Coastal hardening and what it means for the world's sandy beaches

Abstract

Sea level rise is expected to cause chronic shoreline retreat, affecting over 10% of the world's population in low-elevation coastal zones (LECZ). Sandy beaches supported by man-made structures are more vulnerable to substantial beach area loss due to restricted landward migration, affecting ecosystem services. However, the global extent of 'hardened' sandy coastlines at risk of 'severe beach loss' has not been comprehensively quantified. Here, using products and geospatial datasets derived from satellite imagery, we find that, 33% of the world's sandy coastline is currently hardened. Analysis of the results by IPCC AR6 regions show that the most hardened coastal regions are the Bay of Bengal, Western & Central Europe, the Mediterranean, Western North America, and East Asia. Linking coastal hardening information with existing shoreline retreat projections, we find that under SSP5-8.5, up to 26% of the world's sandy coastline (~52,100 km) is likely to face severe beach loss by the end of the 21^st century. Under a moderate emissions scenario (SSP2-4.5), this projection decreases to 21% (~42,080 km), highlighting the potential mitigation gain. The results also show that the vast majority of projected severe sandy beach losses are in high and upper-middle-income countries, with up to 82% of severe losses under SSP5-8.5 and up to 81% under SSP2-4.5 by the end of the 21^st century, relative to the total length of sandy coastline, including both hardened and natural coastlines. The IPCC AR6 regions with the highest projected sandy beach losses include Eastern North America, Northern South America, the Mediterranean, the Bay of Bengal, Western Africa, and South-east Asia.

Fig. 1. Percentage of sandy coastline that is hardened per IPCC AR6 reference region and the corresponding available erodible beach widths.

a Shows the percentage of sandy coasts with rigid human-made (semi-)impermeable structures that alter the natural landscape, and potentially obstruct shoreline retreat and landward translation per IPCC AR6 reference region. The results are based on GISA-10m plus OSM. The global average value is shown in the text box. b shows the probability distribution of erodible hardened beach widths per region (i.e., the distance between the shoreline and the nearest gray infrastructure). The results are based on the average distance obtained from GISA-10m plus OSM. The blue box represents the 5^th–95^th percentile range, the red vertical line shows the median values, and the black whiskers on each side of the blue box extends to minimum and maximum values. Map data from OpenStreetMap.

Fig. 2. Percentages of the sandy coast in each IPCC AR6 region projected to face severe beach losses under SSP2-4.5 and SSP5-8.5 by mid- and end-21^st century.

Severe beach loss is defined as either a hardened sandy beach facing shoreline retreat >100 m or a hardened sandy beach facing up to or more than the distance from the present-day shoreline to the nearest gray infrastructure when shoreline retreat <100 m. The bars represent median estimates, indicative of a ‘most likely’ scenario using GISA-10m plus OSM, along with the error bars showing the 5^th–95^th percentile ranges. Globally averaged values, together with the associated 5^th–95^th percentile ranges, are shown in the text box next to each subplot. (The bottom subplot shows percentages of sandy beaches that are hardened in each IPCC AR6 region (gray bars) to illustrate the relationship between hardening and severe beach loss projections.) Map data from OpenStreetMap.

Fig. 3. Income status of sandy coasts that are hardened and most likely to face severe beach loss.

a Shows the percentage of the sandy coastline that is hardened (based on GISA-10m plus OSM), and b depicts the percentage of sandy coastline that is most likely (i.e., median values) to face severe sandy beach loss by mid- and end-21^st century, classified by country income type based on the 2022 Gross National Income (GNI) per capita from the World Bank.

Fig. 4. Workflow for projecting severe beach loss.

This flowchart illustrates the methodology used to identify sandy locations along a global ice-free coastline that are at risk of severe beach loss. At each of the 1,053,613 analyzed shore-normal transects (spaced 500 m apart), the flowchart first identifies sandy beach transects. For each sandy beach transect, it assesses whether gray infrastructure exists within a 1 km search sector (i.e., a 90° shore-normal quadrant with a 1 km radius). If infrastructure is present, the coastline is considered “hardened”. Further, shoreline retreat is evaluated. If the projected retreat exceeds 100 m, severe beach loss is expected. For retreats less than 100 m, it is checked whether the retreat meets or exceeds the distance to the nearest infrastructure. If so, severe beach loss is expected.