National coastal wetland mapping over the last four decades: An annual classification with high accuracy

As a critical component of global wetland ecosystems, coastal wetlands are facing dual pressures from climate change and human activities, which have driven the drastic evolutions of coastal wetlands, highlighting the urgent need for a precise classification dataset with large spatial scale and high temporal resolution. The national coastal wetland mappings year by year over the last four decades are crucial for coastal integrated management, but such studies are scarce due to the limitations on classification accuracy. In this study, a robust and automated classification method was developed based on coastline delineation and integrations of multiple algorithms. By analyzing more than 60,000 Landsat images of China's coastal areas and incorporating field survey validation data, an annual coastal wetland spatial distribution dataset with 30-meter spatial resolution spanning from 1985 to 2024 was established. The dataset obtained an average Kappa coefficient of 0.957 and a range from 0.978 to 0.935, and the average overall accuracy was 97.6% with a range of 97.12% - 97.85%, demonstrating the high accuracy and reliability of the proposed method. The results revealed that China's coastal wetland area in 2024 was 8443.88 km², comprising 4120.286 km² salt marshes, 272.854 km² mangrove forests, and 4050.74 km² tidal flats. Temporal analysis indicates an initial decline, followed by a gradual recovery in recent years, with recent ecological restoration projects playing a significant role in this recovery. Particularly, areas of mangrove distinctly increased under the extensive ecological restoration. The coastal wetland patterns remarkably spatially varied across areas within China. The proposed method has achieved high-precision annual coastal wetland spatial datasets and effectively monitored the impacts of invasive species and ecological restoration projects on interannual changes in coastal wetlands, thereby providing fundamental support for scientific management and sustainable development of China's coastal wetlands.