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WIREs Clim Change

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

Focus Article

Virginie K. E. Duvat

Published Online: Oct 25 2018

DOI: 10.1002/wcc.557

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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 larger 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. This article is categorized under: Assessing Impacts of Climate Change > Observed Impacts of Climate Change Paleoclimates and Current Trends > Earth System Behavior

Contrasting between‐ and within‐island shoreline changes: Examples of Tuamotu atolls. (a), (b) and (c) show 1966–2013 shoreline (i.e., stability line) changes on the eight most populated and developed islands of Rangiroa, while (d) shows 1962–2014 shoreline changes on the settled island of Tikehau. (a) and (d) illustrate the major role of land reclamation in lagoon shoreline advance. (a) and (c) show hoa infilling, which led to the aggregation of islands Nos 1 and 2. (a) and (c) show examples of large settled islands that exhibited ocean shoreline retreat. Of note, the latter was caused by extensive sediment mining on islands Nos 1 and 4

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Decadal change in island land area for 709 Pacific and Indian Ocean islands. The blue dotted lines correspond to the ±3% threshold. Twenty islands are excluded from this graph, that is, 17 islands >200 ha (7 islands from French Polynesia, 3 from the Republic of the Marshall Islands, 2 from Kiribati, 4 from Tuvalu and 1 from the Maldives) and three islands exhibiting extreme (falling between 125.5% and − 23.2%) values of change (1 from the Republic of the Marshall Islands, 1 from the Maldives and 1 from Tuvalu). The amplitude and direction of change vary with island size. Importantly, none of the islands larger than 10 ha underwent a reduction in size

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Behavior of Pacific and Indian Ocean atolls and islands under sea‐level rise (see Table S1 for detailed results). Stability, increase and decrease in land area are defined based on the commonly used ±3% threshold. Island land area was obtained using the vegetation or stability line shoreline proxy (see Table for details). Atoll land area corresponds to the sum of the land areas of the documented islands of a given atoll. Of note, no atoll exhibited a decrease in land area over the past decades to century. Between‐ and within‐atoll island behavior varied significantly, but it shows no relationship with the rate of sea‐level rise

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Spatial distribution of documented atoll islands (see Table for complementary information)

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Changes affecting islets: Examples of Tuamotu atolls. (a) and (b) show changes affecting some of Tikehau's southern islets, while (c) shows changes affecting Takaroa's western islets. Of note, (a) shows the disappearance of two islets (Nos 12 and 14), and major changes in some islets' configuration and position (Nos 5 and 10). (a) and (b) show that most islets were rather stable in position. (b) Illustrates the retreat of the hoa shorelines of an islet severely affected by the July 1996 distant‐source southern swell. (c) Shows significant ocean shoreline advance (islets Nos 16 to 19) and embayment infilling (islets Nos 17 and 19)

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