Coastal wetlands are located at the intersection of land and sea and provide extremely important ecological services. The coastal wetlands of estuarine harbors are representative parts of the coastal wetlands. Changes that occur in estuarine harbor wetlands are microcosms of the changes occurring in the coastal wetlands more generally. The coastal wetlands of Hainan Island, China, are coastal wetlands typical of tropical islands and are extremely sensitive to climate change. In the context of global sea level rise, studying the characteristics of spatial and temporal distribution of coastal wetlands on Hainan Island, as well as changes in their vulnerability, could provide scientific and technological support to address the adverse effects of climate change. Using nine typical estuarine harbor wetlands as target areas, this study systematically studies the spatial–temporal evolution of coastal wetlands on Hainan Island from 1990 to 2020. The results suggest the following: (1) The total area of coastal wetlands has remained relatively stable, but the area of artificial wetlands, especially aquaculture ponds, has increased significantly. There is a clear spatial variability in the changes in mangrove wetlands, with a clear increase in the area of areas with a high degree of protection, such as Dongzhai Harbor (DZG). The area of the areas with a high intensity of human activity has been significantly reduced, such as Bamen Bay (BMG). (2) The overall ecological risk of coastal wetlands is low, with the average wetland risk index (WRI) of all harbors being below 0.15. The higher the degree of protection, the lower the ecological risk of the area, such as DZG. Human activities are the main factor causing increased ecological risk in wetlands. (3) Climate-change-induced sea level rise and the intensification of human activities are the main determinants of future trends in the spatial distribution of coastal wetlands and wetland ecosystem stability. The results of this study provide guidance on the conservation and restoration of coastal wetlands.

Sub-cloud evaporation changes the isotopic composition of precipitation, which greatly reduces the reliability of precipitation isotopic data as precipitation simulation data. This study employed the precipitation isotope datasets of Haikou in northern Hainan Island from June 2020 to February 2024 to quantitatively study the influence of sub-cloud evaporation on precipitation isotopes in tropical islands. Due to the sub-cloud evaporation, the slope of the local meteoric water line (LMWL: δ²H = 8.33δ¹⁸O + 14.33) is lower than the average slope of the theoretical LMWL (8.48). The average value of the residual ratios of raindrop after evaporation (f) is 86%. The complex and unstable sources of water vapor result in no obvious seasonal variations in the atmospheric humidity, which in turn leads to no obvious seasonal variations in Δd and f. The humid and hot environmental conditions reduced the impact of sub-cloud evaporation on precipitation isotopes. The two main uncertainties in the simulation of below-cloud evaporation are the influence of recycled water vapor on precipitation isotopes and the Stewart model’s assumption that raindrops at the cloud base achieve isotopic equilibrium with the surrounding water vapor, as it is difficult to realize. The results of this study are of great significance for improving the accuracy of precipitation simulation in tropical monsoon islands.