Global drivers of mangrove loss in protected areas.

Despite increasing efforts and investment in mangrove conservation, mangrove cover continues to decline globally. The extent to which protected area (PA) management effectively prevents mangrove loss globally across differing management objectives and governance types is not well understood. We combined remote sensing data with PA information to identify the extent and the drivers of mangrove loss across PAs with distinct governance types and protection levels based on categories developed by the International Union for Conservation of Nature (IUCN). Mangrove loss due to storms and erosion was prevalent across all governance types and most IUCN categories. However, the extent of human-driven loss differed across governance types and IUCN categories. Loss was highest in national government PAs. Private, local, shared arrangement, and subnational government agencies had low human-driven mangrove loss. Human-driven loss was highest in PAs with the highest level of restrictions on human activities (IUCN category I) due to mangrove conversion to areas for commodity production (e.g., aquaculture), whereas PAs that allowed sustainable resource use (e.g., category VI) experienced low levels of human-driven mangrove loss. Because category I PAs with high human-driven loss were primarily governed by national government agencies, conservation outcomes in highly PAs might depend not only on the level of restrictions, but also on the governance type. Mangrove loss across different governance types and IUCN categories varied regionally. Specific governance types and IUCN categories thus seemed more effective in preventing mangrove loss in certain regions. Overall, we found that natural drivers contributed to global mangrove loss across all PAs, whereas human-driven mangrove loss was lowest in PAs with subnational- to local-level governance and PAs with few restrictions on human activities.

Factores globales en la pérdida de manglares en las áreas protegidas Resumen A pesar del incremento en los esfuerzos e inversión de la conservación de los manglares, su cobertura sigue disminuyendo en todo el mundo. No se conoce muy bien el grado al que el manejo de las áreas protegidas (AP) previene eficientemente la pérdida mundial de los manglares en los diferentes objetivos de manejo y tipos de gestión. Combinamos los datos de teledetección con información de las AP para identificar el grado y los factores de la pérdida de manglares en las AP con tipos de gestión claros y niveles de protección basados en las categorías desarrolladas por la Unión Internacional para la Conservación de la Naturaleza (UICN). La pérdida por tormentas y erosión fue común en todos los tipos de gestión y en la mayoría de las categorías de la UICN. Sin embargo, el grado de pérdida antropogénica difirió entre los tipos de gestión y las categorías de la UICN. La pérdida fue mayor en las AP de gobiernos nacionales. Las agencias privadas, locales, de acuerdo compartido y las gubernamentales subnacionales tuvieron una pérdida antropogénica baja. La pérdida antropogénica fue mayor en la AP con el nivel más alto de restricción para las actividades humanas (categoría I de la UICN) debido a la conversión del manglar en áreas de producción de mercancía (p. ej.: acuacultura), mientras que las AP que permiten el uso sostenible de los recursos (p. ej.: categoría VI) tuvieron niveles bajos de pérdida antropogénica. Ya que las AP de categoría I con mayor pérdida antropogénica están gestionadas principalmente por agencias gubernamentales, puede que los resultados de conservación en las AP con mayor pérdida dependan no sólo del nivel de restricciones sino también del tipo de gestión. La pérdida del manglar en los diferentes tipos de gestión y en las categorías de la UICN varió en cada región. Por lo tanto, los tipos específicos de gestión y las categorías de la UICN parecen ser más eficientes en la prevención de la pérdida de manglares en ciertas regiones. En general, encontramos que los factores naturales contribuyen a la pérdida mundial del manglar en todas las AP, mientras que la pérdida antropogénica fue más baja en las AP con un nivel de subnacional a local de gestión y en las AP con pocas restricciones para la actividad humana.

Global hotspots of salt marsh change and carbon emissions.

Salt marshes provide ecosystem services such as carbon sequestration1, coastal protection2, sea-level-rise (SLR) adaptation3 and recreation4. SLR5, storm events6, drainage7 and mangrove encroachment8 are known drivers of salt marsh loss. However, the global magnitude and location of changes in salt marsh extent remains uncertain. Here we conduct a global and systematic change analysis of Landsat satellite imagery from the years 2000-2019 to quantify the loss, gain and recovery of salt marsh ecosystems and then estimate the impact of these changes on blue carbon stocks. We show a net salt marsh loss globally, equivalent to an area double the size of Singapore (719 km2), with a loss rate of 0.28% year-1 from 2000 to 2019. Net global losses resulted in 16.3 (0.4-33.2, 90% confidence interval) Tg CO2e year-1 emissions from 2000 to 2019 and a 0.045 (-0.14-0.115) Tg CO2e year-1 reduction of carbon burial. Russia and the USA accounted for 64% of salt marsh losses, driven by hurricanes and coastal erosion. Our findings highlight the vulnerability of salt marsh systems to climatic changes such as SLR and intensification of storms and cyclones.

Future carbon emissions from global mangrove forest loss.

Mangroves have among the highest carbon densities of any tropical forest. These 'blue carbon' ecosystems can store large amounts of carbon for long periods, and their protection reduces greenhouse gas emissions and supports climate change mitigation. Incorporating mangroves into Nationally Determined Contributions to the Paris Agreement and their valuation on carbon markets requires predicting how the management of different land-uses can prevent future greenhouse gas emissions and increase CO2 sequestration. We integrated comprehensive global datasets for carbon stocks, mangrove distribution, deforestation rates, and land-use change drivers into a predictive model of mangrove carbon emissions. We project emissions and foregone soil carbon sequestration potential under 'business as usual' rates of mangrove loss. Emissions from mangrove loss could reach 2391 Tg CO2 eq by the end of the century, or 3392 Tg CO2 eq when considering foregone soil carbon sequestration. The highest emissions were predicted in southeast and south Asia (West Coral Triangle, Sunda Shelf, and the Bay of Bengal) due to conversion to aquaculture or agriculture, followed by the Caribbean (Tropical Northwest Atlantic) due to clearing and erosion, and the Andaman coast (West Myanmar) and north Brazil due to erosion. Together, these six regions accounted for 90% of the total potential CO2 eq future emissions. Mangrove loss has been slowing, and global emissions could be more than halved if reduced loss rates remain in the future. Notably, the location of global emission hotspots was consistent with every dataset used to calculate deforestation rates or with alternative assumptions about carbon storage and emissions. Our results indicate the regions in need of policy actions to address emissions arising from mangrove loss and the drivers that could be managed to prevent them.

Global declines in human-driven mangrove loss.

Global mangrove loss has been attributed primarily to human activity. Anthropogenic loss hotspots across Southeast Asia and around the world have characterized the ecosystem as highly threatened, though natural processes such as erosion can also play a significant role in forest vulnerability. However, the extent of human and natural threats has not been fully quantified at the global scale. Here, using a Random Forest-based analysis of over one million Landsat images, we present the first 30 m resolution global maps of the drivers of mangrove loss from 2000 to 2016, capturing both human-driven and natural stressors. We estimate that 62% of global losses between 2000 and 2016 resulted from land-use change, primarily through conversion to aquaculture and agriculture. Up to 80% of these human-driven losses occurred within six Southeast Asian nations, reflecting the regional emphasis on enhancing aquaculture for export to support economic development. Both anthropogenic and natural losses declined between 2000 and 2016, though slower declines in natural loss caused an increase in their relative contribution to total global loss area. We attribute the decline in anthropogenic losses to the regionally dependent combination of increased emphasis on conservation efforts and a lack of remaining mangroves viable for conversion. While efforts to restore and protect mangroves appear to be effective over decadal timescales, the emergence of natural drivers of loss presents an immediate challenge for coastal adaptation. We anticipate that our results will inform decision-making within conservation and restoration initiatives by providing a locally relevant understanding of the causes of mangrove loss.