Positional and Morphological Adjustment of Coral Reef Islands Due to Sea Level Rise

Low-lying coral reef islands are considered physically vulnerable to erosion in response to sea-level rise. Webb and Kench (2010) analyzed the physical change in 27 atoll islands located in the central Pacific Ocean over the past 20 to 60 yr, a period over which instrumental records indicate an increase in sea level of the order of 2.0 mm yr− 1. They found that 86% of islands remained stable or increased in area over the timeframe of analysis. Only 14% of study islands exhibited a net reduction in island area. Despite small net changes in area, islands exhibited larger gross changes in island surface configuration and location on the reef platform. Over 65% of islands examined have migrated toward the lagoon. These results contradict widespread perceptions that all reef islands are eroding in response to recent sea level rise. The data illustrate that reef islands are geomorphically resilient landforms that thus far have predominantly remained stable or grown in area over the last 20–60 years. Given this positive trend, reef islands may not disappear from atoll rims and other coral reefs in the near future. However, islands will undergo continued geomorphic change. The pace of geomorphic change may increase with future accelerated sea level rise. The style and magnitude of geomorphic change will likely vary between islands. Therefore, island nations must better understand the pace and diversity of island morphological changes and reconsider the implications for adaptation.¾Michelle Schulte
Webb, A., Kench, P.S., 2010. The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis of island change in the Central Pacific. Global and Planetary Change 72, 234–246.

This study examines the morphological change of 27 atoll islands located in the central Pacific. The islands are located in three Pacific countries, in four atolls, and span 15° of latitude from Mokil atoll in the north (6°41.04′ N) to Funafuti in the South (8°30.59′S). The atolls examined include Funafuti, Tarawa, Pingelap, and Mokil. The atolls vary significantly in terms of size, structure and number of islands distributed on the atoll rim. The atolls also vary in potential exposure to tropical cyclones. All 27 islands in the study are located on atoll reef rims of Holocene age. A total of 27 islands were examined using comparative analysis of historical aerial photography and remotely sensed images. The timeframe of analysis varied from 19 to 61 years depending on aerial photograph coverage and availability. Using ERDAS Imagine 8.4 software and Quickbird satellite imagery, the images were all rectified and ground control points for each island were established. The analysis involved the overlay of the historical time series for each island. Webb and Kench analyzed the islands for areas of accretion or erosion as well as the configuration and position of the island on reef platforms. Changes in island area were calculated and compared to establish change through time.
Webb and Kench show that all islands have undergone physical change over the respective timeframes of analysis and over the period in which the instrumental records indicate an increase in sea level. The data indicate that islands have undergone contrasting morphological adjustments over the period of analysis. Furthermore, the magnitude and styles of island change show considerable variation both within and between atolls in the study. Only 43% of islands have increased in area by more than 3% while 15% of islands underwent net reduction. The net changes in island area mask larger gross changes in surface configuration and location on the reef platform. Modes of island change include: ocean shoreline displacement toward the lagoon, lagoon shoreline progradation, and, extension of the ends of elongate islands. Over 50% of the examined islands experienced ocean shoreline adjustment via erosion. Additionally, accretion of lagoon shorelines was detected in 70% of the islands. Collectively these adjustments represent net lagoon-ward migration of islands in 65% of cases. The results show that a significant number of islands exhibit ocean shoreline erosion which may reflect shore readjustment to increased sea levels over the study period and potentially increased wave energy incident at shorelines.
Webb and Kench illustrate that reef islands are morphologically dynamic features that can change their position on reef platforms at a range of timescales. The mechanisms that drive island change can include a combination of sea-level rise, decadal-scale variations in wind and wave climate and anthropogenic impacts. This study contradicts existing paradigms of island response and has significant implications for the consideration of island stability under ongoing sea-level rise in the central Pacific. First, islands are geomorphologically persistent features on atoll reef platforms and can increase in island area despite sea-level change. Second, islands are dynamic landforms that undergo a range of physical adjustments in responses to changing boundary conditions, of which sea level is just one factor. Third, erosion of island shorelines must be reconsidered in the context of physical adjustments of the entire island shoreline as an island may experience both erosion and accretion at opposite points. The authors conclude that the style and magnitude of geomorphic change will likely vary between islands. Therefore, island nations must place a high priority on resolving the precise styles and rates of change that will occur over the next century and reconsider the implications for adaptation.

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