Prioritising local sea level rise adaptation pathways in Shenzhen and Hong Kong using cost-benefit analysis

Local governments face urgent challenges from sea-level rise (SLR), impacting coastal urban areas significantly. Despite a shared commitment to adapting to climate change and SLR-induced hazards, strategies for SLR adaptation vary significantly between cities, reflecting a spectrum of political, socio-economic, and cultural contexts. This paper examines potential SLR adaptation strategies in Shenzhen and Hong Kong, cities with different political and socio-economic frameworks, to understand the effectiveness of varied local adaptation pathways to meet adaptation objectives. Through an analysis of shoreline adaptation infrastructures, including natural shelters and hard defense structures, alongside historical coastal reclamation data from 1985 to 2020, we identify the most cost-effective adaptation pathway. Specifically, our cost-benefit analysis reveals that Shenzhen ' s " Hybrid management " pathway offers superior economic efficiency, increasing benefits by up to 63% for only a 7% - 16% rise in protection costs. Conversely, Hong Kong ' s " Relative-risk Reduction " pathway shows less timely adaptation actions, indicating a need for more dynamic approaches. The findings highlight the pitfalls of overly rigid SLR adaptation strategies and the necessity for flexible, transition-capable adaptation pathways to better manage future uncertainties. Lessons learned from this study will enable refinement of local SLR adaptation pathway approaches and could significantly improve decision-making under uncertainty in other coastal cities based on investment perspective.