Small Island Developing States under threat by rising seas even in a 1.5 °C warming world

Small Island Developing States (SIDS), encompassing diverse island nations across the Caribbean, Atlantic, Pacific, and Indian Oceans, are disproportionately vulnerable to climate change impacts, especially rising sea levels and extreme coastal events. Their unique geographic characteristics, limited resources, and high dependence on coastal assets make them particularly susceptible to such threats. Since the 1990s, SIDS have been recognized as frontline communities experiencing the brunt of climate change, playing a vital role in international discussions pushing for ambitious mitigation targets and addressing loss and damage associated with climate-related events. Despite this global recognition, quantitative estimates of loss and damage under differing emissions scenarios remain largely lacking. This research addresses this gap by providing a comprehensive assessment of future coastal flood risk in all SIDS, examining impacts under various climate change scenarios, including the ambitious 1.5°C warming target. Understanding the potential economic losses and displacement of populations under these scenarios is critical for effective adaptation and resource allocation strategies for these vulnerable nations.

Existing studies on climate risks in SIDS often focus on individual states, groups of states, or specific regions. The inherent diversity amongst SIDS in terms of geographical characteristics (atolls, volcanic islands, continental territories), governance systems, economic development levels, and data availability hinders the creation of a comprehensive global picture. While some global coastal hazard assessments encompass SIDS, they often lack the detailed analysis or specific focus needed to understand the unique vulnerabilities of these nations. This research aims to synthesize existing literature concerning coastal flood risks in SIDS while expanding upon previous studies by providing a globally comprehensive assessment incorporating a more detailed methodology and broader data range.

This study employs the LISCOAST (Large-scale Integrated Sea-level and Coastal Assessment Tool) framework, a modular system combining state-of-the-art global modeling tools and datasets. The analysis considers five Shared Socioeconomic Pathways (SSPs) representing a spectrum of emissions scenarios, from ambitious mitigation (1.5°C world, SSP1-1.9) to very high emissions (SSP5-8.5). The methodology integrates probabilistic dynamic simulations of relative sea-level rise, tides, waves, and surges to estimate extreme sea levels (ESLs) along SIDS coastlines. Two-dimensional (2D) hydrodynamic modeling is used to determine permanently inundated and episodically flooded areas, taking into account artificial flood protection and natural protection provided by reefs and mangroves. The assessment quantifies human exposure and direct economic losses by incorporating flood inundation maps with detailed exposure data (population, land use, GDP per capita) and empirical vulnerability relations. The study utilizes improved data sets, including the GLO-30 DEM, which mitigates known limitations in previous global-scale assessments. It accounts for wave and tropical cyclone effects, nonlinear interactions between water-level components, and the attenuating effects of reefs and mangroves. The analysis considers various return periods (1, 2, 5, 10, 20, 50, 100, 200, 500, 1000, and 5000 years) for extreme sea levels to generate probabilistic risk assessments. The study also incorporates population and economic projections under the different SSP scenarios to further refine the estimates of future coastal flood impacts, going beyond simple climate change modeling to include dynamic socioeconomic factors. A sensitivity analysis is conducted to assess the impact of natural protection provided by ecosystems.

The study projects a substantial increase in coastal flood risk across SIDS by 2100. Under high-emission scenarios (SSP5-8.5), the expected annual flooded area (EAFA) could be over 29,515 km² (median estimate), more than eight times greater than the present-day EAFA. Even under the 1.5°C scenario (SSP1-1.9), EAFA would exceed 19,213 km². The area permanently flooded (below future high-tide water levels) could range from 7,653 km² (1.5°C) to 16,274 km² (very high emissions). The Expected Annual Number of People Exposed (EAPE) to coastal flooding also increases significantly across all scenarios. Expected Annual Damage (EAD) is projected to increase substantially, reaching hundreds of billions of US dollars by 2100, depending on the emissions scenario. Mitigating emissions to moderate levels (SSP2-4.5) could reduce end-of-century coastal flood damage by 23%, with further reductions possible by limiting warming to 1.5°C. Certain countries are identified as particularly vulnerable, with projected economic losses significantly impacting their GDP. These include countries like the Maldives, Bahrain, Guinea-Bissau, Belize, Bahamas, Vanuatu, and others. The study also found that economic growth amplifies the projected damage estimates across all scenarios, demonstrating the compounding impact of socioeconomic factors on future coastal flood risks. The findings show that even under the ambitious 1.5°C scenario, significant coastal flood impacts are unavoidable, highlighting the crucial need for adaptation strategies.

The findings emphasize the urgent need for adaptation strategies in SIDS. Even under stringent mitigation scenarios, coastal flood risks will increase dramatically, necessitating significant investments in coastal protection and sustainable development. The high projected economic losses relative to the GDP of many SIDS underscore the economic vulnerability of these nations. The integration of socioeconomic scenarios into the flood impact assessment provides a more realistic portrayal of future risks, considering factors like population growth and economic development, which can either exacerbate or mitigate climate-induced changes. The study highlights the benefits of ambitious climate mitigation, showcasing a significant reduction in projected damages even under the most optimistic scenario, further bolstering the call for international cooperation to reduce greenhouse gas emissions. The model’s limitations related to future ecosystem dynamics and uncertainties in socioeconomic projections should be considered when interpreting the results.

This study presents a comprehensive assessment of coastal flood risks in SIDS, revealing substantial and unavoidable increases under all considered climate change scenarios. Even the most ambitious mitigation targets won't entirely prevent significant losses. The study highlights the importance of investing in adaptation measures, including both engineered coastal defenses and ecosystem-based approaches. Future research should focus on improving the prediction of ecosystem dynamics and enhancing the integration of social and economic variables into coastal flood risk assessments at increasingly localized scales. International cooperation is crucial to provide financial and technical support for SIDS to implement necessary adaptation and mitigation measures.

The study acknowledges several limitations, including uncertainties in future sea-level rise projections, particularly beyond the 21st century, and uncertainties in socioeconomic development pathways. The assumption of static vulnerability and the limited consideration of potential adaptations such as managed retreat and evolving coastal protection strategies could underestimate or overestimate the extent of future impacts. The model’s reliance on existing datasets presents data-related uncertainties, and the lack of high-resolution data across all SIDS could lead to some inaccuracies in the assessment at a very localized level. Although the methodology is significantly improved over previous work, further advancements in modeling techniques and data availability are needed to reduce uncertainty in future risk estimates.