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Coral reef islands are considered to be among the most vulnerable environments to future sea level rise. However, emerging data suggest that different island types, in contrasting locations, have formed under different conditions in relation to past sea level. Uniform assumptions about reef island futures under sea level rise may thus be inappropriate. Using chronostratigraphic analysis from atoll rim islands (sand- and gravel-based) in the southern Maldives, we show that while island building initiated at different times around the atoll (similar to 2,800 and similar to 4,200 calibrated years before present at windward and leeward rim sites, respectively), higher than present sea levels and associated high-energy wave events were actually critical to island initiation. Findings thus suggest that projected sea level rise and increases in the magnitude of distal high-energy wave events could reactivate this process regime, which, if there is an appropriate sediment supply, may facilitate further vertical reef island building.

Plain Language Summary The habitability of reef island nations under climate change is a debated and controversial subject. Improving understanding of reef island responses to past environmental change provides important insights into how islands may respond to future environmental change. It is typically assumed that all reef islands will respond to environmental change in the same manner, but such assumptions fail to acknowledge that reef islands are diverse landforms that have formed under different sea level histories and across a range of settings. Here we reconstruct reef island evolution in two contrasting settings (in terms of exposure to open ocean swell) in the southern Maldives. Important differences in island development are evident between these settings in the timings, sedimentology, and modes of island building, even at local scales. This implies that island responses to climate change may be equally diverse and site-specific. We present evidence that island initiation was associated with higher than present sea levels and high-energy wave events. Projected increases in sea level and the magnitude of such high-energy wave events could therefore recreate the environmental conditions under which island formation occurred. If there is a suitable sediment supply, this could result in vertical island-building, which may enhance reef island future resilience.