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.