Establishing relative sea level trends where a coast lacks a long term tide gauge

Vulnerability assessment of coastal areas to projected sea level rise requires incorporation of historic trends in relative sea level change as an exposure factor. Most shorelines of developing countries lack long term tide gauges, such as the Pacific Islands region, which are especially vulnerable to climate change impacts. This study has the objective of demonstrating how long-term relative sea level trends can be derived from proxy records, on the tectonically unstable main island of Fiji. At Tikina Wai on the western coast, while elevations of present mangrove zones of Rhizophora stylosa, Rhizophora samoensis and Bruguiera gymnorrhiza were <1.2 m around mean sea level, sediment cores down to 3 m showed mangrove occurrence meters lower than they can grow today. Pollen analysis identified past locations of these mangrove species zones, and the present day elevations of the species were used to reconstruct past sea levels. Results of this study showed that relative sea-level has been slowly rising for the last several centuries at about 2.1 mm a⁻¹, yet mangrove communities have remained resilient with nearly equivalent net sedimentation rates, though with some zone retreat landwards. With such local subsidence, the Tikina Wai district is more exposed to future sea level rise projections than stable coastal areas elsewhere, with additional exposure in having a micro-tidal range. Adaptation actions identified to address this risk include enhancement of sedimentation under mangrove communities through coastal and catchment planning to remove obstructions to sediment supply, reducing non-climate stresses to increase organic production, and replanting of degraded areas. Such information on relative sea level trends can be used to identify where adaptation resources are best concentrated.