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WIREs Clim Change
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Destruction or persistence of coral atoll islands in the face of 20th and 21st century sea‐level rise?

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The future of low‐lying reef islands has been the subject of international concern, scientific debate, and media interest in the last decade. As a result of sea‐level rise, atoll islands are expected to become increasingly unstable and to be susceptible to potential depopulation by the end of the 21st century. Some have suggested that sea‐level rise has already resulted in widespread erosion and inundation of atoll islands. Here, we analyze the physical changes in over 200 islands on 12 atolls in the central and western Pacific in the past few decades when sea level in the region increased at rates three to four times the global average. Results show little evidence of heightened erosion or reduction in island size. Instead island shores have adjusted their position and morphology in response to human impacts such as seawall construction and to variations in climate–ocean processes. These changes are reviewed and the role of sea‐level rise is evaluated. The implications of this analysis are addressed in two parts. First, we consider the proposition that future sea‐level rise will destabilize atoll islands to such an extent that their inhabitants will be forced to migrate offshore. And second, we identify a series of new challenges relating to risk reduction and adaptation policy for atoll island governments, international agencies, and island communities. These require a substantial shift away from the present adaptation paradigm of external migration and focus on the persistence of atoll islands and in‐country solutions. WIREs Clim Change 2015, 6:445–463. doi: 10.1002/wcc.350

Global and Pacific sea levels. (a) Global mean sea‐level rise (GMSL) in mm from 1870 to 2009. (Reprinted with permission from Refs and . Copyright 2011 Wiley Blackwell; Springer). (b) Total sea‐level rise in mm/year at stations across the Pacific from west to east estimated over the period 1950–2009 based on annual reconstructed sea level (RESL) and continuous global positioning system (GPS) records of vertical land motion. The horizontal line represents the global mean sea‐level trend over this period of 1.8 mm/year with error band of ±0.5 mm/year. (Reprinted with permission from Ref . Copyright 2012 Elsevier Ltd.). (c) Sea‐level records from the 1970s for three tide gauge stations Pohnpei, Funafuti, and Papeete in the western and central Pacific. The sites are close to the atolls where studies of multidecadal island shoreline changes have been undertaken (see Figure for locations). (Source: Monthly data from Permanent Service for Mean Sea Level; http://www.pol.ac.uk/pmsl/)
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Island area change (%) between 1897 and 2013 for a sample of 12 islands on Funafuti atoll, Tuvalu. The four island types located on the map of Funafuti atoll (Figure (a)) are color coded. The trend of global mean sea‐level rise from 1890 to 2010 is also shown (see Figure (a)).
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Summary data of island area and percentage decadal change at seven atoll sites. Island‐change data within the highlighted ±3.0% band width is not considered significant and following Ref islands in that group are regarded as being stable. (Data sources from tables as follows: Funafuti, Tarawa, Pingelap, Mokil ,Table 2, Ref ; Majuro, Table 2, Ref ; Nadikdik, Table , Ref ; North Tarawa, Table 5, Ref ; Wotje, Table 3, Ref ).
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Map of the central and western Pacific showing reconstructed mean sea‐level trends in mm/year between 1950 and 2009. (Reprinted with permission from Ref . Copyright 2012 Elsevier Ltd.). Includes location of tide gauge stations for sites identified in Figure (b) and (c) and the location of atolls where the studies of multidecadal island shoreline changes reviewed here have been undertaken.
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Atoll and reef island morphology and structure. (a) Funafuti atoll in the central Pacific with its reef perimeter and lagoon. The atoll is approximately 25 km N–S and 18 km E–W. There are 33 islands on the reef rim, the boxes showing locations of the four types of atoll islands found in the central Pacific: Type 1 islands are composed of sand, roughly symmetrical to oval in shape, typically located on leeward atoll rims near reef passages (Box I). Type 2 islands are developed at convex‐seawards bends of atoll rim (Box II), commonly boomerang or horseshoe shaped with coral gravel ridges to seaward and sand ridges along lagoon shores. Type 3 islands are narrow and elongate, occur on straight narrow reefs (Box III), and comprise sediments and topography similar to Type 2 islands. Type 4 islands are complex structures of mixed coral gravel and sand, developed on cemented rubble, rectangular in shape, separated by narrow passages (Box IV). (Source from Ref ). (b) Cross‐section of a typical atoll showing major structural elements including deep lagoon (20–50 m) and reef rim with islands, reef flat, and reef crest identified. (c) Atoll island commonly 50–100 m wide with a high ridge on the ocean side (right) and lower ridge on the lagoon side (left) showing vegetated core and mobile beaches along both shores and cemented beach sand (beachrock) and cemented coral rubble (conglomerate). ((b) and (c) Reprinted with permission from Ref . Copyright 2009 Cambridge University Press).
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