TS.D.7.5 Adaptation is costly, but the benefit-to-cost ratio is high for urbanised coastal areas with high concentrations of assets . Protection has a high benefit-cost ratio during the 21st century but can become unaffordable and insufficient to reduce coastal risk , reaching technical limits . Hard protection sets up lock-in of assets and people to risks and reaches limits by the end of the century or sooner, depending on the scenario, local sea level rise effects and community tolerance thresholds . Considering coastal retreat as part of the solution space could lower global adaptation costs but would result in large land losses and high levels of migration for South and Southeast Asia in particular and in relative terms, small island nations would suffer most . Solutions include disincentivising developments in high-risk areas and addressing existing social vulnerabilities now . 
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report