A global assessment of atoll island planform changes over the past decades

Abstract: Over the past decades, atoll islands exhibited no widespread sign of physical destabilization in the face of sea-level rise. A reanalysis of available data, which cover 30 Pacific and Indian Ocean atolls including 709 islands, reveals that no atoll lost land area and that 88.6% of islands were either stable or increased in area, while only 11.4% contracted. Atoll islands affected by rapid sea-level rise did not show a distinct behavior compared to islands on other atolls. Island behavior correlated with island size, and no island smaller than 10 ha decreased in size. This threshold could be used to define the minimum island size required for human occupancy and to assess atoll countries and territories’ vulnerability to climate change. Beyond emphasizing the major role of climate drivers in causing substantial changes in the configuration of islands, this reanalysis of available data indicates that these drivers explain subregional variations in atoll behavior and within-atoll variations in island and shoreline (lagoon vs. ocean) behavior, following atoll-specific patterns. Increasing human disturbances, especially land reclamation and human structure construction, operated on atoll-to-shoreline spatial scales, explaining marked within-atoll variations in island and shoreline behavior. Collectively, these findings highlight the heterogeneity of atoll situations. Further research needs include addressing geographical gaps (Indian Ocean, Caribbean, north-western Pacific atolls), using standardized protocols to allow comparative analyses of island and shoreline behavior across ocean regions, investigating the role of ecological drivers, and promoting interdisciplinary approaches. Such efforts would assist in anticipating potential future changes in the contributions and interactions of key drivers.

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Conclusion

This review first confirms that over the past decades to century, atoll islands exhibited no widespread sign of physical destabilization by sea-level rise. The global sample considered in this paper, which includes 30 atolls and 709 islands, reveals that atolls did not lose land area, and that 73.1% of islands were stable in land area, including most settled islands, while 15.5% of islands increased and 11.4% decreased in size. Atoll and island areal stability can therefore be considered as a global trend.

Importantly, islands located in ocean regions affected by rapid sea-level rise showed neither contraction nor marked shoreline retreat, which indicates that they may not be affected yet by the presumably negative, that is, erosive, impact of sea-level rise. Second, this review reaffirms that atoll island areal change was mainly influenced by island size. While the smallest islands (<5 ha, 52.90% of islands) exhibited contrasting areal changes (i.e., stability, increase, or decrease in size) and highly variable values of areal change (from −22.7 to +125.5%), the islands larger than 5 ha (47.10% of islands) generally experienced areal and positional stability.

It is noteworthy that no island larger than 10 ha decreased in size, making this value a relevant threshold to define atoll island areal stability. We therefore propose to use this threshold, first, to define the minimum island size required for human occupancy or exploitation, and second, to assess atoll and atoll countries and territories’ vulnerability to climate change. Using this threshold for future island development (e.g., resort island) would considerably limit the risk for new developments to be negatively affected by island areal and positional instability, on condition of also avoiding any human intervention that may alter island sediment budget (e.g., sediment extraction) and natural dynamics (e.g., obstruction of sediment transport and deposition by constructions). In addition, the physical instability of small islands (<10 ha) suggests that atoll countries and territories’ vulnerability to sea-level rise is inversely proportional to the size of the islands composing them. This for example means that the Republic of Maldives (mainly composed of small islands) is, from a geomorphic perspective, more vulnerable to climate change than the French Tuamotu Archipelago (made up of larger islands).

Assessing atolls’ and atoll countries’ vulnerability to climate change using this threshold would offer a first comprehensive overview of atoll status and of atoll countries’ needs in terms of adaptation to climate change. Because they are the most vulnerable, atolls (at the national scale) and atoll countries (at the global scale) having small islands should be the focus of monitoring and assessment activities, and of adaptation efforts.

Third, this paper confirms the highly dynamic nature of some specific atoll island features, such as sand and gravel spits, island extremities, beaches, hoa shores, and ancient hoa areas, which exhibited marked areal and positional changes over the past decades. These changes occurred over short (i.e., several years) to multidecadal timescales, depending on the climate drivers involved (e.g., short term ENSO-influenced beach changes vs. multidecadal shoreline smoothing and spit extension).

The highly dynamic nature of these features indicates the continuous adjustment of island shores to climatic conditions, which in turn implies that it is imperative to limit as much as possible human interventions that may destabilize the fragile equilibrium of such islands. This once again emphasizes the crucial need for a better consideration of island dynamics in development

projects.

Fourth, this paper shows that over the past decades, atoll islands exhibited highly contrasting behaviors across ocean basins and subregions. No distinct regional (i.e., scale of ocean basins or ocean subregions) or subregional (i.e., scale of atoll groups) profiles emerge from this global review. In some cases, nearby atolls exhibited contrasting behaviors, for example, a majority of expanding vs. a majority of contracting islands, or opposite behaviors of their leeward and/or windward sides.

Likewise, within a given atoll, nearby islands and island shorelines (either ocean-facing, or lagoon-facing) commonly experienced opposite behaviors. The patterns of atoll island planform change are resolutely atoll- and even in some cases island-specific.

This conclusion suggests that the atoll and island “shadow effects” (Andrefouët et al., 2012), which contribute to the contrasting responses of nearby atolls and islands to rather similar climatic conditions, play a major role in explaining the contrasting behaviors of atolls, atoll sides, islands and island shorelines, within a given atoll group.