Coastal cities, vital for economic growth and innovation, are increasingly vulnerable to climate change impacts like rising sea levels, extreme weather events, and heatwaves. These impacts interact with existing urban vulnerabilities such as inequality and inadequate infrastructure, creating diverse risk profiles across cities. The urgent need for adaptation is undeniable, particularly for transformative adaptation that fundamentally alters social-ecological systems. While the conceptual need for transformative adaptation has been highlighted, a comprehensive global assessment of its actual progress in coastal cities is lacking. This is crucial for tracking climate action and informing policy, especially in the context of the Global Stocktake under the Paris Agreement. Previous studies have focused on specific adaptation types, actors, regions, or planning approaches. However, a systematic global analysis of empirical evidence on implemented coastal urban adaptation, encompassing its types, actors, and level of transformation, is still absent. This study aims to fill this gap by providing a global analysis of empirical evidence of adaptation in coastal cities, identifying shortcomings, and informing policy and practice for improved adaptation strategies.

The study leverages a systematic review of peer-reviewed literature (2013-2020) on coastal urban adaptation, expanding upon existing assessments by focusing on empirical evidence of implemented responses. The research builds on previous work examining various aspects of urban adaptation, including institutional or ecosystem-based approaches, specific actors, regional contexts, and adaptation planning. However, it addresses the lack of a global, systematic assessment that integrates response types, actors, and the extent of transformative change. This systematic approach, while acknowledging potential biases related to language and data sources (e.g., underrepresentation of low- and middle-income countries), provides valuable insights into both adaptation research and actual adaptation activities.

The study employed a systematic map approach following the ROSES protocol and the Global Adaptation Mapping Initiative (GAMI) process. The research involved a four-tiered literature search across three databases (Web of Science, Scopus, and the GAMI database), focusing on peer-reviewed, English-language publications from 2013 to 2020. This period was chosen to encompass the end of the IPCC's fifth assessment cycle and the beginning of the sixth. After removing duplicates, 683 publications underwent screening based on pre-defined inclusion/exclusion criteria, resulting in 183 publications covering 199 coastal cities for qualitative coding. A structured questionnaire with 30 questions was used for data extraction. The coding process ensured inter-coder reliability through a detailed code book, pre-coding discussions, and double-coding of 10% of the dataset. Data analysis involved descriptive statistics, correlation tests (Spearman's rank correlation and χ² test), and assessment of transformative adaptation based on depth, speed, and scope of reported activities. The study also used publicly available population data to assess the geographical representation of the studied cities in relation to global population distributions in low-elevation coastal zones (LECZ).

The study revealed significant geographical and economic disparities in the available literature on coastal urban adaptation. While literature covers cities across all regions and income groups, high-income countries are overrepresented, especially North America, while Asia, with a large LECZ population, is underrepresented. The literature predominantly focuses on adaptation in smaller cities rather than megacities. Regarding hazards, adaptation activities primarily address sea-level rise, different types of flooding, and storm surges, often considering multiple hazards simultaneously. However, the majority of studies considered past and current events, with limited quantitative assessments of future hazard trends. Similarly, adaptation measures often focused on past and current patterns of exposure and vulnerability, primarily concerning population, vulnerable groups, and residential buildings. The types of adaptation reported are predominantly technological/infrastructural and behavioral/cultural. Ecosystem-based responses were less frequently reported, especially in low- and middle-income countries. High-income countries are more likely to feature institutional responses implemented by city governments, whereas lower-income countries more often rely on households implementing behavioral adaptation. City size also influenced the type of actor involved: larger cities show more government-led adaptations, whereas individual/household-led actions were more prevalent in smaller cities. Across all regions and income groups, reported adaptation measures are characterized by low depth, scope, and speed, indicating a lack of transformative change. There is limited evidence of risk reduction, with some cases even showing maladaptation. While some instances of deeper adaptation were observed (e.g., resettlement, integration of adaptation into broader planning), these remained exceptions.

The findings highlight critical gaps in knowledge and action concerning coastal urban adaptation. The uneven geographical coverage of research underscores the need for greater attention to adaptation in low- and middle-income countries, especially in Africa, Asia, and Central and South America, where the interplay of urbanization, vulnerable settlements, and climate change impacts is projected to be highly dynamic. The reliance on past and current data for adaptation planning raises concerns about the relevance and efficacy of strategies given accelerating climate change. The disparity in adaptation approaches across income groups highlights inequalities in burden-sharing, with low-income communities disproportionately relying on individual adaptation measures due to limited institutional and technological support. While technology-based solutions are crucial in the short term, a long-term strategy needs to consider alternative and complementary measures, including ecosystem-based adaptations, which remain underreported. The overall lack of transformative adaptation is alarming given the escalating risks. While incremental actions may contribute cumulatively, their pace may be insufficient to manage the rapid changes underway.

This study emphasizes the urgent need for transformative adaptation in coastal cities globally. The significant gaps in knowledge and the predominantly incremental nature of observed adaptation measures underscore the critical need for a substantial shift in approaches. Future research should prioritize addressing the underrepresentation of low- and middle-income cities in the literature and promoting research on effective, equitable, and transformative adaptation strategies. Enhanced data collection and analysis, encompassing various data sources beyond peer-reviewed publications, are crucial for building a more comprehensive understanding of adaptation efforts and informing evidence-based policies. Further research should explore the cumulative effects of incremental adaptation and investigate the conditions that foster transformative change.

The study's reliance on peer-reviewed, English-language publications introduces potential biases, particularly underrepresenting low- and middle-income countries and potentially overlooking adaptation activities not documented in academic literature. The study's timeframe (2013-2020) limits the scope of the analysis to recent adaptation actions, and future studies could benefit from more up-to-date data. The use of self-reported data from studies may also introduce reporting biases. While efforts were made to address these issues using multiple methods, they present limitations that should be acknowledged in interpreting the results.