Introduction Coastal communities around the world are increasingly alarmed by the rising sea levels and frequent coastal floods. Since 1900, average sea levels have risen by nearly 21 centimeters, at an average rate of 1.7 millimeters per year, which has doubled over the past two decades reaching a rate of up to 3.7 millimeters per year. Without climate policies and investments, projections indicate that mean sea levels could rise by up to 1 meter (m) by 2100.[1] Even more alarming is that more than 2.5% of the global population lives in coastal areas less than 3 m above sea level. In Asia and the Pacific, 44.1 million people live in areas with elevation below 1 m, and 112.3 million people live in areas below 3 m. The rising sea level and flooding are especially concerning for residents in low-lying deltas like Bangladesh, Viet Nam, and atoll countries where a large number of people and assets reside in vulnerable coastal zones. Rapid urbanization and population growth in coastal areas further increase the risks. Economic opportunities and limited land drive populations toward the coast, where many megacities are located. This demographic shift increases exposure to flood risks, particularly in countries such as Indonesia, Philippines, the PRC, and Viet Nam, where rapid coastal development often outpaces the implementation of adequate flood protection and adaptation measures, with possible risks of lock in development pathways. The Asian Development Bank (ADB) conducted a study to assess the impacts of sea level rise and coastal flooding across its developing member countries, evaluating the costs and benefits of adaptation. The study used state-of-the-art flood inundation models, extreme sea level estimates, digital elevation data, remote sensing data on flood protection levels, and gridded exposure datasets. Socioeconomic Impacts of Coastal Flooding The study indicates that average annual damage from coastal flooding under the current climate situation is substantial—estimated at $26.8 billion or 0.1% of the region's gross domestic product (GDP). Southeast Asia bears the brunt of this impact, accounting for over 42% of the total damage, followed by South Asia and East Asia. Countries like the PRC and Indonesia are particularly affected, each facing potential losses exceeding $6 billion annually. The actual economic repercussions extend beyond direct losses, disrupting businesses and agriculture and further straining these economies. Nearly six million people in ADB developing member countries are at risk of coastal flooding every year, representing about 0.15% of the region's total population. Figure 1: Average Yearly Damage Expected annual damage from coastal floods in developing member countries until the year 2100 under different shared socioeconomic pathways (SSPs). The lines express the ensemble median projections and the colored areas the 5th-95th confidence interval. SSPs are used to derive GHG emission scenarios under different climate policies.Note: SSP1-1.9: sustainability, SSP1-2.6: low-emissions, SSP2-4.5: moderate-emissions, SSP3-7.0: high-emissions, and SSP5-8.5: very-high-emissions.Source: ADB. Projected Impacts and Economic Estimates The projected impacts of coastal flooding are concerning. By the end of the century, the average annual flooded area among the developing member countries is estimated to reach 72,600 square kilometers, nearly four times the current affected areas. Average annual economic damage is expected to rise between 12.5 and 27.4 times by 2100, translating to $336.8 billion to $735.5 billion, depending on future climate and shared socioeconomic pathways. These estimates underscore the urgent need for robust adaptation strategies, as the financial burden could grow exponentially to 2.8% of these countries’ combined GDP under a high-emission scenario. Aside from economic impacts, projections indicate that at least 49.3 million people will be affected by coastal flooding annually by 2100. This represents a substantial portion of the population at 1.9% to 4.9%. Residents of the Pacific and Southeast Asia regions are at higher risk, emphasizing the need for targeted adaptation and mitigation efforts. Low-lying atolls (Kiribati, Maldives, Marshall Islands, and Tuvalu), which are inherently prone to coastal erosion and inundation, will face extremely high impacts. By 2050, even under a low emission scenario, annual economic impacts from flooding are projected to exceed 3% of their GDP. Implementing net-zero emission policies could help avoid almost $400 billion in direct economic damages and protect an average of 55 million people from annual flooding by 2100. Such mitigation benefits corresponds to 54% of the anticipated impacts under the high emissions scenario. Nevertless, due to the inertia of the system, even a stabilization of GHG in the atmosphere will not preven sea level to continue to rise for centuries to millenia, making adaptation efforts unavoidable. Table 1. Present and Future Impacts of Coastal Flooding along the Coastlines of Developing Member Countries. 2020 2050 2100 Annually Flooded Area(km2 x 1000) 18.6 45.0-52.7 72.6-129.8 Permanently Flooded Area(km2 x 1000) - 1.6-2.3 4.7-11.2 People Annually Affected(millions) 6.0 22.7-30.2 49.3-104.0 Mean Annual Damage(billionUSD) 26.8 143.7-197.7 336.8-735.5 The table provides information on annually and permanently flooded areas, mean number of people affected, and annual damage.Source: ADB. Adaptation Strategies: Building a Resilient Future Implementing adaptation initiatives comes at a significant cost but is necessary. ADB estimates annual costs for coastal adaptation among developing member countries could range from $9 billion to $17 billion. However, the benefits far outweigh the costs, with net economic benefits projected at least 10 times the investments. Adaptation strategies should include the following considerations: Proactive investmentsProactive investments in resilience to inform development choises are critical to mitigate future impacts of climate change and sea level rise on coastal communities. Transformative approachAddressing coastal flooding and sea level rise requires transformative approaches that go beyond short-term fixes, including long-term planning, investments, and decision making. Integrated and coordinated approachApproaches should be based on system understanding of risks and opportunities and combining different forms of coastal adaptation (e.g., nature-based, hybrid, and gray), depending on coastal characteristics and land use. This also includes other forms of adaptation, such as improved land use and settlement planning, resilient infrastructure systems, governance, policy integration, disaster preparedness, and community resilience. Several adaptation strategies will be applied in ADB climate-related projects. The proposed India: Maharashtra Sustainable Climate Resilient Coastal Protection and Management Project will implement hybrid and nature-based solutions to counteract erosion and flooding and include initiatives for livelihood, capacity building, and improvement of shoreline management practices. Meanwhile, the proposed Fiji Enhancing Climate Resilience of Coastal Communities Project explores a combination of hard and nature-based solutions to mitigate coastal flooding and erosion, enhance biodiversity, support the livelihood of local communities, and increase stakeholder awareness and capacity on integrated coastal management and conservation strategies. [1] relative to 2020. Resources Asian Development Bank (ADB). 2022. Sea-Level Change in the Pacific Islands Region: A Review of Evidence to Inform Asian Development Bank Guidance on Selecting Sea-Level Projections for Climate Risk and Adaptation Assessments. ADB. 2024. An Innovative and Climate-Resilient Approach to Coastal Management and Adaptation for India. M. Vousdoukas et al. 2018. Global Probabilistic Projections of Extreme Sea Levels Show Intensification of Coastal Flood Hazard. Nature Communications. 9. p. 2360. M. Vousdoukas et al. 2023. Small Island Developing States Under Threat by Rising Seas Even in a 1.5 °C Warming World. Nature Sustainability. 6. pp. 1552–1564. Ask the Experts Alessio Giardino Senior Climate Change Specialist (Coastal Adaptation), Climate Change and Sustainable Development Department, Asian Development Bank Dr. Alessio Giardino is a senior climate and coastal adaptation specialist with over 20 years’ experience in climate adaptation, disaster risk management, and coastal resilience. He leads the knowledge and innovation work to inform the design of resilient investments in the water sector, specifically on coastal adaptation. Prior to ADB, he worked at Deltares in The Netherlands. He holds a MSc in Environmental Engineering (cum laude) from the University of Turin in Italy, and a PhD in Coastal Engineering from KU Leuven in Belgium. Follow Alessio Giardino on Michalis I. Vousdoukas Climate Change Specialist, Department of Marine Sciences, University of the Aegean, Greece Dr. Michalis I. Vousdoukas is a coastal oceanographer specializing in the impacts of climate change and adaptation strategies along global coastlines. With experience across research centers and universities in seven countries, he has authored over 80 scientific journal papers. He frequently serves as an expert for the United Nations and was a contributing author to Chapter 12 of the IPCC Working Group I Sixth Assessment Report, which focuses on climate change information for regional impact and risk assessment. Asian Development Bank (ADB) The Asian Development Bank is committed to achieving a prosperous, inclusive, resilient, and sustainable Asia and the Pacific, while sustaining its efforts to eradicate extreme poverty. Established in 1966, it is owned by 68 members—49 from the region. Its main instruments for helping its developing member countries are policy dialogue, loans, equity investments, guarantees, grants, and technical assistance. 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