here we show that a range of ecosystems contain 'irrecoverable carbon' that is vulnerable to release upon land use conversion and, once lost, is not recoverable on timescales relevant to avoiding dangerous climate impacts. Globally, ecosystems highly affected by human land-use decisions contain at least 260 gigatonnes of irrecoverable carbon, with particularly high densities in peatlands, mangroves, old-growth forests and marshes. To achieve climate goals, we must safeguard these irrecoverable carbon pools through an expanded set of policy and finance strategies. Main TextScientific assessments provide increasingly strong evidence that global warming in excess of 1.5 ˚C above pre-industrial levels may trigger irreversible changes to the Earth system, with far-reaching social and economic costs for human societies around the world 1 . Limiting warming to 1.5 ˚C, according to the Intergovernmental Panel on Climate Change (IPCC), requires the world to slow global emissions immediately and reach net zero carbon dioxide (CO 2 ) emissions by around 2050.To do this, the IPCC estimates that our remaining carbon budget as of 2017, or the amount of CO 2 we can add to the atmosphere between now and mid-century, is about 420 gigatonnes (Gt), equal to about 114 Gt of carbon, for a two-thirds chance of staying below 1.5 ˚C1 . Given emissions have not slowed since 2017, as of 2020, this carbon budget will be spent in approximately eight years at current emissions rates 2 . Staying within this carbon budget will require a rapid phase-out of fossil fuels in all sectors as well as maintaining and enhancing carbon stocks in natural ecosystems, all pursued urgently and in parallel 3-6 . Natural climate solutions, which promote conservation, restoration, and improved land management to increase carbon sequestration or reduce emissions from ecosystems and agricultural lands, could provide a quarter or more of the cost-effective mitigation (i.e. ≤USD100 / t CO 2 e) needed by 2030 [7][8][9] .These natural climate solutions focus on either turning down the 'dial' of emissions, for example by preventing the conversion of ecosystems to other land-uses, or turning up the dial on ecosystems' ability to remove CO 2 from the atmosphere via restoration or enhanced productivity. Yet uncertainty remains regarding the responsiveness of various ecosystem carbon stocks to management actions and regarding the relative reversibility of their loss. Are there ecosystem carbon stocks that, if lost, could not recover within a time scale meaningful to the remaining carbon budget? Any loss of such 'irrecoverable' carbon stocks would represent an effectively permanent debit from our remaining carbon budget. Ecosystems containing irrecoverable carbon may thus warrant distinct and unwavering conser...

The global significance of carbon storage in Indonesia's coastal wetlands was assessed based on published and unpublished measurements of the organic carbon content of living seagrass and mangrove biomass and soil pools. For seagrasses, median above-and below-ground biomass was 0.29 and 1.13 Mg C ha -1 respectively; the median soil pool was 118.1 Mg C ha -1 . Combining plant biomass and soil, median carbon storage in an Indonesian seagrass meadow is 119.5 Mg C ha -1 . Extrapolated to the estimated total seagrass area of 30,000 km 2 , the national storage value is 368.5 Tg C. For mangroves, median above-and below-ground biomass was 159.1 and 16.7 Mg C ha -1 , respectively; the median soil pool was 774.7 Mg C ha -1 . The median carbon storage in an Indonesian mangrove forest is 950.5 Mg C ha -1 . Extrapolated to the total estimated mangrove area of 31,894 km 2 , the national storage value is 3.0 Pg C, a likely underestimate if these habitats sequester carbon at soil depths [1 m and/or sequester inorganic carbon. Together, Indonesia's seagrasses and mangroves conservatively account for 3.4 Pg C, roughly 17 % of the world's blue carbon reservoir. Continued degradation and destruction of these wetlands has important consequences for CO 2 emissions and dissolved carbon exchange with adjacent coastal waters. We estimate that roughly 29,040 Gg CO 2 (eq.) is returned annually to the atmosphere-ocean pool. This amount is equivalent to about 3.2 % of Indonesia's annual emissions associated with forest and peat land conversion. These results highlight the urgent need for blue carbon and REDD? projects as a means to stem the decline in wetland area and to mitigate the release of a significant fraction of the world's coastal carbon stores.

Ecosystems around the world are already threatened by land‐use and land‐cover change, extraction of natural resources, biological disturbances, and pollution. These environmental stressors have been the primary source of ecosystem degradation to date, and climate change is now exacerbating some of their effects. Ecosystems already under stress are likely to have more rapid and acute reactions to climate change; it is therefore useful to understand how multiple stresses will interact, especially as the magnitude of climate change increases. Understanding these interactions could be critically important in the design of climate adaptation strategies, especially because actions taken by other sectors (eg energy, agriculture, transportation) to address climate change may create new ecosystem stresses.

Globally, carbon-rich mangrove forests are deforested and degraded due to land-use and land-cover change (LULCC). The impact of mangrove deforestation on carbon emissions has been reported on a global scale; however, uncertainty remains at subnational scales due to geographical variability and field data limitations. We present an assessment of blue carbon storage at five mangrove sites across West Papua Province, Indonesia, a region that supports 10% of the world's mangrove area. The Additional supporting information may be found online in the Supporting Information section. How to cite this article: Sasmito SD, Sillanpää M, Hayes MA, et al. Mangrove blue carbon stocks and dynamics are controlled by hydrogeomorphic settings and land-use change. Glob

Abstract1. Oceans and coasts provide a wide array of services to humans, including climate regulation, food security, and livelihoods. Managing them well is vital to human well-being as well as the maintenance of marine biodiversity and ocean-dependent economies.2. Carbon sequestration and storage is increasingly recognized as a valuable service provided by coastal vegetation. Carbon sequestered and stored by mangrove forests, tidal marshes, and seagrass meadows is known as 'blue' carbon. These habitats capture and store carbon within the plants themselves and in the sediment below them. When the habitats are destroyed, much of their carbon is released back to the atmosphere and ocean contributing to global climate change.3. Therefore, blue carbon ecosystem protection is becoming a greater priority in marine management and is an area of interest to scientists, policy makers, coastal communities, and the private sector including those that contribute to ecosystem degradation but also those that are looking to reduce their carbon footprint. A range of policy and management responses aim to reduce coastal ecosystem loss, including the establishment of marine protected areas (MPAs).4. This paper explores how MPA design, location, and management could be used to protect and increase carbon sequestration and ensure integrity of carbon storage through conservation and restoration activities. While additional research is necessary to validate the proposed recommendations, this paper describes much needed first steps and highlights the potential for blue carbon finance mechanisms to provide sustainable funding for MPAs.

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A revised guide to supporting coastal wetland programs and projects using climate finance and other financial mechanisms Coastal "blue" carbon A revised guide to supporting coastal wetland programs and projects using climate finance and other financial mechanisms This revised report has been written by D. Herr and, in alphabetic order, T. Agardy, D. Benzaken, F. Hicks, J. Howard, E. Landis, A. Soles and T. Vegh, with prior contributions from E. Pidgeon, M. Silvius and E. Trines.The designation of geographical entities in this book, and the presentation of the material, do not imply the expression of any opinion whatsoever on the part of IUCN, Conservation International and Wetlands International concerning the legal status of any country, territory, or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries.The views expressed in this publication do not necessarily reflect those of IUCN, Conservation International, Wetlands International, The Nature Conservancy, Forest Trends or the Nicholas Institute for Environmental Policy Solutions.Copyright: © 2015 International Union for Conservation of Nature and Natural Resources, Conservation International, Wetlands International, The Nature Conservancy, Forest Trends and the Nicholas Institute for Environmental Policy Solutions.Reproduction of this publication for educational or other non-commercial purposes is authorized without prior written permission from the copyright holder provided the source is fully acknowledged.Reproduction of this publication for resale or other commercial purposes is prohibited without prior written permission of the copyright holder.