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A systematic global stocktake of evidence on human adaptation to climate changeAssessing global progress on human adaptation to climate change is an urgent priority. Although the literature on adaptation to climate change is rapidly expanding, little is known about the actual extent of implementation. We systematically screened >48,000 articles using machine learning methods and a global network of 126 researchers. Our synthesis of the resulting 1,682 articles presents a systematic and comprehensive global stocktake of implemented human adaptation to climate change. Documented adaptations were largely fragmented, local and incremental, with limited evidence of transformational adaptation and negligible evidence of risk reduction outcomes. We identify eight priorities for global adaptation research: assess the effectiveness of adaptation responses, enhance the understanding of limits to adaptation, enable individuals and civil society to adapt, include missing places, scholars and scholarship, understand private sector responses, improve methods for synthesizing different forms of evidence, assess the adaptation at different temperature thresholds, and improve the inclusion of timescale and the dynamics of responses.

Retreat from some areas will become unavoidable under intensifying climate change. Existing deployments of managed retreat are at small scale compared to potential future needs, leaving open questions about where, when, and how retreat under climate change will occur. Here, we analyze more than 40,000 voluntary buyouts of flood-prone properties in the United States, in which homeowners sell properties to the government and the land is restored to open space. In contrast to model-based evaluation of potential future retreat, local governments in counties with higher population and income are more likely to administer buyouts. The bought-out properties themselves, however, are concentrated in areas of greater social vulnerability within these counties, pointing to the importance of assessing the equity of buyout implementation and outcomes. These patterns demonstrate the challenges associated with locally driven implementation of managed retreat and the potential benefits of experimentation with different approaches to retreat.

Adaptive capacity is an important element of long-term adaptation to climate change and is the focus of a rapidly growing body of research. Interdisciplinary growth has the potential to introduce new methods and insights, but it could also cause fragmentation and hamper methodological development or limit transfer of academic insights to climate change adaptation practice. This article uses qualitative content, bibliometric, and citation network analyses to systematically review the scope, methods, and findings of 276 studies on adaptive capacity of social and social-ecological systems. The review demonstrates that adaptive capacity research is highly interdisciplinary; covers a wide range of sectors, geographic locations, and scales of analysis; and is highly fragmented. The majority of empirical studies are isolated by lack of comparative work and cross-field citation. Forty-six percent of studies reviewed do not cite prior works on adaptive capacity: even those on similar topics in the same geographic location. Methods to assess adaptive capacity have proliferated to include more than 64 indicator-based indices or frameworks and 37 proxy outcome measures. The article argues that lack of either consensus or debate across the literature raises concerns that scientific progress in the field may be constrained and the ability of adaptive capacity research to inform adaptation practice may be limited. To promote consistency and transparency in future work, 158 determinants of adaptive capacity are defined and illustrated with common assessment indicators and examples. Additional opportunities for progress are noted with suggestions for future research. This article is categorized under: Vulnerability and Adaptation to Climate Change > Institutions for Adaptation K E Y W O R D S adaptive capacity, climate change adaptation, citation network, fragmentation, systematic review

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F aced with global warming, rising sea levels, and the climate-related extremes they intensify, the question is no longer whether some communities will retreat-moving people and assets out of harm's way-but why, where, when, and how they will retreat. To the extent that retreat is already happening, it is typically ad hoc and focused on risk reduction in isolation from broader societal goals. It is also frequently inequitable and often ignores the communities left behind or those receiving people who retreat. Retreat has been seen largely as a last resort, a failure to adapt, or a one-time emergency action; thus, little research has focused on retreat, leaving practitioners with little guidance. Such a narrow conception of retreat has limited decision-makers' perception of the tools available and stilted innovation. We propose a reconceptualization of retreat as a suite of adaptation options that are both strategic and managed. Strategy integrates retreat into long-term development goals and identifies why retreat should occur and, in doing so, influences where and when. Management addresses how retreat is executed. By reconceptualizing retreat as a set of tools used to achieve societal goals, communities and nations gain additional adaptation options and a better chance of choosing the actions most likely to help their communities thrive.We argue for strategy that incorporates socioeconomic development and for management that is innovative, evidence-based, and context-specific. These are not radical alterations to adaptation practice-adaptation planning often starts with identifying the goals people have, and context-specific implementation has long been a central tenet of adaptation-but they have been underapplied to retreat. Retreat is hard to do and even harder to do well, for many reasons: short-term economic gains of coastal development; subsidized insurance rates and disaster recovery costs; misaligned incentives between residents, local officials, and national governments; imperfect risk perceptions; place attachment; and preference for the status quo (1-6). A reconceptualization could make strategic, managed retreat an efficient and equitable adaptation option.

Human societies will transform to address climate change and other stressors. How they choose to transform will depend on what societal values they prioritize. Managed retreat can play a powerful role in expanding the range of possible futures that transformation could achieve and in articulating the values that shape those futures. Consideration of retreat raises tensions about what losses are unacceptable and what aspects of societies are maintained, purposefully altered, or allowed to change unaided. Here we integrate research on retreat, transformational adaptation, climate damages and losses, and design and decision support to chart a roadmap for strategic, managed retreat. At its core, this roadmap requires a fundamental reconceptualization of what it means for retreat to be strategic and managed. The questions raised are relevant to adaptation science and societies far beyond the remit of retreat.

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