Significant coastal dune loss challenges California's climate resilience and biodiversity goals

Coastal sand dunes support unique biodiversity and buffer beaches and communities against storm impacts. However, these sensitive and dynamic ecosystems are increasingly threatened by erosion, sea-level rise (SLR), and encroaching coastal development. Restoration projects designed to re-establish the eco-geomorphic functions of degraded dunes and associated ecosystem services present a viable pathway to relieve these stressors. Yet, even for populated coasts like California, insufficient information on the location of dunes and patterns of change over time constrain regional efforts to identify suitable restoration/conservation sites, and forecast future impacts on vulnerable beach–dune systems. Here, we present a comprehensive assessment of coastal dune extent and loss, spanning 165 years and 9.5° of latitude across California. Areas and drivers of change were quantified by comparing historical and current dune extents generated using historical surveys, high-resolution aerial imagery, and LiDAR, combined with a machine-learning tool. Coastal dunes currently extend along 352 km (22%) of California's 1,625 km coastline, covering almost 300 km². This represents a 60% (442 km²) decrease in area since the mid-1800s with the loss, degradation, and fragmentation of many dune ecosystems attributed to urban development, altered land-uses, and erosion. Strong regional patterns of dune loss were evident, reflecting population demographics and land uses. Densely populated areas of southern California experienced the greatest dune losses (95%, 108 km²), although substantial reductions were also found in central California (60%, 331 km²). Our results highlight the magnitude of coastal change in California over time and can inform future conservation, restoration, and resilience-planning efforts and priorities.