Shifting baselines and maritime jurisdiction in the Pacific

Abstract Sea-level rise (SLR) creates uncertainty for maritime jurisdiction under the United Nations Convention on the Law of the Sea (UNCLOS), particularly where coastal baselines may change over time. This study develops a regional geospatial dataset from deposited baseline coordinates, official charts and delimitation records to assess how maritime zones across the Pacific could contract under 1 m, 2 m and 5 m SLR scenarios if baselines were treated as ambulatory. Using CoastalDEM90 elevation data and maritime boundary modelling in ArcGIS Pro, the analysis estimates potential changes to the outer limits of jurisdiction across Pacific Island states and selected overseas territories. Results show uneven projected exposure. Regional contraction remains modest at lower scenarios, reaching approximately 1.19% at 1 m and 1.29% at 2 m, but increases to 6.50% under 5 m SLR. Some jurisdictions face substantially higher proportional contraction due to reliance on low-lying peripheral features that contribute to present maritime limits. These findings do not predict legal outcomes. They identify the spatial consequences that could arise under one contested legal interpretation and show why emerging stability-oriented legal practice matters. The study demonstrates the value of integrating geospatial analysis with legal scholarship and highlights implications for baseline stability, fisheries governance and equity in a changing ocean.