Tidal wetlands provide critical ecosystem functions for coastal communities including flood protection, water filtration, carbon sequestration and aquatic nursery habitat. However, New York City's salt marshes, including our study site at Pelham Bay Park's Turtle Cove, are rapidly disappearing due to accelerating relative sea-level (RSL) rise and coastal development. Field research, mapping and satellite imagery reveal significant loss of this similar to 10 hectare (ha) wetland, as perturbations from human activity prevent marsh landward migration, impede tidal flows and threaten marsh survival. We extracted three sediment cores and conducted 20 m transects across a gradient of disturbed marsh areas. We present the analyses of land-use change, X-ray fluorescence (XRF), loss on ignition (LOI), stable carbon isotopes (delta 13C), foraminifera, and accelerator mass spectrometry (AMS) radiocarbon dating of terrestrial macrofossils to examine the past and to inform future conditions for this rapidly eroding wetland. Moreover, we reconstruct sea level over a millennium to analyze changes in marsh plant communities in response to RSL rise and coastal development. We found that between 1974 and 2018 CE, similar to 65% of marsh disappeared at a rate of 1.5% yr-1 or 800 m2 yr-1. The marsh loss coincided with increasing RSL rates of 3.5 mm yr-1 from 1958-1975 CE to 6.7 mm yr-1 from 1999-2024 CE. Meanwhile, developed areas expanded 568 m2 yr-1 from 1985-2023 CE, replacing wetland areas and disrupting hydrologic processes with hardened shorelines. Marsh loss resulted in the release of soil organic carbon stored over many centuries and a concerning amount of lead (Pb) into Long Island Sound, presenting risks to public health and wildlife. Culvert assessments demonstrated that tidal restriction by built structures contributed to rising tide levels comparable to RSL rise over the past century, which likely exacerbated marsh erosion. Lastly, tidal prism reductions caused enough accumulation of heavy metals to significantly alter peat chemical composition for a century. This study improves our understanding of compounded stressors that prevent the capacity of salt marshes to with stand anthropogenic impacts. Ultimately, our findings inform an adaptive management of these threatened ecosystems in their struggle to keep pace with climate change and urbanization.

Coastal agriculture faces escalating threats from seawater intrusion (SWI), jeopardizing global food security through freshwater scarcity, soil salinization and crop damage. However, research on SWI often fails to consider its impact on coastal agriculture. Linking georeferenced SWI data with cropland presence, this review examines SWI's global distribution and primary drivers. Major attested hotspots include the Mediterranean, South and South-East Asia, and the Bohai Sea region in China. Approximately 87 Mha of cropland globally are vulnerable due to low elevation and coastal proximity, including regions where little to no literature has documented SWI. Main drivers include sea-level rise (SLR), drought, groundwater depletion, river modifications, tidal flooding and subsidence. Projections of SLR indicate cropland of North America, the Indian Subcontinent, and South-East Asia as high-risk for SWI. Additionally, regions like South-East Asia and the Indian Subcontinent are expected to experience significant demographic growth in coastal areas. Understanding present and future SWI dynamics is crucial for designing effective mitigation and adaptation strategies in coastal agriculture to support food supply.

Approximately 11% of the world's population lives within 10 km of an ocean coastline, a percentage that is likely to increase during the remainder of the 21st century due to urbanization and economic development. In the presence of climate change, coastal communities will be threatened by increasing damages due to sea-level rise (SLR), accompanied by hurricanes, storm surges and coastal inundation, shoreline erosion, and seawater intrusion into the soil. While the past decade has seen numerous proposals for coastal protection using adaptation methods to deal with the deep uncertainties associated with a changing climate, our review of the potential impact of SLR on the resilience of coastal communities reveals that these adaptation methods have not been informed by community resilience or recovery goals. Moreover, since SLR is likely to continue over the next century, periodic changes to these community goals may be necessary for public planning and risk mitigation. Finally, community policy development must be based on a quantitative risk-informed life-cycle basis to develop public support for the substantial public investments required. We propose potential research directions to identify effective adaptation methods based on the gaps identified in our review, culminating in a decision framework that is informed by community resilience goals and metrics and risk analysis over community infrastructure life cycles.