Shoreline changes in coral reef islands of the Federated States of Micronesia since the mid-20th century

Abstract

With projections of accelerating sea-level rise, changing weather and wave regimes and increasing intensity of storm events associated with anthropogenic climate change, the physical persistence and consequently habitability of coral reef islands is a global concern. This study presents an analysis of shoreline changes on 104 coral reef islands from 16 atolls in the western equatorial Pacific nation of the Federated States of Micronesia across a period coincident with rising local sea level and a high frequency of storm events. Aerial photographs from the mid-1940s and 1970s were analysed alongside recent high-resolution satellite imagery to document shoreline changes and planform morphological adjustments in islands. Results revealed accretion has been the predominant mode of shoreline change, with 46% of the studied shorelines showing statistically significant accretion leading to a net increase of 64.37 ha (~3%) of planform land area across the archipelago. Three new islands were formed with a cumulative land area of 2.05 ha, conversely, three islands were completely eroded leading to a loss of net land area of 3.20 ha, providing a record of concurrent island formation and loss within a high storm frequency setting. Reef islands in the FSM demonstrated a range of styles of planform geomorphic adjustments including island rotation, expansion, contraction, spit extension, and migration of islands on the reef platform. Results provide empirical evidence of shoreline accretion despite local sea-level rise as well as of heterogeneity in magnitude and style of geomorphic responses of reef islands across FSM with considerable variability at inter and intra-atoll scales.