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 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.

Contrasting potential for nature-based solutions to enhance coastal protection services in atoll islands

“…10.7%), while the islands < 10 ha underwent more contrasting behaviours, with 65.1% of these islands experiencing stability, while respectively 17.0% and 17.9% showed contraction and expansion. The high instability of very small islands makes some atoll countries, such as the Maldives, particularly vulnerable to climate change (Aslam & Kench, 2017;Duvat, 2019). More generally, these findings highlight, first, that climate change, particularly sea-level rise, does not constitute the main driver of atoll island change to date, and second, that until now, most atoll islands were able to naturally adapt to ocean-climate related pressures (McLean & Kench, 2015;Duvat, 2019).…”

“…rates ranging from 2.0 ±0.6 mm/yr (on Pingelap and Mokil atolls, the Federated States of Micronesia) to 5.1 ±0.7 mm/yr (on Funafuti, Tuvalu), with most values lying between 2 and 3 mm/yr (Becker et al, 2012). Despite accelerated sea-level rise over the past decades, only 11.4% of the 709 sample islands decreased in size, while 15.5% increased and 73.1% remained stable in area (Duvat, 2019). In addition, all of the islands > 10 ha experienced either areal stability (209/234 islands, i.e.…”

“…Since 2010, the scientific community has quantitatively assessed multi-decadal (i.e. for the past three or four decades to a century) land area change for 709 islands from 30 atolls in the Pacific and Indian Oceans, thereby providing data on island behaviour under contemporary sea-level rise (Duvat, 2019). Results put an end to the already-sinking-island scenario by proving the persistence of atoll islands and the maintenance of atoll countries' landmass despite higher than average (1.8 mm/yr) rates of sea-level rise over the past decades, i.e.…”

“…Male' in the Maldives (Naylor, 2015), South Tarawa in Kiribati (Biribo & Woodroffe, 2013;Duvat, Magnan, & Pouget, 2013), and Fongafale in Tuvalu (Yamano et al, 2007), but also in rural and even in unsettled islands Duvat, Salvat, & Salmon, 2017). In most cases, human-driven changes mainly consisted in land reclamation, carried out to face land shortage, and in coastal infrastructure (airport and harbour) development (Duvat, Salvat, & Salmon, 2017;Duvat, 2019). For example, in the Maldives, land reclamation caused the highest rates of island and atoll expansion recorded (Aslam & Kench, 2017;Fallati, Salvini, Strelacchini, & Galli, 2017), while also causing widespread shoreline armouring (due to the stabilisation of reclaimed areas by seawalls, dykes, and rip-rap) and extended reef degradation (due to the burial or mechanical destruction of reef flats by aggregate mining).…”

“…Male', which is protected by emerged breakwaters and massive rock revetments along most of its shoreline (Naylor, 2015), is the exception. In atoll environments, the use of hard defence significantly also contributes to the degradation of the reef ecosystem, because the material used to build protection structures is often directly extracted from nearby reef flats and beaches (Biribo & Woodroffe, 2013;Duvat, 2013), while also undermining the capacity of islands to naturally adjust to ocean-climate related pressures through sediment reorganisation by obstructing reef-to-island sediment transport pathways (McLean & Kench, 2015;Duvat, 2019). This increases not only current, but also future climate-related risks.…”