A global analysis of how human infrastructure squeezes sandy coasts

Coastal zones, encompassing diverse landforms, provide crucial ecosystem services. Sandy coasts, covering one-third of ice-free shorelines, support interconnected habitats offering services like flood defense, carbon storage, and biodiversity. Human dependence on these services is increasing, with approximately 40% of the global population residing within 100 km of the shoreline. However, human activities are rapidly degrading these ecosystems. Global threats such as sea-level rise and extreme weather events are exacerbated by local impacts like eutrophication and pollution. A major local disturbance is infrastructure development close to the shore, restricting space for ecosystems and impeding cross-ecosystem processes. This space reduction hinders adaptation to sea-level rise through landward retreat, a phenomenon known as 'coastal squeeze'. While shoreline changes have been extensively studied, a global assessment of infrastructure's impact on coastal squeeze has been lacking. This research addresses this gap by analyzing the proximity of human infrastructure to the world's sandy shores to quantify the extent of coastal squeeze and identify key drivers.

Existing research highlights the significant threats of coastal erosion and sea-level rise to shoreline ecosystems. Studies have focused on shoreline changes and the impacts of climate change, but a comprehensive global assessment of how human infrastructure constricts coastal ecosystems from the landward side remained absent. This study builds on previous work investigating the value of coastal ecosystem services, the degradation of these services due to human activities, and the challenges posed by sea-level rise and extreme weather events. It addresses the need for a global-scale analysis of coastal squeeze caused by infrastructure development.

This study analyzed the proximity of human infrastructure to sandy shores using a global dataset of 235,469 transects (25 km long, 1 km apart), representing 29% of the world's ice-free shoreline. Data from OpenStreetMap and Global Urban Footprint were used to determine the distance from the shoreline to the nearest paved road or building, defining the infrastructure-free width. Transects with natural barriers (cliffs) were identified and excluded. Socio-economic variables (population density and GDP) were used in a multiple regression model to explain variations in infrastructure-free width. The study also compared infrastructure-free widths in areas with and without nature reserve protection. Finally, the percentage of remaining infrastructure-free space under various sea-level rise projections was calculated using data from Vousdoukas et al. (2020). The analysis involved careful selection of datasets to ensure the highest accuracy and detail, considering aspects such as coastline definition (OpenStreetMap), infrastructure data (OpenStreetMap and Global Urban Footprint), and elevation data (CoastalDEM) to identify natural barriers. Statistical methods included multiple linear regression to assess the impact of socioeconomic factors and Wilcoxon rank-sum tests and Kruskal-Wallis tests to compare infrastructure-free widths in protected and unprotected areas, and in urban and rural settings. Sea-level rise projections were integrated to estimate the future loss of infrastructure-free space.

The analysis revealed a widespread presence of human structures along sandy shores. Of the analyzed transects, 28% were naturally limited by coastal geometry before encountering infrastructure. Of the remaining transects, 93% had infrastructure within 25 km of the coast. The median distance to the nearest structure was only 392 meters, with 33% of global sandy shores having less than 100 meters of infrastructure-free space. When considering only heavy infrastructure (buildings and highways), the median width increased to 1.6 km, but 28% of shores still had such structures within 100 meters of the waterline. Infrastructure proximity was greater in densely populated areas, particularly between 32 and 45 degrees North latitude. Europe had the most severe coastal squeeze (median: 131 m), followed by Asia, North America, South America, Africa, and Oceania. A multiple regression model showed that population density and GDP explained 35–39% of the variance in infrastructure-free width. Protected shores had a significantly greater infrastructure-free width (four to seven times greater depending on the type of infrastructure considered) than unprotected shores. Most protected areas were located in rural areas. Projections using sea-level rise data suggest that 23–30% of the world's sandy shores will lose their remaining infrastructure-free space by 2100.

The findings demonstrate that human-made infrastructure significantly impacts the world's sandy coasts, severely limiting their ability to adapt to sea-level rise. The strong correlation between coastal squeeze and population density and GDP highlights the increasing pressure from development and population growth. The limitation of space hinders natural processes such as sediment transport and landward migration, threatening ecosystem functions. The significant protective effect of nature reserves underscores the importance of conservation efforts in preserving coastal resilience. While nature reserves provide some buffer, their limited coverage (16% of sandy shores) highlights the need for broader policy interventions. The study's limitations include the reliance on existing datasets, which may not capture all infrastructure or shoreline changes accurately. The projections for sea-level rise also contain uncertainties. Despite these limitations, the findings emphasize the urgent need for integrated spatial planning that considers both development and conservation needs to address coastal squeeze effectively.

This global analysis demonstrates the severe impact of human infrastructure on the world's sandy coasts, restricting their ability to adapt to sea-level rise and threatening ecosystem services. The strong influence of socioeconomic factors and the significant protective role of nature reserves highlight the need for policy changes integrating nature protection into spatial planning. Future research could refine the analysis by incorporating high-resolution data, exploring alternative infrastructure development strategies, and investigating the effectiveness of various coastal management techniques in mitigating coastal squeeze.

The study's reliance on publicly available datasets may have limitations. The accuracy of the infrastructure data and the representation of all types of coastal infrastructure might vary across regions. The projections for future sea-level rise incorporate uncertainties, and the responses of beaches to these changes may differ based on local geomorphological conditions. The correlative nature of the relationship between protected areas and reduced coastal squeeze prevents definitive causal inferences. The study's focus on sandy shores also limits its generalizability to other coastal ecosystems.