UC Santa Cruz

Climate change threatens established communities on low-lying Pacific atoll islands by increasing their vulnerability to wave-driven flooding and shoreline instability. Atoll islands are dynamic features whose morphology can quickly respond to changing climate, sea level, and wave conditions. Results from this thesis provide new information about how future wave climates may change near Pacific atoll islands, specific projections for flooding and erosion at two particular atolls, and insights into how atoll morphology affects shoreline susceptibility to erosion. The first study projects how wave climates may change in the western tropical Pacific by 2100 at 25 island locations under two greenhouse gas emissions scenarios. Output from four global climate models were used to run a dynamically-downscaled numerical wave model. The mean of the top 5% of future boreal winter and summer wave heights, and their associated directions and frequencies, were calculated. Large winter wave heights were projected to decrease throughout the study area, and summer large wave directions were projected to change by as much as 30°. These changes were incorporated into the second study, which modeled run-up and erosion along island shorelines at Wake and Midway Atolls under differing oceanographic forcing conditions. Sea-level rise was projected to be the dominant control on flooding and morphological change on these two atolls, indicating that atoll islands are at great risk even if future wave heights decrease. Small changes in the direction of large incident waves also had a significant impact on shoreline flooding and erosion. Better-protected shorelines (e.g., lagoon shorelines) were disproportionately affected by increasing sea level and changing wave directions. While these findings were site specific for two atypical atolls, the third study involved modeling shoreline erosional change on a schematic atoll and evaluating how initial morphology affects shoreline response. Bathymetry for a generic atoll was generated with varying morphological parameters and wave-driven erosion was calculated. The parameters that had a significant impact on shoreline erosion were: reef flat width, reef flat depth, island width, and atoll diameter. Atolls with narrower, deeper reef flats, narrower islands, and smaller diameters were most susceptible to shoreline instability with sea level rise. Windward islands are projected to lengthen and migrate toward the lagoon, leeward islands are projected to lengthen and migrate toward the reef rim, and oblique islands are projected to migrate leeward and toward the lagoon. Findings from all three studies elucidate shoreline processes along atoll islands and provide critical information that can guide aid allocation and help communities plan for future hazards.