Elsevier

Anthropocene

Volume 33, March 2021, 100282
Anthropocene

Global-scale changes in the area of atoll islands during the 21st century

https://doi.org/10.1016/j.ancene.2021.100282Get rights and content

Highlights

  • Since 2000, land area on 221 atolls examined had increased by 61.74 km2 (6.1 %).

  • 153 atolls increased in size and 68 decreased in size between 2000 and 2017.

  • Most of the increase in land area occurred in the Maldives and South China Sea.

  • Land reclamation was primarily responsible for land area increases on atolls.

Abstract

The long-term persistence of atoll islands is under threat due to continued sea level rise driven by anthropogenic climate change. One widely discussed potential impact of sea level rise is the widespread, chronic erosion of atoll islands. Despite concerns of erosion driven by sea level rise, no published evidence exists of pervasive erosion of atoll islands at a global scale. Existing studies of atoll island change have been based on small, temporally sparse samples of islands on a limited number of atolls. As a result, the global response of atoll islands coincident with sea level rise remains uncertain. Using rich collections of Landsat imagery, this study analyses changes in land area on 221 atolls in the Indian and Pacific Oceans. Results show that, between 2000 and 2017, the total land area on these atolls has increased by 61.74 km2 (6.1 %) from 1007.60 km2 to 1069.35 km2. Most of the change in land area resulted from island building within the Maldives and on atolls in the South China Sea. Since 2000, the Maldives have added 37.50 km2 of land area, while 16.57 km2 of new islands have appeared within the South China Seas Spratly and Paracel chains. Understanding the extent of land area change at the global scale improves insights into the variation in responses, and how the manipulation of land is shaping the potential habitability trajectory for some atoll communities. Results highlight the anthropogenic imprint on the size of atoll islands, thereby providing a better understanding of variations in the future trajectories of human settlement and adaptation within atoll settings.

Introduction

Many of the world’s coasts are considered threatened by erosion as the result of a multitude of anthropogenic and natural stressors (Zhang et al., 2004; Mentaschi et al., 2018). These stressors, generated at both local and global scales, are expected to accelerate as sea level rise driven by climate change places pressure on coastal systems and the ecosystems, communities and economies in which they support. Coastal areas, particularly those along heavily urbanised coasts, are engineered at an accelerating rate, pushing coastlines seaward as wetlands are converted to land, shorelines are advanced and armoured and in places entirely new islands are constructed (Bisaro, 2019). At the forefront of discussions surrounding the erosional impacts of sea level rise on coastal systems are low-lying atoll islands. Atoll islands are typically low-lying landforms, largely found within the world’s tropical oceans. For the nations of Tuvalu, the Maldives and the Republic of the Marshall Islands are the only sites of habitable land. The islands are a product of the deposition of reef-derived material and are typically only a few meters in elevation above sea level. As a result of their small size and low elevation, atoll islands are often considered among the most vulnerable systems with respect to the impacts of climate change (McLean and Kench, 2015).

The future persistence of atoll islands both as geologic features and as sites of human habitation is uncertain. Anthropogenic climate change is expected to increase sea level and ocean temperature and alter the frequency and intensity of tropical storms (Bacmeister et al., 2018). Similarly, changes in ocean temperature and chemistry are expected to reduce the protection services provided by surrounding coral reefs (Harvey et al., 2018) and reduce the generation of biogenic sediment (Perry et al., 2011), leaving islands vulnerable to shoreline erosion, threatening coastal infrastructure and the stability of the islands themselves (Kench and Mann, 2017). Sea level rise, in concert with changes in ocean swell, is expected to increase the frequency of overtopping and inundation of islands, resulting in damage to infrastructure and the intrusion of saltwater into groundwater reserves (Storlazzi et al., 2018). The exposure to these changing environmental conditions has led to the grim prognosis that many atoll-dwellers will be climate change refugees in the coming decades, as their islands become uninhabitable or even disappear (Betzold, 2015).

Many atoll islands have been subjected to the same engineering interventions observed along continental coasts, including land reclamation, shoreline armouring and island building. The human modification of island shorelines is widespread within populated atoll islands, as nations seek to protect urban infrastructure and create more land to accommodate internal migration and economic development (Duvat, 2020; Duvat and Magnan, 2019; Fallati et al., 2017). The active manipulation of island shorelines has been proposed, and in several cases implemented, as an adaptation option for atoll nations in the face of continued sea level rise (Esteban et al., 2019). The scale and drivers of such interventions varies, from local scale reclamations for the development of individual dwellings, to the construction of entirely new land masses at a scale to support populations of several thousand people (Biribo and Woodroffe, 2013; Duvat and Magnan, 2019). Several studies have documented the construction of islands within the South China Sea, where several nations have built islands as tensions over marine boundary disputes have increased (Barnes and Hu, 2016; Asner et al., 2017). Similarly, noteworthy island building has occurred within the Maldives, where islands have been expanded to house tourist resorts, as well as increase the availability of land for the local population and infrastructure (Speelman et al., 2017). Such large-scale reclamation projects have created new islands, such as Hulhumalé Island in the Maldives, which is 1.80-1.90 km2 and currently expanding further (Naylor, 2015; Stojanov et al., 2017). In contrast, most land reclamation and shoreline armouring on Pacific atolls has been smaller in scale to the island building observed in the South China Sea and Maldives. The reclamation has generally been restricted to piecemeal hardening of the shoreline of the densely populated atolls, although in rare cases, the construction of airports (Ford, 2011) and military bases (Hamylton and East, 2012) has resulted in significant island-building.

The prospect of coastal erosion is a key concern for atoll nations and features prominently in political and popular media discussions of the impacts of sea level rise on atoll islands (Donner, 2012). Such proclamations underpin the ‘disappearing islands’ narrative when discussing atoll futures. Despite the widely circulated assertion that atoll islands are actively eroding, however, little scientific evidence exists to support such blanket claims. Records do not exist of widespread, chronic erosion of atoll islands under current rates of sea level rise (Duvat, 2019). Rather, studies have shown that islands have persisted, and in many cases increased in size, since the mid-20th century (Kench et al., 2018; Duvat, 2019). These studies have generally revealed considerable intra- and inter-atoll variability with respect to observed changes in planform island configuration, with no discernible pattern of island change described to date, although in some studies, smaller islands have shown greater variability in change spatially and temporally (Kench et al., 2018; Aslam and Kench, 2017). Most studies have speculated on the role that reef-generated sediment delivery to the island shorelines must play in driving changes on the islands. Robustly identifying and attributing the specific processes responsible for island growth, however, remains a challenge (Duvat, 2019).

Over the past decade, multiple studies have adopted remote sensing approaches to investigate changes in the position of shorelines and the planform area of atoll islands (Table 1). These studies are underpinned by the comparison of planform island morphology interpreted from historic aerial photographs and recent satellite imagery. Studies rely on limited collections of mapping-quality aerial photographs that provide fragmentary records of island morphology from the mid-20th century. These photographs are compared with island morphology interpreted from modern, high-resolution (i.e. < 1 m) satellite imagery. The methodologies applied to analyse atoll island change using remote imagery rely on the interpretation and digitisation of islands within a Geographic Information System (GIS). This approach, while robust, relies on the manual interpretation of shorelines and is therefore subjective, time-consuming and costly. The constraints imposed by the difficulty and cost of acquiring historic and recent remote imagery, along with the interpretation costs, have contributed to a collection of studies with limited spatio-temporal coverage. These limitations further confound attempts to quantify global-scale island change, and to detect and attribute the drivers of change.

Despite concerns surrounding the vulnerability of atoll-islands and numerous local and national-scale assessment of shoreline change, few assessments have adequately assessed how the land area of atolls has changed in the 21st century at a global scale.

In this paper we address the following four questions: (1) To what extent has the land area changed on atolls over the 21 st century? (2) Is there regional variability in the changes in land area on atolls? (3) To what extent are anthropogenic influences key drivers of land area change on atoll islands (4) How can freely available, moderate-resolution satellite imagery contribute to global-scale monitoring of changes on low-lying atoll islands?

Section snippets

Field setting

Global estimates indicate at least 439 atolls, found largely in the tropical Pacific and Indian Oceans (Goldberg 2016). Three countries comprise entirely low-lying atolls: The Maldives, Marshall Islands and Tuvalu. Except for one raised coral island (Banaba), Kiribati comprises exclusively atolls, whereas the Federated States of Micronesia (FSM), Cook Islands and French Polynesia are home to several atolls. This study examined changes on land areas on 279 atolls throughout the Pacific and

Data sources

Several studies have utilised high resolution (i.e. sub-metre pixel size) optical satellite imagery to study land area change on atolls (Table 1). These images are typically commercial, however, which are costly to acquire and analyse at a large scale. In contrast, Landsat provides freely accessible imagery at a coarser resolution (30 m) to that typically employed in such studies. Landsat imagery is widely used for a range of analyses of land cover change and has recently provided global-scale

Global-scale change in the area of atoll islands

Between the oldest (1999–2001 or 1999–2002) and most recent (2017) composite images, the land area on the 221 atolls examined increased by 61.74 km2 from 1007.60 km2 to 1069.35 km2, a 6.1 % increase. Most of this increase, 38.89 km2, occurred between 2013 and 2017 (Fig. 3). The global-scale change in atoll island landmass was largely a product of an increase of island area in the Maldives and South China Sea (SCS), which account for 54.05 km2 (87.56 %) of the global increase in land area.

Global scale island change

This study has examined, for the first time, land area change on atolls at a global-scale. The study measures land area change in the major atoll nations including the Maldives, Marshall Islands, Kiribati and Tuvalu, as well as large numbers of atolls in Micronesia and French Polynesia. This examination of island change covers a spatial extent far greater than previous efforts to monitor land area change on atolls, which have not exceeded ten atolls in any given study (Table 1). Likewise, by

Conclusions

In summary, analysis of change in the land area of islands perched on 221 atolls, using satellite imagery, produced answers to the research questions posed in this paper. First, the study shows a large increase the area of atoll islands, with 61.74 km2 of land added between 2000 and 2017. s, results show that most changes in atoll land area have taken place in the Maldivian archipelago and on atolls in the South China Sea. Of the 61.74 km2 of land area increase observed in the study, 54.07 km2

Data availability

All Landsat imagery used in the study is publicly available. All code used in the analysis is available at https://github.com/geotrip/AtollIslandsInGEE

Declaration of Competing Interest

This research received no funding beyond a student scholarship to Andrew Holdaway. The authors are aware of no conflicts of interest with this research.

Acknowledgements

This work was supported by a University of Auckland Masters scholarship.

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