In addition, the majority of carbonate islands are positioned on coral reefs, where measured vertical reef flat accretion rates are up to an order of magnitude slower than the rates of projected SLR. Most atoll islands have limited adaptation space, land available for human habitation ( 7– 9), and water and food sources ( 10, 11), and most have ecosystems that are vulnerable to seawater inundation ( 12). Furthermore, the tropics are dotted with thousands of low-lying atoll islands, and because many of these islands have maximum elevations less than 4 m and average elevations less than 2 m above present sea level, they are expected to experience disproportionately greater SLR impacts ( 6). Projections indicate that these effects will be amplified in the tropics, where sea level will be higher than the global average ( 5). Although the precise rates and elevations of SLR by 2100 are uncertain, the existing models all suggest that eustatic sea level will be significantly higher by the end of the century and that it will have a profound impact on low-lying coastal areas. Recent observations ( 1, 2) and projections ( 3, 4) of sea level show that eustatic sea-level rise (SLR) by the end of the 21st century could exceed 2.0 m above 2000 levels. This study provides critical information for understanding the timing and magnitude of climate change impacts on atoll islands that will result in significant, unavoidable geopolitical issues if it becomes necessary to abandon and relocate low-lying island states. This annual flooding will result in the islands becoming uninhabitable because of frequent damage to infrastructure and the inability of their freshwater aquifers to recover between overwash events. We show that, on the basis of current greenhouse gas emission rates, the nonlinear interactions between sea-level rise and wave dynamics over reefs will lead to the annual wave-driven overwash of most atoll islands by the mid-21st century. We project the impact of sea-level rise and wave-driven flooding on atoll infrastructure and freshwater availability under a variety of climate change scenarios. However, these have not taken into account the additional hazard of wave-driven overwash or its impact on freshwater availability. Most studies on the resilience of these islands to sea-level rise have projected that they will experience minimal inundation impacts until at least the end of the 21st century.
Sea levels are rising, with the highest rates in the tropics, where thousands of low-lying coral atoll islands are located.
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