Rising mean and extreme sea‐levels and induced increased coastal flooding are expected to lead to massive coastal migration if coasts are not protected. Using a wide range of sea‐level rise (SLR) scenarios, socioeconomic pathways and discount rate assumptions, 21st century coastal migration is assessed at global scale assuming local cost‐benefit optimal protection decisions for about 12,000 coastal segments with homogeneous coastal and socioeconomic characteristics. Costs considered include investment and maintenance cost for protection, migration cost in the case of no protection, and expected annual damage to assets by extreme sea‐level events that over‐top existing protection. Robust decisions in favor of protection over all scenarios are found for about 3% of the global coastline, covering 78% of global coastal population and 92% of global coastal floodplain assets. For the remaining 97% of global coastline cumulative 21st century land loss ranges from 60,000 to 415,000 km² and coastal migration ranges from 17 to 72 million people. Big countries with long uninhabited coastlines suffer the biggest land losses. In absolute terms big countries in South and South‐east Asia account for the highest coastal migration, while in relative terms small island nations suffer most. Global cost of 21st century SLR can be lowered by factor two to four if local cost‐benefit decisions also consider, next to protection, coastal migration as an adaptation option.

中文翻译：

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21世纪海平面上升带来的沿海移民

如果沿海地区得不到保护，平均海平面和极端海平面的上升以及由此引发的沿海洪灾的增加预计将导致大规模的沿海移民。利用广泛的海平面上升（SLR）情景，社会经济路径和折现率假设，在全球范围内对21世纪沿海移民进行了评估，并假设本地成本效益是对约12,000个沿海和社会经济特征均一的沿海地区进行最佳保护的决定。考虑的成本包括用于保护的投资和维护成本，在没有保护的情况下的迁移成本，以及由于超出现有保护范围的极端海平面事件而对资产造成的预期年度损失。在大约3％的全球海岸线上，做出了在所有情况下都支持保护的可靠决策，覆盖了全球78％的沿海人口和92％的全球沿海洪泛区资产。对于其余97％的全球海岸线，21世纪累积的土地损失范围为60,000至415,000 km²，沿海移民的人数从17到7200万人。海岸线长而无人居住的大国遭受的土地损失最大。绝对而言，南亚和东南亚大国是沿海人口迁移最多的国家，而相对而言，小岛屿国家则是受灾最多的国家。如果当地的成本效益决定在保护的基础上还考虑将沿海移民作为一种适应性选择，则可以将21世纪SLR的全球成本降低2到4倍。